MicroBiology MCQs with Answer and Explnation | Chapter 55

Answer: Typhoid fever

Typhoid fever, caused by Salmonella Typhi bacteria, can lead to a chronic carrier state. This means the person can harbor the bacteria without symptoms for months or even years and still shed the bacteria in their stool, potentially infecting others.

The other options are incorrect:

• Campylobacter enterocolitis: While this bacterial infection can cause diarrhea, it typically resolves on its own and doesn’t lead to a chronic carrier state.
• Shigella enterocolitis: Similar to Campylobacter, Shigella infection causes diarrhea but doesn’t usually result in chronic carriage.
• Cholera: Cholera, caused by Vibrio cholerae bacteria, leads to severe watery diarrhea. While some individuals may become asymptomatic carriers for a short period, chronic carriage is uncommon.

Answer: Shigella sonnei

Shigella sonnei, a type of bacteria, causes shigellosis. This illness involves direct invasion of the intestinal mucosa, leading to inflammation, ulceration, and bloody diarrhea.

The other options are incorrect:

• Enterotoxigenic Escherichia coli (ETEC): ETEC produces toxins that act on the intestinal lining, causing diarrhea without directly invading the mucosa.
• Clostridium botulinum: This bacterium produces a potent neurotoxin that affects the nervous system and doesn’t target the intestinal mucosa.
• Vibrio cholerae: Similar to ETEC, Vibrio cholerae causes cholera through toxin production affecting the intestinal lining, not by mucosal invasion.

Answer: Bacteroides fragilis and Klebsiella pneumoniae

In a post-surgical patient with symptoms of fever and abdominal pain suggestive of peritonitis, the most likely causative organisms are Bacteroides fragilis and Klebsiella pneumoniae. Bacteroides fragilis is an anaerobic bacterium commonly found in the gastrointestinal tract and is a frequent cause of intra-abdominal infections such as peritonitis. Klebsiella pneumoniae is a Gram-negative bacterium that can cause nosocomial infections, including postoperative peritonitis, especially in immunocompromised or debilitated patients.

The other options are incorrect:

• Bordetella pertussis and Salmonella enteritidis: Bordetella pertussis causes whooping cough (pertussis) and is not associated with peritonitis. Salmonella enteritidis is a common cause of foodborne gastroenteritis but is less likely to cause peritonitis, especially in the absence of gastrointestinal surgery.
• Clostridium botulinum and Shigella dysenteriae: Clostridium botulinum causes botulism, a neurological illness characterized by muscle weakness and paralysis, and is not associated with peritonitis. Shigella dysenteriae is a bacterium that causes bacillary dysentery (shigellosis) characterized by bloody diarrhea but is not a common cause of postoperative peritonitis.
• Actinomyces israelii and Campylobacter jejuni: Actinomyces israelii is a bacterium associated with actinomycosis, a chronic suppurative infection that can involve various body sites but is rare and not typically associated with postoperative peritonitis. Campylobacter jejuni is a common cause of bacterial gastroenteritis but is not a common cause of peritonitis, especially in the postoperative setting.

Answer: Gastric acid

Gastric acid in the stomach is the most important host defense mechanism for preventing dysentery caused by Salmonella. It has a powerful bactericidal effect, meaning it can kill many bacteria, including Salmonella, before they reach the intestines and cause dysentery.

The other options are incorrect:

• Salivary enzymes: While saliva contains enzymes with some antimicrobial properties, they are not strong enough to significantly impact Salmonella survival.
• Normal flora of the mouth: The normal bacterial flora in the mouth competes with ingested bacteria for resources, but it’s not the primary defense against Salmonella in the context of dysentery.
• Alpha interferon: Interferon is a type of immune signaling molecule, but it’s not the first line of defense in the stomach. It plays a role in the adaptive immune response, which takes longer to activate compared to the immediate barrier provided by gastric acid.

Answer: Enterobacter

Enterobacter is a type of gram-negative bacteria commonly found in the environment and human gut flora. While it can occasionally cause infections, it’s not a well-known cause of diarrhea compared to the other listed options.

The other options are incorrect:

• Salmonella: Salmonella is a common cause of foodborne illness leading to diarrhea, fever, and abdominal cramps.
• Rotavirus: Rotavirus is a highly contagious virus causing severe diarrhea, especially in young children.
• Campylobacter: Campylobacter jejuni is a bacterial pathogen causing diarrhea, often associated with contaminated food or water.

Answer: Shigella sonnei

Shigella sonnei may have caused this outbreak. Shigella species, including S. sonnei, are Gram-negative rods that typically do not ferment lactose, are nonmotile, and do not produce hydrogen sulfide (H2S). They are known to cause bloody diarrhea (dysentery) and are often associated with outbreaks in settings like day care centers.

The other options are incorrect:

• Escherichia coli (E. coli): While some E. coli strains can cause diarrhea, the specific characteristics (non-motility, no H2S production) are less common with diarrheagenic E. coli.
• Salmonella typhimurium: Salmonella can cause diarrhea, but it often presents with additional symptoms like fever and vomiting, which may not be prominent in this case.
• Rotavirus: Rotavirus is a common cause of viral diarrhea in children, but it wouldn’t typically show PMNs and blood in a fecal smear. Additionally, Rotavirus wouldn’t be identified by the bacterial culture mentioned.

Answer: Budding yeast and pseudohyphae

The clinical presentation suggests oral thrush, a fungal infection caused by Candida albicans. Candida is a type of yeast, and under certain conditions, it can form thread-like structures called pseudohyphae. These structures are not true hyphae (seen in molds) but resemble them in appearance.

The other options are incorrect:

• Mold with septate hyphae: Molds have true hyphae with walls divided by cross-walls (septa). While fungal infections can occur in the mouth, oral thrush is typically caused by yeast, not molds.
• Encapsulated yeast: Candida albicans is not typically encapsulated.
• Mold with nonseptate hyphae: This describes a characteristic of some fungal molds, but not Candida albicans associated with oral thrus

Answer: Staphylococcus aureus

Staphylococcus aureus is most likely to cause symptoms of vomiting and diarrhea within a short time after ingestion of contaminated food. This bacterium produces heat-stable enterotoxins that can cause rapid onset of food poisoning symptoms, typically within 1 to 6 hours after consuming contaminated food.

The other options are incorrect:

• Campylobacter jejuni: Campylobacter jejuni is a common cause of bacterial gastroenteritis, but its symptoms typically manifest 2 to 5 days after ingestion of contaminated food or water, not within hours.
• Salmonella enteritidis: Salmonella enteritidis can cause symptoms of gastroenteritis, including vomiting and diarrhea, but the onset of symptoms typically occurs within 6 to 72 hours after ingestion of contaminated food, not within hours.
• Shigella flexneri: Shigella flexneri causes shigellosis, which is characterized by symptoms such as abdominal cramps, fever, and diarrhea with blood or mucus. The onset of symptoms usually occurs within 1 to 7 days after ingestion, making it less likely to cause symptoms within hours after a meal.

Answer: An integrated temperate phage

Pathogenic Corynebacterium diphtheriae contains an integrated temperate phage within its genetic material, which carries the genes for diphtheria toxin production. This toxin is a key virulence factor responsible for the symptoms of diphtheria.

The other options are incorrect:

• Highly repetitive bacterial DNA: Highly repetitive bacterial DNA is not specific to pathogenic Corynebacterium diphtheriae and can be found in various bacterial species, including nonpathogenic diphtheroids.
• A diphthamide on eEF-2: Diphthamide is a post-translationally modified amino acid found on eukaryotic elongation factor 2 (eEF-2). Its presence or absence is not a distinguishing characteristic between pathogenic Corynebacterium diphtheriae and nonpathogenic diphtheroids.
• An episome: An episome is a genetic element that can replicate independently of the bacterial chromosome but can also integrate into it. While certain bacterial pathogens may carry episomes, it is not a specific feature that distinguishes pathogenic Corynebacterium diphtheriae from nonpathogenic diphtheroids.

Answer: Conjugation resulting in transfer of an R (resistance) factor

Conjugation is a process in bacterial genetics that involves the transfer of genetic material, often in the form of plasmids, from one bacterium to another through a sex pilus. This transfer can include genes encoding antibiotic resistance (R factors), which is a significant mechanism for the spread of antibiotic resistance among bacterial populations.

The other options are incorrect:

• Transduction of a chromosomal gene: Transduction involves bacteriophages (viruses that infect bacteria) transferring fragments of bacterial DNA from one bacterium to another. This process doesn’t involve a sex pilus.
• Integration of a temperate bacteriophage: Temperate phages can integrate their DNA into the bacterial chromosome. This doesn’t require a sex pilus.
• Transposition of a mobile genetic element: Transposition is the movement of a segment of DNA within the same genome or to a different plasmid. Sex pili aren’t involved in this process.

Answer: Through site-specific recombination of a temperate phage and bacterial DNA

A prophage is the dormant form of a temperate phage integrated into the bacterial chromosome. This integration happens through a specific recombination event between the phage and bacterial DNA.

The other options are incorrect:

• Through infection of a bacterial cell with lambda phage, lacking the lambda repressor: While lambda phage can lysogenize (become a prophage) under certain conditions, simply lacking the lambda repressor wouldn’t guarantee prophage formation. Without proper integration, the phage might enter the lytic cycle and lyse the host cell.
• Through activation of the recA gene product of an exogenote: The recA gene is involved in DNA repair, and its activation can be a stress response in bacteria. However, it’s not directly linked to prophage formation. An exogenote refers to foreign genetic material outside the chromosome, and its presence wouldn’t necessarily trigger prophage creation.
• Through excision of bacterial DNA and active lytic replication of a bacteriophage: This describes the lytic cycle of a phage, where the phage replicates within the host cell and lyses it to release new phage particles. Prophage formation is the opposite, where the phage integrates its DNA and becomes dormant.

Answer: Conjugation with a cell with a free plasmid carrying drug resistance

Conjugation is the process by which genetic material, often in the form of plasmids carrying drug resistance genes, is transferred from one bacterial cell to another via direct cell-to-cell contact. This mechanism is most likely to be involved in the transfer of multiple drug resistance between bacterial cells.

The other options are incorrect:

• Transformation of chromosomal genes: Transformation involves the uptake of naked DNA from the environment by a bacterium. While it can transfer genes, it’s less common for multiple drug resistance genes to be packaged together for transformation.
• Transposition: Transposition involves the movement of a genetic element within the same chromosome or to a different plasmid. It can be involved in spreading resistance genes, but it wouldn’t necessarily transfer them between different bacteria.
• Specialized transduction of a chromosomal gene for drug resistance: Specialized transduction involves a bacteriophage (virus) transferring a specific bacterial gene between cells. While it can transfer drug resistance genes, it typically targets a single gene, not multiple genes on a plasmid.

Answer: is a change in pathogenicity due to the presence of a prophage.

Conjugation with a cell with chromosomal drug resistance, resulting in the transfer of a prophage carrying drug resistance genes, can lead to a change in pathogenicity of the recipient bacterium. This change occurs because the prophage may carry additional virulence factors or alter the expression of bacterial genes, resulting in an increase in pathogenicity.

The other options are incorrect:

• Is the conversion of a virulent phage into a temperate phage: This option describes lysogeny, where a virulent (lytic) phage integrates its DNA into the host bacterium’s chromosome, becoming a prophage. It does not specifically involve conjugation or chromosomal drug resistance.
• Is the immunity that a prophage confers on a bacterium: This option describes lysogenic immunity, where the presence of a prophage in a bacterial cell prevents superinfection by the same or related phage. It does not specifically relate to conjugation or chromosomal drug resistance.
• Refers to the incorporation of a prophage into the chromosome: This option describes lysogeny, where a prophage integrates into the bacterial chromosome. It does not specifically involve conjugation or chromosomal drug resistance.

Answer: A non-encapsulated strain of Streptococcus pneumoniae acquires a gene for capsule formation from the extract of an encapsulated strain.

Bacterial transformation involves the uptake and incorporation of foreign DNA by a bacterial cell, leading to genetic changes. In this scenario, the non-encapsulated strain of Streptococcus pneumoniae acquires a gene for capsule formation from the extract of an encapsulated strain through transformation, resulting in the acquisition of the ability to produce capsules.

The other options are incorrect:

• A formerly non-toxigenic strain of Corynebacterium diphtheriae becomes toxigenic: This event is more likely due to lysogeny, where a bacteriophage carrying the gene for diphtheria toxin infects the bacterium, leading to the integration of the toxin gene into the bacterial chromosome.
• A gene for gentamicin resistance from an Escherichia coli chromosome: This event is more likely due to conjugation or transduction, where the gene for gentamicin resistance is transferred from one bacterium to another via a plasmid (conjugation) or a bacteriophage (transduction).
• A strain of Neisseria gonorrhoeae starts producing a plasmid-encoded beta-lactamase similar to that another Gram-negative strain: This event is more likely due to conjugation, where the plasmid encoding beta-lactamase is transferred from one bacterium to another through direct cell-to-cell contact.

Answer: Recombinant DNA technology provides a high degree of risk to the health of the general population.

While there are potential biosafety concerns associated with recombinant DNA technology, strict guidelines and regulations are in place to minimize risks. There have been no documented cases of illnesses in the general population directly attributed to recombinant DNA technology.

The other options are incorrect:

• Thus far, no illnesses in laboratory workers have been traced to genetic recombinants: This statement is generally true. Rigorous safety protocols are followed to minimize the risk of exposure to potentially hazardous organisms during research.
• Recombinant DNA technology offers specific benefits to the scientific, medical, and general population: This is accurate. Recombinant DNA has led to advancements in medicine (e.g., production of insulin), agriculture (e.g., pest-resistant crops), and scientific research (e.g., gene function studies).
• Mutant strains of bacteria produced by genetic recombination are often unable to survive in the natural environment: This can be true in some cases. Genetic modifications might not be advantageous for survival outside the laboratory environment.

Answer: Transformation

Transformation is the process by which a cell takes up naked DNA from the environment and incorporates it into its own genome. This ability to bind to and import DNA is essential for transformation to occur.

The other options are incorrect:

• Homologous recombination: This process involves the exchange of genetic material between two similar DNA strands. It doesn’t require a cell to take up external DNA.
• Conjugation: This process involves the transfer of genetic material between two bacteria through a physical connection (sex pilus). No external DNA needs to be bound or imported.
• Specialized transduction: This is a process where a bacteriophage (virus that infects bacteria) transfers a specific bacterial gene between cells. The phage doesn’t require the recipient cell to actively bind or import DNA; the phage injects the genetic material.

Answer: Myeloma cells

Hybridomas are formed by fusing antibody-producing white blood cells, also known as B lymphocytes, with myeloma cells. Myeloma cells are cancerous plasma cells, which are mature B lymphocytes that normally produce antibodies.

The other options are incorrect:

• Bone marrow cells: Bone marrow is the tissue where B lymphocytes develop, but it doesn’t contain the specific antibody-producing cells needed for hybridoma formation.
• Viruses: Viruses are not used in hybridoma technology.
• Red blood cells: Red blood cells are not involved in antibody production and wouldn’t be used for hybridoma creation.

Answer: Transformation

Deoxyribonuclease (DNase) is an enzyme that breaks down DNA. Transformation involves the uptake of naked DNA from the environment by a bacterium and its incorporation into the recipient’s genome. If DNase is present in the environment, it can degrade the free DNA molecules before they can be taken up by bacteria, effectively preventing transformation.

The other options are incorrect:

• Transduction: This process involves a bacteriophage (virus) transferring bacterial DNA between cells. The DNA is usually packaged within the phage capsid, protecting it from degradation by DNase.
• Transfection: While not commonly used in bacteria, transfection refers to the introduction of foreign DNA directly into a eukaryotic cell (like an animal cell). In some transfection methods, the DNA might be protected from degradation by DNase.
• Conjugation: This process involves the transfer of genetic material between bacteria through a physical connection (sex pilus). The DNA is typically transferred within a plasmid or a chromosomal segment, protected from the external environment and not susceptible to DNase.

Answer: Transduction

This is the process where a bacteriophage (virus that infects bacteria) acts as a vector to transfer bacterial DNA from one bacterium (donor) to another (recipient). The phage packages some of the donor’s DNA along with its own genetic material and injects it into the recipient cell.

The other options are incorrect:

• Transformation: Transformation involves the uptake of naked DNA from the environment by a recipient bacterium. No virus is involved.
• Transfection: Transfection typically refers to the introduction of foreign DNA directly into a eukaryotic cell (like an animal cell), not bacteria. It doesn’t involve a bacterial virus.
• Conjugation: Conjugation involves the transfer of genetic material between bacteria through a physical connection (sex pilus). No virus is involved.

Answer: sex pilus

During conjugation in E. coli, a sex pilus plays a critical role in forming a temporary connection between the donor and recipient bacteria. This pilus acts like a bridge for transferring genetic material from the donor to the recipient.

The other options are incorrect:

• Restriction endonuclease: Restriction enzymes are involved in cutting DNA at specific recognition sequences. While they might play a role in some bacteria to restrict foreign DNA, they aren’t essential for forming the mating pair during E. coli conjugation.
• Transfer of both strands of DNA: Typically, only one strand of the donor DNA is transferred during E. coli conjugation. The recipient strand replicates to form a complete double-stranded DNA molecule.
• Integration of Transposon: Transposons are mobile genetic elements that can move within the same genome or to a different plasmid. While they can be transferred during conjugation, their integration isn’t necessarily required for forming the initial mating pair.

Answer: Conjugation, eg, transfer of an R (resistance) factor

Conjugation involves the transfer of genetic material between bacteria, typically through a sex pilus. This transfer can involve plasmids carrying genes for antibiotic resistance (R factors) or other traits. However, conjugation doesn’t necessarily involve recombination. The transferred DNA molecule usually remains separate from the recipient’s chromosome.

The other options are incorrect:

• Transduction of a chromosomal gene: This process involves a bacteriophage (virus) transferring a fragment of bacterial DNA from one bacterium to another. This can lead to the incorporation of the transferred DNA into the recipient’s chromosome, which is a form of recombination.
• Integration of a temperate bacteriophage: When a temperate phage infects a bacterium, it can integrate its DNA into the host chromosome. This is a form of recombination.
• Transposition of a mobile genetic element: Transposons can move around within the same genome or to a different plasmid. This movement involves the cutting and pasting of DNA sequences, which is a type of recombination.

Answer: organized aggregates of immune cells that surround foci of infected tissues

Tubercular granulomas are a hallmark feature of tuberculosis infection. These are not simply damaged tissues or collections of dead cells. They are a complex organized structure formed by the body’s immune system in response to Mycobacterium tuberculosis infection.

The other options are incorrect:

• Damaged tissue cells and bacilli only: While damaged tissue and bacteria might be present, granulomas are more than just that. They involve a specific immune response.
• Fibrous and damaged tissue and bacilli only: Similar to the previous option, granulomas involve organized immune cells, not just fibrous tissue.
• Necrotic tissue and damaged tissue and bacilli only: Necrosis (cell death) can be a part of a granuloma, but it’s not the only component. The organized immune cell response is a defining feature.

Answer: isoniazid and rifampin irrespective of resistance to any other drug

MDR TB (Multidrug-Resistant Tuberculosis) is defined as tuberculosis that is resistant to at least isoniazid and rifampin, irrespective of resistance to any other drugs. Isoniazid and rifampin are two of the most effective first-line therapeutic drugs used to treat tuberculosis.

The other options are incorrect:

• more than three anti-tubercular drugs: This option does not accurately define MDR TB. MDR TB specifically refers to resistance to at least isoniazid and rifampin, two of the most effective first-line therapeutic drugs used to treat tuberculosis.
• fluoroquinolones and at least one of the three injectable second-line drugs used to treat TB: This definition corresponds to extensively drug-resistant tuberculosis (XDR TB), not MDR TB. XDR TB is resistant to at least isoniazid and rifampin (like MDR TB) but is also resistant to fluoroquinolones and at least one of the three injectable second-line drugs used to treat TB.
• Isoniazid, Pyrazinamide, and Rifampicin: This option lists three of the first-line therapeutic drugs used to treat tuberculosis but does not specify resistance to any particular combination of drugs, as required for the definition of MDR TB.

Answer: Virulent strains

Cord growth, characterized by the formation of cord-like structures, is typically observed in cultures of virulent strains of Mycobacterium tuberculosis. These structures are composed of tightly packed bacterial cells and are associated with the virulence of the organism.

The other options are incorrect:

• Saprophytic strains: These are Mycobacterium strains that live freely in the environment and don’t typically cause disease. They might not exhibit cord formation as prominently as virulent strains.
• Avirulent strains: These are Mycobacterium strains that lack the virulence factors to cause disease in humans. Cord formation might be less frequent or absent in avirulent strains.
• Atypical mycobacteria: This is a broad category of Mycobacterium species that are not M. tuberculosis or M. leprae. While some atypical mycobacteria can cause disease, cord formation isn’t a universal characteristic for all of them.

Answer: Robert Koch

Robert Koch was a German physician and microbiologist who made significant contributions to the field of bacteriology. In 1882, he announced the discovery of Mycobacterium tuberculosis, the bacterium responsible for tuberculosis. This discovery revolutionized our understanding of infectious diseases and paved the way for the development of diagnostic tests and treatments for TB.

The other options are incorrect:

• Louis Pasteur: Another famous microbiologist from France, Louis Pasteur is known for his work on vaccines, pasteurization, and the germ theory of disease. While he made crucial advancements in microbiology, he wasn’t the one who discovered M. tuberculosis.
• Jean-Antoine Villemin: A French physician, Jean-Antoine Villemin played a key role in establishing the infectious nature of tuberculosis through his experiments in the 1860s. However, he didn’t isolate the specific bacterium responsible for the disease.
• Calmette and Guerin: These are the names of two scientists, Albert Calmette and Camille Guérin, who co-developed the BCG vaccine for tuberculosis. They weren’t involved in the initial discovery of M. tuberculosis.

Answer: Examine transparent adhesive tape for the presence of eggs

Pinworms lay their eggs around the anus at night, causing itching. Examining a stool sample or blood smear wouldn’t detect these eggs. An IgE antibody test wouldn’t be specific to pinworms. Examining transparent adhesive tape (commonly called the “Scotch tape test”) pressed against the anus in the morning is the simplest and most effective way to diagnose pinworms.

The other options are incorrect:

• Examine a blood smear for the presence of microfilaria: Microfilaria are the larval stage of filarial worms, which are different from pinworms. Blood smears wouldn’t show pinworm eggs.
• Examine the stool for the presence of cysts: Pinworms don’t form cysts. Stool examination is used for diagnosing infections with other parasites that produce cysts in their life cycle.
• Determine the titer of IgE antibody against the organism: An IgE antibody test is used to diagnose allergies, not specifically for pinworm infections. While an allergic reaction to pinworms is possible, it’s not the primary diagnostic method.

Answer: Gram-negative curved rod; microaerophilic

The positive urease breath test points towards Helicobacter pylori (H. pylori) infection. H. pylori is a gram-negative, curved rod-shaped bacterium that thrives in the low-oxygen environment of the stomach lining (microaerophilic).

The other options are incorrect:

• Gram-negative rod; aerobic: While some gram-negative rods are aerobic (thrive in oxygen), H. pylori specifically requires a low-oxygen environment.
• Gram-negative rod; facultative anaerobe: Facultative anaerobes can survive with or without oxygen. H. pylori prefers a microaerophilic environment.
• Gram-positive rod; aerobic: H. pylori is gram-negative and microaerophilic, not gram-positive and aerobic.

Answer: Botulism is caused by a toxin that hydrolyzes lecithin (lecithinase), thereby destroying nerve cells.

Botulinum toxin works by blocking the release of acetylcholine, a neurotransmitter, at neuromuscular junctions. This disrupts nerve signaling to muscles, leading to paralysis. Lecithinase is not involved in botulism.

The other options are incorrect:

• Some strains of Escherichia coli produce an enterotoxin that causes diarrhea: This is true. Certain E. coli strains produce heat-labile or heat-stable enterotoxins that disrupt intestinal fluid balance, causing diarrhea.
• Cholera toxin acts by stimulating adenylate cyclase: This is true. Cholera toxin increases cyclic AMP (cAMP) levels in intestinal cells, leading to excessive fluid secretion and diarrhea.
• Diphtheria is caused by an exotoxin that inhibits protein synthesis by inactivating an elongation factor: This is true. Diphtheria toxin inactivates elongation factor-2 (EF-2), an essential protein for ribosomal protein synthesis, leading to cell death and tissue damage.

Answer: V. alginolyticus

While some Vibrio species can be associated with opportunistic infections, Vibrio alginolyticus is the one most commonly linked to ear infections, particularly otitis externa (swimmer’s ear) in warm, coastal waters.

The other options are incorrect:

• V. parahemolyticus: This species is primarily associated with foodborne illness caused by consuming contaminated seafood.
• V. fulnifcus: This species is more commonly linked to wound infections, especially those involving exposure to saltwater.
• Vibrio fluvialis: While it can cause some skin and soft tissue infections, Vibrio fluvialis is less commonly implicated in ear infections compared to Vibrio alginolyticus.

Answer: Infection will resolve only with a combination of antituberculous drugs, and then it may take weeks

Acid-fast structures found in the stool suggest Mycobacterium avium complex (MAC) infection, a common opportunistic infection in HIV patients with low CD4+ counts. These infections require treatment with a combination of antibiotics for weeks or months.

The other options are incorrect:

• Even with the best treatment, the infection may be unrelenting: While some MAC infections can be difficult to treat, with proper medication most cases resolve.
• Infection is short lasting and self-resolving and requires no treatment: MAC infections are not short-lived and require treatment.
• Infection could have been prevented by avoiding cat feces and undercooked or raw meat: While some acid-fast bacteria can be transmitted through these means, MAC is not typically contracted in this way. It is an opportunistic infection that takes advantage of a weakened immune system.

Answer: Clostridium difficile

The most likely cause of fever and severe diarrhea following antibiotic treatment, especially with ampicillin, is Clostridium difficile infection. Ampicillin is known to disrupt the normal gut microbiota, allowing C. difficile to proliferate and cause diarrhea. This condition is known as antibiotic-associated diarrhea or pseudomembranous colitis.

The other options are incorrect:

• Bacteroides fragilis: While this bacteria can cause diarrhea, it’s less common as a cause of antibiotic-associated diarrhea compared to C. diff.
• Proteus mirabilis: This bacterium can cause urinary tract infections but is not typically associated with diarrhea.
• Bordetella pertussis: This bacterium causes whooping cough, which does not present with diarrhea.

Answer: Gram-negative curved rod; toxin that increases cAMP

This description points towards Vibrio cholerae, the bacteria responsible for cholera. Vibrio cholerae is a gram-negative, curved rod-shaped bacterium. It produces a toxin that disrupts the intestinal lining, leading to profuse watery diarrhea. This toxin specifically increases cyclic adenosine monophosphate (cAMP) levels in intestinal cells, causing excessive fluid secretion.pen_spark

The other options are incorrect:

• Gram-negative rod; toxin that inhibits protein synthesis: This description is not applicable to Vibrio cholerae. Cholera toxin does not inhibit protein synthesis; instead, it increases cAMP levels in host cells.
• Gram-negative rod; toxin that increases cAMP: This is the correct description of Vibrio cholerae and its toxin. It is not an incorrect option.
• Intoxication with a heat-labile toxin that blocks the release of acetylcholine: This description does not fit Vibrio cholerae. It might be referring to botulism, caused by Clostridium botulinum, where the toxin blocks the release of acetylcholine at neuromuscular junctions, leading to flaccid paralysis, but it does not cause profuse watery diarrhea.

Answer: Vibrio cholera

While some Vibrio species can be associated with opportunistic infections, Vibrio alginolyticus is the one most commonly linked to ear infections, particularly otitis externa (swimmer’s ear) in warm, coastal waters.

The other options are incorrect:

• V. parahemolyticus: This species is primarily associated with foodborne illness caused by consuming contaminated seafood.
• V. fulnifcus: This species is more commonly linked to wound infections, especially those involving exposure to saltwater.
• Vibrio fluvialis: While it can cause some skin and soft tissue infections, Vibrio fluvialis is less commonly implicated in ear infections compared to Vibrio alginolyticus.

Answer: Cytomegalovirus

Cytomegalovirus (CMV) is the most common virus that can be transmitted from a mother to her baby in utero (during pregnancy) in the United States. It affects approximately 0.5% to 1.5% of births.

The other options are incorrect:

• Rubella virus: While a serious concern during pregnancy, thanks to widespread vaccination programs, rubella is uncommon in the US.
• Hepatitis B virus: Hepatitis B can be passed from mother to baby, but routine screening and vaccination for pregnant women significantly reduces the risk.
• Respiratory syncytial virus (RSV): RSV typically infects infants after birth, not in utero.

Answer: Inapparent disease, manifested only by virus shedding, is common

While both Varicella-zoster virus (VZV) and Herpes Simplex Virus (HSV) can cause inapparent (asymptomatic) infections, it’s not as common as the other listed characteristics. These viruses typically cause noticeable symptoms during primary infection.

The other options are incorrect:

• Linear, double-stranded DNA genome: Both VZV and HSV share this genetic structure.
• Vesicular rash: Both viruses can cause a blistering rash, although the location and appearance might differ slightly.
• Persistence of latent virus after recovery from acute disease: A hallmark characteristic of both VZV and HSV is their ability to establish latency within nerve cells after the initial infection. This allows the virus to reactivate later in life under certain conditions, causing recurrent outbreaks (shingles for VZV and cold sores/genital herpes for HSV).

Answer: Establishment of latent infection

After a primary herpes simplex virus (HSV) infection, the virus doesn’t get completely eliminated from the body. Instead, it establishes a latent infection within nerve cells. In this state, the virus remains dormant but can reactivate later in life under certain triggers like stress, illness, or sun exposure, causing recurrent outbreaks (cold sores or genital herpes).

The other options are incorrect:

• Persistent asymptomatic viremia: Viremia refers to the presence of virus particles in the bloodstream. While HSV can shed in saliva or genital secretions during reactivation, persistent asymptomatic viremia (virus constantly present in the blood) is not typical for HSV infections.
• Persistent cytopathic effect in infected cells: A cytopathic effect describes how a virus damages and kills host cells. During an active HSV infection, there might be cell death, but the virus doesn’t permanently destroy all infected cells. It establishes latency within some cells.
• Complete eradication of virus and virus-infected cells: The human immune system can’t completely eliminate HSV. While it can control the infection and prevent outbreaks, the latent virus persists within nerve cells.

Answer: Acyclovir reduces the number of recurrent disease episodes by eradicating latently infected cells.

Acyclovir is an antiviral medication that cannot eliminate the latent reservoir of HSV. While it can shorten outbreaks and reduce viral shedding, it doesn’t eradicate infected cells.

The other options are incorrect:

• Initial disease episodes are generally more severe than recurrent episodes: This statement is generally accurate. Primary HSV infections tend to be more severe with longer healing times compared to recurrent outbreaks.
• Multinucleated giant cells with intranuclear inclusions are found in the lesions: This is also accurate. These are microscopic features characteristic of HSV infection.
• Genital herpes can be transmitted in the absence of apparent lesions: This is true. HSV shedding can occur even without visible sores, making transmission possible during asymptomatic periods.

Answer: Parvovirus B19

Parvovirus B19 is the most likely agent in this scenario. It commonly presents with a lacy body rash, known as erythema infectiosum or “fifth disease.” The rash typically starts on the face (giving a “slapped cheek” appearance) before spreading to the trunk and extremities. Fever may also accompany the rash.

The other options are incorrect:

• Rubella: While rubella can cause a rash, it typically starts on the head and spreads downward, unlike the progression described (face first, then trunk and extremities).
• Varicella zoster virus (chickenpox): Chickenpox causes itchy, fluid-filled blisters, not a “lacy” rash.
• Measles: Measles rash is typically red and blotchy, starting at the hairline and spreading downward. It may also present with conjunctivitis (pink eye) and cough, which are not mentioned in the scenario.

Answer: Ability to attach in sialic acid receptor

The ability to attach to the sialic acid receptor is not responsible for the genetic diversity of Influenza A virus. Instead, it’s a feature related to its infection mechanism. Genetic diversity in Influenza A virus primarily arises from its segmented genome, which undergoes frequent reassortment and mutation events.

The other options are incorrect:

• RNA Virus: Influenza A is an RNA virus, and RNA viruses have higher mutation rates compared to DNA viruses due to the lack of proofreading mechanisms during replication. This inherent characteristic contributes to the generation of genetic variations.
• Ability to grow in many species: Influenza A viruses can infect various mammals and birds. This ability allows for the mixing of viral genes from different hosts during co-infection, leading to a process called reassortment. Reassortment is a major mechanism for generating new influenza A strains with different properties.
• Segmented genome: The segmented genome of influenza A virus is another crucial factor for its diversity. The eight separate RNA segments can be reassorted during co-infection, creating new combinations and potentially leading to novel virus strains.

Answer: Influenza

Influenza viruses have segmented RNA genomes, which allows for genetic reassortment during co-infection with different strains. This reassortment can lead to dramatic changes in surface antigens (antigenic shift) in addition to the gradual mutations that occur over time (antigenic drift). These changes allow influenza to evade the immune system and create new strains that can cause widespread illness.

The other options are incorrect:

• HIV: HIV undergoes antigenic drift, but not antigenic shift. Its mutations are slower and more targeted to evade the immune system, but they don’t create entirely new subtypes.
• Rabies: Rabies is a single-stranded RNA virus and primarily mutates through antigenic drift.
• Rotavirus: Rotavirus has a double-stranded RNA genome and primarily evolves through antigenic drift.

Answer: Hemagglutinin

Hemagglutinin (HA) is a glycoprotein spike found on the surface of the influenza virus. There are 18 different subtypes of HA, designated H1 through H18, and these subtypes determine the influenza A virus strain. The immune system recognizes these HA proteins, and antibodies against specific HA subtypes can protect you from infection.

The other options are incorrect:

• Lipid in viral envelope: The viral envelope is a fatty layer that surrounds the virus but doesn’t contain type-specific antigens.
• Major structural protein: This is a broad category, and influenza has several major structural proteins. The type-specific antigen is not shared by all structural proteins.
• Nucleocapsid: The nucleocapsid is the core of the virus and contains the viral RNA genome. It doesn’t contain type-specific antigens.

Answer: The virus enters the host in airborne droplets

Influenza is a highly contagious respiratory illness spread primarily through airborne droplets produced by coughing, sneezing, or talking. These droplets can be inhaled by another person, introducing the virus into their respiratory tract.

The other options are incorrect:

• Pneumonia is not associated with secondary bacterial infections: While influenza itself is a viral infection, it can damage the respiratory epithelium, making the host more susceptible to secondary bacterial infections like pneumonia.
• The virus frequently establishes persistent infections in the lung: Influenza infections are acute, meaning they are short-lived and the body typically clears the virus within a few weeks.
• Viral infection does not kill cells in the respiratory tract: Influenza replicates within respiratory epithelial cells, ultimately leading to cell death and contributing to symptoms like cough and sore throat.

Answer: Glycoproteins on influenza virus that contribute to virulence

Hemagglutinin (HA) and neuraminidase (NA) are two crucial glycoproteins embedded in the envelope of the influenza virus. They play a significant role in the virus’s ability to infect host cells and cause illness (virulence).

The other options are incorrect:

• Proteins found in the nucleus of influenza virus: The influenza virus has a segmented RNA genome located in the viral core, not the nucleus. HA and NA are found on the viral envelope, not inside the core.
• Proteins that surround each segment of the nucleic acid in influenza: The viral RNA segments are associated with viral proteins (nucleoproteins) but not HA and NA. These proteins help package the RNA for replication.
• Glycoprotein receptors on influenza’s target cells: These are actually sialic acids, sugar molecules on the host cell surface that HA binds to during infection. HA and NA are viral proteins, not cellular receptors.

Answer: Single stranded positive sense RNA virus

It is a single-stranded negative-sense RNA virus. Influenza A virus, like other influenza viruses, carries a segmented genome consisting of single-stranded negative-sense RNA molecules.

The other options are incorrect:

• Viral transcription occurs in the nucleus: Influenza A virus replicates and transcribes its RNA genome in the nucleus of infected host cells.
• Mature virions are released by budding: Influenza A virus exits host cells by budding from the plasma membrane, a process that occurs without cell lysis.
• Attaches to cell-surface sialic acid: Influenza A virus attaches to host cells by binding to cell-surface sialic acid residues via the viral hemagglutinin protein.

Answer: Hot air oven

Oils and fats are non-aqueous (don’t dissolve in water) and can degrade at high temperatures. Hot air ovens provide dry heat sterilization which is effective against microorganisms at a lower temperature compared to other methods, minimizing damage to the oil/fat.

The other options are incorrect:

• Pasteurization: This method uses milder heat, effective for some bacteria but not for complete sterilization.
• Autoclaving: Uses high-pressure steam, unsuitable for oils as it can cause them to hydrolyze (break down) and lose their properties.
• Tyndallisation: This involves repeated cycles of heating and incubation, effective for some bacteria but time-consuming and may not eliminate all spores in oils.

Answer: thermal death point

The thermal death point is the lowest temperature at which all microorganisms in a liquid suspension are killed within a specified time (usually 10 minutes). It is an important parameter for assessing the effectiveness of heat-based sterilization processes.

The other options are incorrect:

• Thermal death time: Thermal death time refers to the time required to kill all microorganisms in a liquid suspension at a specific temperature. It does not specify the lowest temperature.
• Thermal death temperature: Thermal death temperature refers to the temperature at which all microorganisms in a liquid suspension are killed, but it does not specify a specific time duration.
• Decimal reduction time: Decimal reduction time is the time required to reduce the microbial population by 90% (or one log cycle) at a specific temperature. It is not necessarily the temperature at which all microorganisms are killed.

Answer: D value

D-value specifically refers to the time required at a specific temperature to kill 90% (one log cycle) of a microbial population. It’s a widely used parameter for sterilization processes.

The other options are incorrect:

• Thermal death temperature (TDT): This defines the specific temperature needed to kill all microorganisms in a set time (usually 10 minutes). It’s a single temperature, not a time-based measure like D-value.
• F-value: F-value represents the total time required under specific conditions (temperature, etc.) to achieve a desired level of sterility, often targeting a 12-log reduction (99.99999999%) of the most resistant microbe. It considers factors beyond just temperature and incorporates the entire sterilization process.

Answer: Joseph Lister

While both Louis Pasteur and Joseph Lister played crucial roles in the development of sterilization techniques, Lister is credited with pioneering “antiseptic surgery” in the 1860s. He used carbolic acid to disinfect instruments and dressings, significantly reducing surgical infections.

The other options are incorrect:

• Louis Pasteur: Though Pasteur’s work on germ theory provided the foundation for understanding infection, Lister’s practical application of disinfection in surgery had a more direct impact on sterilization techniques.
• John Needham: Needham was involved in early debates about spontaneous generation, not directly related to sterilization techniques.
• Robert Koch: Koch made significant contributions to microbiology, but Lister’s work on antiseptic surgery predates Koch’s major discoveries.

Answer: Filtration

Filtration is the method most commonly used for the sterilization of antisera. Antisera, which are serum preparations containing antibodies, are often heat-sensitive and cannot withstand the high temperatures of autoclaving, tyndallisation, or hot air oven sterilization. Filtration allows for the removal of bacteria and other microorganisms from antisera without subjecting them to heat.

The other options are incorrect:

• Autoclaving: As mentioned earlier, high temperatures in autoclaving can damage antibodies in antisera.
• Tyndallisation: Though effective for some solutions, it’s time-consuming and may not eliminate all spores.
• Hot air oven: Similar to autoclaving, hot air ovens also use high temperatures that can damage antibodies.

Answer: Bacteroides fragilis

Bacteroides fragilis is not typically associated with urinary tract infections (UTIs). It is an anaerobic bacterium commonly found in the gastrointestinal tract and is more often associated with intra-abdominal infections, such as peritonitis or abscesses.

The other options are incorrect:

• Proteus mirabilis: This bacterium is a known cause of UTIs, particularly in patients with urinary catheters or blockages.
• Escherichia coli (E. coli): This is the most frequent bacterial cause of UTIs, especially in women.
• Klebsiella pneumoniae: While less common than E. coli, Klebsiella pneumoniae can also cause UTIs, particularly in healthcare settings.