MicroBiology MCQs with Answer and Explnation | Chapter 54

Answer: Acid production from glucose

The methyl red test is a biochemical test used to differentiate between bacteria based on their ability to produce and maintain acidic end products during glucose fermentation.

The other options are incorrect:

• Indole production: This is detected using a different test called the indole test.
• Urease production: This is detected using the urease test.
• Hydrogen sulfide production: This is detected using the sulfide indole motility (SIM) test.

Answer: Mueller Hinton agar

Mueller Hinton agar is the recommended agar for antibiotic sensitivity testing (also known as antibiotic susceptibility testing). It provides a standardized and consistent medium for the diffusion of antibiotics, allowing for accurate assessment of bacterial sensitivity or resistance.

The other options are incorrect:

• Nutrient agar: Nutrient agar is a general-purpose medium used for the cultivation of a wide variety of microorganisms, but it is not specific for antibiotic sensitivity testing.
• Potato dextrose agar: Potato dextrose agar is primarily used for the cultivation of fungi, especially molds, and is not suitable for antibiotic sensitivity testing.
• MacConkey agar: MacConkey agar is selective for gram-negative bacteria, particularly Enterobacteriaceae. It is used for the isolation and differentiation of these bacteria based on lactose fermentation but is not ideal for antibiotic sensitivity testing.

Answer: Sarcina

Sarcina are a type of cocci bacteria that specifically arrange themselves in cubical packets of eight cells. This packet-like arrangement is a key characteristic used for their identification.

The other options are incorrect:

• Micrococci: Micrococci are also cocci, but they typically grow in single spheres or irregular clusters, not in organized packets of eight.
• Gonococci: Gonococci are diplococci, meaning they appear in pairs. They are the bacteria responsible for gonorrhea.
• Pneumococci: Pneumococci are streptococci, forming chains of cocci. They are responsible for diseases like pneumonia.

Answer: Streptococcus pyogenes

Streptococcus pyogenes, also known as Group A Streptococcus (GAS), is the organism associated with both rheumatic fever and glomerular nephritis. These can develop as rare complications following an untreated or inadequately treated GAS infection, like strep throat or impetigo.

The other options are incorrect:

• Staphylococcus haemolyticus: While some Staphylococcus species can cause various infections, Staphylococcus haemolyticus is not typically linked to rheumatic fever or glomerular nephritis.
• Streptococcus viridans: Streptococcus viridans are a group of oral commensal bacteria. While some strains can be opportunistic pathogens, they are not typically associated with rheumatic fever or glomerular nephritis.
• Staphylococcus aureus: Similar to Staphylococcus haemolyticus, Staphylococcus aureus can cause various infections but is not a common culprit for rheumatic fever or glomerular nephritis.

Answer: Bordetella

Bordet-Gengou (BG) agar, supplemented with fresh blood, is specifically designed to isolate Bordetella species, particularly Bordetella pertussis, the causative agent of whooping cough. The blood provides essential nutrients and allows for the detection of hemolytic reactions by Bordetella.

The other options are incorrect:

• Pasteurello: While Pasteurella and Bordetella are related bacteria, BG agar is not the primary isolation medium for Pasteurella species. Different media with specific nutrients are used for Pasteurella depending on the particular species.
• Haemophilus: Haemophilus requires enriched media with factors like X factor and V factor that BG agar lacks. Chocolate agar or Thayer-Martin agar are preferred for Haemophilus isolation.
• Yersinia: Similar to Pasteurella, Yersinia also requires specific media depending on the species. BG agar wouldn’t be the optimal choice for isolating Yersinia.

Answer: d

According to the Lancefield scheme of classifying Streptococcus species, the enterococci are placed in Group D. This classification is based on the antigenic characteristics of the bacterial cell wall, specifically the carbohydrate antigen present.

The other options are incorrect:

• A: Group A Streptococcus (e.g., Streptococcus pyogenes) is different from the enterococci and is not associated with Lancefield Group D.
• B: Group B Streptococcus (e.g., Streptococcus agalactiae) is distinct from enterococci and is classified under Lancefield Group B.
• C: Lancefield Group C includes Streptococcus equisimilis and Streptococcus equi, among others, but it does not encompass enterococci. Enterococci belong to Group D.

Answer: Sporulation and the arrangement of spores on the hyphae

Fungal identification primarily relies on characteristics related to spore formation (sporulation) and how the spores are arranged on the reproductive structures (hyphae). This information helps to differentiate between various fungal genera and species.

The other options are incorrect:

• Solubility in 20% potassium hydroxide: While KOH can be used for some preliminary tests to differentiate between fungal and non-fungal elements, it’s not the primary method for identification.
• Serologic and biochemical characteristics: These tests are more commonly used for bacterial identification, although some specific fungal tests may involve serological or biochemical analysis.
• Staining properties with polychrome dyes: Staining with dyes like lactophenol cotton blue helps visualize fungal structures, but it’s not the sole method for identification. Staining characteristics might provide clues, but the focus is on spore morphology and arrangement.

Answer: Haemophilus ducreyi

Haemophilus ducreyi is the bacterial culprit behind chancroid, a sexually transmitted infection causing painful genital ulcers.

The other options are incorrect:

• Haemophilus aegyptius: This bacterium is rarely encountered and not associated with chancroid.
• Bordetella pertussis: Bordetella pertussis is the causative agent of whooping cough, a respiratory illness, and not linked to chancroid.
• Haemophilus influenzae: While Haemophilus influenzae can cause respiratory infections, it’s not the organism responsible for chancroid.

Answer: Haemophilus influenzae

Haemophilus influenzae is a fastidious bacterium, meaning it has specific growth requirements. It requires enriched media supplemented with X factor (hemin) and V factor (NAD) for proper growth. These factors are essential for essential cellular functions in Haemophilus influenzae.

The other options are incorrect:

• Escherichia coli: E. coli is a common inhabitant of the human gut and a laboratory workhorse. It’s not fastidious and grows readily on basic media like LB agar, which doesn’t contain X and V factors.
• Listeria monocytogenes: Listeria monocytogenes is another bacterium but is not as fastidious as Haemophilus influenzae. It can grow on enriched media but doesn’t necessarily require X and V factors for survival.
• Bacillus anthracis: Bacillus anthracis is a Gram-positive bacterium. The question specifies Gram-negative rods, eliminating Bacillus anthracis from the choices.

Answer: Coagulase

The coagulase test is the primary method used to differentiate Staphylococcus aureus from other staphylococcal species. Staphylococcus aureus produces coagulase, an enzyme that can clot plasma. Other staphylococcal species typically lack this enzyme.

The other options are incorrect:

• Oxidase test: This test differentiates between bacteria that can produce the enzyme cytochrome oxidase (oxidase-positive) and those that cannot (oxidase-negative). Staphylococcus aureus is oxidase-negative, just like most other staphylococci.
• Fibrinolysin test: This test detects the presence of fibrinolysin, an enzyme that can break down fibrin clots. While some Staphylococcus aureus strains produce fibrinolysin, it’s not specific enough to differentiate S. aureus from all other staphylococci, as some other species can also produce it.
• Catalase test: This test differentiates between bacteria that produce the enzyme catalase (catalase-positive) and those that cannot (catalase-negative). Both Staphylococcus aureus and most other staphylococci are catalase-positive.

Answer: Bordetella

Bordet-Gengou (BG) agar, supplemented with fresh blood, is specifically designed for isolating Bordetella species, particularly Bordetella pertussis, the causative agent of whooping cough. The blood provides essential nutrients and allows for the detection of hemolytic reactions by Bordetella.

The other options are incorrect:

• Yersinia: Yersinia requires specific media depending on the species. BG agar wouldn’t be the optimal choice for isolating Yersinia.
• Pasteurello: While Pasteurella and Bordetella are related bacteria, BG agar is not the primary isolation medium for all Pasteurella species. Different media with specific nutrients are used for Pasteurella depending on the particular species.
• Haemophilus: Haemophilus requires enriched media with factors like X factor and V factor that BG agar lacks. Chocolate agar or Thayer-Martin agar are preferred for Haemophilus isolation.

Answer: Capsules are produced by virulent strians

Streptococcus pneumoniae, the bacterium responsible for causing pneumonia, produces capsules, particularly by virulent strains. The presence of capsules is associated with increased virulence and evasion of the host’s immune response.

The other options are incorrect:

• Is motile: Streptococcus pneumoniae is non-motile. Motility is not a characteristic feature of this bacterium.
• Is consistently gram-positive even in old cultures: While Streptococcus pneumoniae is gram-positive, it may appear gram-negative in older cultures or when subjected to certain laboratory conditions. This phenomenon is known as the “L-form” or “variant” appearance.
• Grows best at slightly acid pH: Streptococcus pneumoniae grows best at a slightly alkaline pH, typically around 7.8. It is not favored by slightly acidic conditions.

Answer: Retain their moisture

Most media, especially those containing organic compounds, can degrade or become unusable if they dry out. Maintaining moisture is crucial for their long-term storage.

The other options are incorrect:

• Maintain them in an incubator: Incubators are used for growing bacteria and other cultures, not for long-term storage. High temperatures in an incubator can accelerate spoilage.
• Avoid exposing them to light: While light exposure can affect some media types, it’s not the primary concern for most media storage.
• Maintain them at room temperature: While some media can be stored at room temperature, others require cooler temperatures (around 4°C) for extended periods.

Answer: Sporulation and the arrangement of spores on the hyphae

Spores are the reproductive units of fungi. Their characteristics, like shape, size, color, and how they are produced on the hyphae (filaments), are key identifiers for fungal species. Examining these features helps narrow down the possibilities and differentiate between fungi.

The other options are incorrect:

• Serologic and biochemical characteristics: These methods are more commonly used for identifying bacteria, not fungi. While some fungal identification techniques involve biochemical tests, they are often secondary to spore analysis.
• Staining properties with polychrome dyes: Staining can reveal certain structures within fungi, but it’s not generally used as the primary identification method. Spore characteristics are more distinctive and reliable for classification.
• Solubility in 20% potassium hydroxide: This test might be used in some specific situations, but it’s not a common or reliable way to identify fungi. Spore morphology offers a more definitive approach.

Answer: Clostridium botulinum

Clostridium botulinum is a strict anaerobe, meaning it cannot survive in the presence of oxygen. It produces a potent neurotoxin that causes botulism, a serious illness.

The other options are incorrect:

• Bacillus anthracis: Bacillus anthracis is a facultative anaerobe, meaning it can survive and grow with or without oxygen.
• Nocardia asteroides: Nocardia asteroides is an aerobic organism, requiring oxygen for its growth and survival.
• Listeria monocytogenes: Listeria monocytogenes is a facultative anaerobe, similar to Bacillus anthracis.

Answer: Euglena

Euglena is a single-celled organism with a complex structure. It has a nucleus, chloroplasts for photosynthesis, and flagella for movement. Viruses, on the other hand, are much simpler. They lack the cellular machinery for independent survival and rely on host cells to reproduce.

The other options are incorrect:

• Satellites: These are small pieces of nucleic acid that can associate with viruses but cannot replicate on their own. They are simpler than viruses.
• Viroids: These are even simpler than viruses, consisting solely of a small strand of RNA without a protein coat. They rely on a host cell’s machinery for replication.
• Prions: While prions are unusual infectious agents, they are not technically alive. They are misfolded proteins that can cause other proteins to misfold as well, leading to disease. They lack the complexity of a living organism like Euglena.

Answer: Tuberculosis

Tuberculosis (TB) is a bacterial infection caused by Mycobacterium tuberculosis. Viruses are different pathogens entirely.

The other options are incorrect:

• Influenza: Influenza, commonly known as the flu, is a respiratory illness caused by influenza viruses.
• Polio: Poliomyelitis, or polio for short, is a highly infectious viral disease that can cause paralysis.
• Hepatitis: Hepatitis is inflammation of the liver, and there are several types caused by different viruses, such as hepatitis A, B, and C.

Answer: Secondary stain

In a Gram stain, safranin is used as a counterstain, which is another term for a secondary stain. The primary stain (usually crystal violet) differentiates between gram-positive and gram-negative bacteria. Safranin helps visualize gram-negative bacteria that lose the primary stain during decolorization.

The other options are incorrect:

• Primary stain: Crystal violet or methylene blue is typically used as the primary stain in a Gram stain.
• Mordant: A mordant is a chemical that helps bind a dye to tissue. It’s not used in a Gram stain.
• Decolorizer: Alcohol or a mixture of acetone and alcohol is used as the decolorizer to remove the primary stain from gram-negative bacteria.

Answer: Capsules are produced by virulent strians

Streptococcus pneumoniae can produce a polysaccharide capsule, but it’s not always present and virulence (disease-causing ability) is not the only factor determining capsule production. Other factors like growth conditions can influence capsule formation.

The other options are incorrect:

• Grows best at slightly acid pH: Streptococcus pneumoniae prefers a near-neutral pH (around 7.0-7.5) for optimal growth.
• Is motile: Streptococcus pneumoniae is non-motile, meaning it cannot move on its own.
• Is consistently gram-positive even in old cultures: While S. pneumoniae is typically gram-positive, some older cultures might show inconsistencies in staining due to cell wall changes.

Answer: Streptococcus pyogenes

Streptococcus pyogenes, also known as Group A Streptococcus (GAS), is the organism associated with both rheumatic fever and glomerulonephritis. These complications can develop after an untreated or improperly treated S. pyogenes infection like strep throat or impetigo.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus is a gram-positive bacterium that can cause various skin infections, pneumonia, and food poisoning. It is not linked to rheumatic fever or glomerulonephritis.
• Staphylococcus haemolyticus: This is a less common species of Staphylococcus and while it can cause some infections, it’s not typically associated with rheumatic fever or glomerulonephritis.
• Streptococcus viridans: Streptococcus viridans are a group of green streptococci commonly found in the mouth and upper respiratory tract. While they can occasionally cause opportunistic infections, they are not known to cause rheumatic fever or glomerulonephritis.

Answer: Haemophilus ducreyi

Haemophilus ducreyi is a gram-negative bacterium and the sole cause of chancroid, a sexually transmitted infection (STI) characterized by painful genital ulcers.

The other options are incorrect:

• Haemophilus aegyptius: This is a rarely encountered species of Haemophilus and is not associated with any known human diseases.
• Bordetella pertussis: This bacterium causes whooping cough, a respiratory illness, and is not transmitted sexually.
• Haemophilus influenzae: While some strains of Haemophilus influenzae can cause respiratory infections, it is not the causative agent of chancroid.

Answer: They grow best under anaerobic conditions.

Neisseria species are aerobic or facultatively anaerobic, meaning they can survive with or without oxygen. They prefer to grow in the presence of oxygen, not strictly anaerobic conditions.

The other options are incorrect:

• They produce IgA protease as a virulence factor: This statement is correct. Neisseria species, like Neisseria gonorrhoeae (causing gonorrhea) and Neisseria meningitidis (causing meningitis), produce IgA protease to evade the immune system’s defenses.
• They are oxidase-positive: This statement is correct. Neisseria possess cytochrome c oxidase, an enzyme involved in the electron transport chain, and thus test positive for oxidase activity.
• They are gram-negative diplococci: This statement is correct. Neisseria are classified as gram-negative bacteria and typically appear as pairs of cocci (diplococci) under a microscope.

Answer: Haemophilus influenzae

Haemophilus influenzae is primarily transmitted through respiratory droplets from an infected person. While it can cause infections in newborns, it’s less likely to be directly transmitted during gestation or birth compared to the other options listed.

The other options are incorrect:

• Treponema pallidum: This bacterium causes syphilis, which can be transmitted from a mother to her fetus during pregnancy, leading to congenital syphilis.
• Neisseria gonorrhoeae: This bacterium causes gonorrhea, a sexually transmitted infection (STI) that can be passed from mother to baby during childbirth, leading to ophthalmia neonatorum (eye infection) in the newborn.
• Chlamydia trachomatis: This bacterium is another STI that can be transmitted from mother to baby during childbirth, causing conjunctivitis (pink eye) and pneumonia in the newborn.

Answer: Pyoderma (impetigo)

Lancefield group B streptococci (S. agalactiae), also known as Group B Strep (GBS), are not typically associated with pyoderma (impetigo). Impetigo is usually caused by Staphylococcus aureus or Staphylococcus pyogenes.

The other options are incorrect:

• Vaginal carriage in 5% to 25% of normal women of childbearing age: This statement is correct. GBS can colonize the vagina and rectum in healthy women without causing any symptoms.
• Neonatal sepsis and meningitis: This statement is correct. GBS is a leading cause of sepsis and meningitis in newborns, especially within the first week of life.
• Beta-hemolysis: This statement is correct. S. agalactiae are beta-hemolytic bacteria, meaning they break down red blood cells on blood agar plates, resulting in a clear zone around the colonies.

Answer: The definitive diagnosis can be made by detecting antibodies to Neisseria gonorrhoeae in the patient’s serum.

Antibody tests for gonorrhea are not very reliable for definitive diagnosis. They can be positive in past infections or due to non-gonococcal infections. Other tests, like culture or nucleic acid amplification tests (NAATs), are more accurate for confirming current infection.

The other options are incorrect:

• Gonococcal conjunctivitis of the newborn rarely occurs in the United States, because silver nitrate or erythromycin is commonly used as prophylaxis. This statement is correct. Prophylactic eye drops with antibiotics significantly reduce the risk of this serious infection in newborns.
• A presumptive diagnosis can be made by finding gram-negative kidney bean-shaped diplococci within neutrophils in a urethral discharge. This statement is correct. Identifying these characteristics in a smear can suggest gonorrhea, but further testing like culture or NAAT is needed for confirmation.
• Infection in men is more frequently symptomatic than in women. This statement is correct. Women are more likely to have asymptomatic gonorrhea infections compared to men.

Answer: Infection can be diagnosed by finding antichlamydial antibody in a serum specimen.

While antibody tests can detect past or current chlamydial infections, they are not the most reliable method for diagnosing a current infection. They may not differentiate between a current or past infection, and some people may not develop a strong antibody response. More accurate tests for chlamydia involve detecting chlamydial DNA or antigen directly from a swab of the infected area.

The other options are incorrect:

• There is no vaccine against these infections. This statement is correct. There is currently no commercially available vaccine for chlamydia.
• Infection can persist after administration of penicillin. This statement is correct. Penicillin and other antibiotics commonly used for bacterial infections are ineffective against chlamydia because it is a bacterium-like organism. Chlamydia requires specific antibiotics like azithromycin or doxycycline.
• Symptomatic infections can be associated with urethral or cervical discharge containing many polymorphonuclear leukocytes (PMNs). This statement is correct. PMNs, also called white blood cells, are part of the immune response and their presence in discharge can indicate inflammation caused by chlamydia.

Answer: Infections are usually diagnosed by detecting a significant rise in antibody titer.

Antibody tests are not very reliable for diagnosing Actinomyces or Nocardia infections. These infections are often slow-growing and antibody responses may be weak or develop slowly. Definitive diagnosis usually relies on isolating and identifying the organism through culture or other methods like PCR (polymerase chain reaction).

The other options are incorrect:

• A. israelii is an anaerobic rod found as part of the normal flora in the mouth. This statement is correct. Actinomyces israelii is a commensal organism in the mouth and can sometimes cause opportunistic infections.
• Both Actinomyces and Nocardia are branching, filamentous rods. This statement is correct. Both genera share this morphological characteristic.
• N. asteroides causes infections primarily in immunocompromised patients. This statement is correct. Nocardia asteroides is a common species that can cause infections in people with weakened immune systems.

Answer: Their life cycle consists of a metabolically inactive particle in the extracellular phase.

Chlamydiae have a unique biphasic lifecycle. One stage is an infectious elementary body (EB) that is metabolically inactive and can survive outside the host cell. This is the most accurate statement about their life cycle.

The other options are incorrect:

• They replicate in the nucleus of infected cells, where they form inclusions that are useful diagnostically. While Chlamydia do replicate within host cells and form inclusions, these inclusions are typically cytoplasmic, not nuclear. These inclusions can indeed be helpful for diagnosis.
• They exhibit swarming motility on a blood agar plate. Chlamydiae are non-motile bacteria and cannot swarm on agar plates.
• They are gram-positive rods that do not form spores. Chlamydiae are not classified as gram-positive or gram-negative because they have a unique cell wall structure. They are also obligate intracellular parasites and do not form spores.

Answer: Trichomonas vaginalis

This is a flagellated protozoan parasite that can cause vaginal infections with symptoms like itching, frothy yellow discharge, and painful urination. It can be diagnosed by wet mount microscopy, which is consistent with the scenario where organisms are visible on low power.

The other options are incorrect:

• Candida albicans: This is a fungus that causes yeast infections. While it can cause vaginal itching and discharge, the discharge is typically white and clumpy, not frothy and yellow.
• Chlamydia trachomatis: This bacterium can cause chlamydia, a sexually transmitted infection (STI) that can sometimes be asymptomatic, but wouldn’t typically cause the classic symptoms of Trichomoniasis. Cultures can also be negative for chlamydia in some cases.
• Trichophyton rubrum: This is a fungus that causes dermatophyte infections like ringworm. It wouldn’t cause vaginal symptoms and wouldn’t be visible on a wet mount.

Answer: It is an important cause of subacute bacterial endocarditis.

Chlamydia trachomatis is not typically associated with subacute bacterial endocarditis (SBE). SBE is an infection of the inner heart lining and heart valves, usually caused by bacteria like Staphylococcus aureus or Streptococcus species.

The other options are incorrect:

• It is an important cause of nongonococcal urethritis. This statement is correct. Nongonococcal urethritis refers to urethral inflammation not caused by gonorrhea, and Chlamydia trachomatis is a leading cause.
• It is the cause of lymphogranuloma venereum. This statement is correct. Lymphogranuloma venereum is a sexually transmitted infection caused by specific strains of Chlamydia trachomatis.
• It is an important cause of conjunctivitis. This statement is correct. Chlamydia trachomatis can cause conjunctivitis, particularly trachoma, which is a major public health concern in some regions.

Answer: A Gram stain reveals many neutrophils and gram-negative diplococci, and culture on Thayer-Martin medium reveals oxidase-positive colonies.

The clinical presentation of mucopurulent discharge, tenderness on palpation of the right fallopian tube, and the presence of gram-negative diplococci on Gram stain, along with oxidase-positive colonies on Thayer-Martin medium, is indicative of a Neisseria gonorrhoeae infection. This bacterium is a common cause of sexually transmitted infections and can lead to pelvic inflammatory disease.

The other options are incorrect:

• A Gram stain reveals many neutrophils and gram-variable rods, and culture on Thayer-Martin medium reveals ß-hemolytic colonies: ß-hemolytic colonies on Thayer-Martin medium are not typical for Neisseria gonorrhoeae. Gram-variable rods may suggest other bacterial species, but this presentation is not consistent with gonorrhea.
• A Gram stain reveals many neutrophils but no gram-negative diplococci are seen, and culture on Thayer-Martin medium reveals coagulase-positive colonies: Coagulase-positive colonies are indicative of Staphylococcus aureus, not Neisseria gonorrhoeae. The absence of gram-negative diplococci on Gram stain is not consistent with gonorrhea.
• A Gram stain reveals many neutrophils and spirochetes, and culture on Thayer-Martin medium reveals no colonies: The presence of spirochetes suggests a possible infection with Treponema pallidum, the causative agent of syphilis. Thayer-Martin medium is not suitable for Treponema pallidum culture.

Answer: Most chlamydiae are transmitted by arthropods.

Chlamydiae are not primarily transmitted by arthropods (insects and their relatives) like mosquitoes or ticks. Their transmission routes vary depending on the species.

The other options are incorrect:

• Chlamydia trachomatis has multiple serotypes that can cause different diseases. This statement is correct. Different strains (serotypes) of C. trachomatis can cause infections like trachoma (eye infection), urogenital infections, and lymphogranuloma venereum (a sexually transmitted infection).
• Chlamydiae are strict intracellular parasites because they lack the enzymes necessary to synthesize ATP. This statement is correct. Chlamydiae lack a complete metabolic pathway and rely on host cells for energy production (ATP) and other essential functions.
• Chlamydiae possess both DNA and RNA and are bounded by a cell wall. This statement is correct. Chlamydiae have a unique single-circular chromosome (DNA) and some ribosomal RNA. They are surrounded by a cell wall, although it differs structurally from typical bacterial cell walls.

Answer: The specimen must be stained with osmium or other heavy metal.

While staining can enhance contrast and visualize certain cellular structures in TEM, it’s not absolutely necessary for all samples. Unstained samples can still be imaged under TEM, especially for high-contrast materials. However, staining is a common technique used to improve image quality and differentiate cellular components.

The other options are incorrect:

• The beam is focused by electromagnetic lenses: This statement is correct. Electromagnetic lenses are essential for focusing and directing the electron beam in a TEM.
• The specimens are placed in a high vacuum for viewing: This statement is correct. A high vacuum is necessary to prevent the electron beam from scattering off air molecules, which would distort the image.
• The specimens must be sliced very thin, 20-100 nm in thickness: This statement is correct. Because electrons have a short wavelength, samples need to be extremely thin (in the nanometer range) to allow the beam to penetrate and generate a clear image.

Answer: After adding crystal violet and Gram’s iodine, both gram-positive bacteria and gram-negative bacteria will appear blue.

After adding crystal violet and Gram’s iodine in the Gram stain procedure, both gram-positive and gram-negative bacteria initially appear purple-blue. The crucial differentiation occurs during the decolorization step, where the choice of counterstain (safranin or basic fuchsin) determines the final color. Gram-positive bacteria retain the crystal violet-iodine complex and appear purple, while gram-negative bacteria lose the complex and take up the counterstain, appearing pink or red.

The other options are incorrect:

• If you forget to stain with the red dye (safranin or basic fuchsin), both gram-positive bacteria and gram-negative bacteria will appear blue. This is incorrect. Gram-positive bacteria would still appear blue, but gram-negative bacteria wouldn’t be stained and wouldn’t be visible.
• One reason why bacteria have a different color in this stain is because the gram-positive bacteria have lipid in their membrane, whereas gram-negative bacteria do not. While the cell wall structure plays a role, the key difference lies in the thickness of the peptidoglycan layer. Gram-positive bacteria have a thicker peptidoglycan layer that retains the crystal violet-iodine complex during decolorization.
• If you forget to heat-fix, both gram-positive and gram-negative bacteria will appear blue. Heat fixation helps ensure the crystal violet stain adheres to the bacteria. Without it, some bacteria might lose the stain during subsequent steps, leading to inaccurate results.

Answer: Capsule

Negative staining is a technique used to visualize the capsule surrounding some bacteria and fungi. The stain does not penetrate the cells but instead stains the background, leaving the unstained capsule as a clear halo around the cell.

The other options are incorrect:

• Motility: Motility requires observing live, motile bacteria moving in a liquid medium. Negative staining creates a fixed image and wouldn’t be suitable for detecting motility.
• Mycolic acid: Mycolic acid is a component of the cell wall in some bacteria, particularly mycobacteria. Negative staining wouldn’t specifically target or reveal the presence of mycolic acid.
• Flagella: While flagella are appendages for bacterial movement, negative staining wouldn’t be the optimal method for visualizing them due to their small size. Electron microscopy is a better choice for flagella observation.

Answer: Crystal violet, iodine, alcohol, safranin

The Gram stain procedure involves a specific sequence of staining steps to differentiate between gram-positive and gram-negative bacteria. Here’s the order:

• Crystal violet: This stains all bacteria purple.
• Gram’s iodine: This solution acts as a mordant, helping trap crystal violet within the cells.
• Alcohol (decolorizer): This step differentiates the two groups. Gram-negative bacteria have a thinner peptidoglycan layer and outer membrane, allowing the crystal violet-iodine complex to be washed away by the alcohol. Gram-positive bacteria retain the complex due to their thicker peptidoglycan layer.
• Safranin (counterstain): This stains the decolorized gram-negative bacteria pink or red, making them visible under the microscope.

The other options are incorrect:

• Alcohol, crystal violet, iodine, safranin: This order would stain everything first with alcohol, potentially preventing subsequent staining steps from working effectively.
• Crystal violet, safranin, alcohol, iodine: Using safranin before the decolorization step wouldn’t differentiate the bacteria.
• Alcohol, crystal violet, iodine, safranin: Similar to the first incorrect option, using alcohol first would interfere with subsequent staining.

Answer: Gram stain

The Gram stain is a cornerstone technique in microbiology specifically designed to classify bacteria based on their cell wall composition. It differentiates between Gram-positive and Gram-negative bacteria based on their ability to retain a crystal violet-iodine complex during a decolorization step.

The other options are incorrect:

• Methylene blue: While methylene blue is a common stain used for general bacterial visualization, it doesn’t differentiate bacteria based on cell wall structure.
• Capsular stain: This stain targets and visualizes the capsule surrounding some bacteria and fungi, not the cell wall itself.
• Spore stain: This stain is used to identify the presence and location of endospores, a dormant stage formed by some bacteria, and doesn’t provide information about general cell wall content.

Answer: Negative staining

Negative staining is a technique specifically designed to visualize capsules surrounding bacteria and some fungi. The stain doesn’t penetrate the cells but instead stains the background, leaving the unstained capsule as a clear halo around the cell. This allows for easy observation of the capsule’s presence and size.

The other options are incorrect:

• Scanning electron microscopy (SEM): While SEM can be used to image bacteria, it requires complex sample preparation and doesn’t provide clear visualization of capsules due to the drying process involved.
• Ziehl-Neelsen staining: This stain is specifically used for identifying acid-fast bacteria like Mycobacterium tuberculosis and wouldn’t be used for visualizing capsules.
• Gram staining: Gram stain differentiates bacteria based on cell wall composition and wouldn’t necessarily reveal the presence of a capsule.

Answer: Clostridium tetani

Clostridium tetani is the bacterium known for producing endospores with a characteristic “drumstick” appearance under microscopy. This morphology is due to the positioning of the spore within the elongated bacterial cell.

The other options are incorrect:

• Clostridium perfringens: While Clostridium perfringens can also form spores, they typically appear oval or round, not drumstick-shaped.
• Clostridium difficile: Clostridium difficile can form spores, but they are generally oval and wouldn’t exhibit the distinct drumstick morphology.
• Clostridium botulinum: Similar to C. perfringens and C. difficile, Clostridium botulinum spores are typically oval and wouldn’t have the drumstick appearance.

Answer: Streptococcus pyogenes

Streptococcus pyogenes is least likely to cause nonbloody diarrhea. This bacterium is primarily associated with respiratory infections and skin infections, and it is not a common cause of gastrointestinal symptoms leading to diarrhea.

The other options are incorrect:

• Clostridium difficile: As explained, C. diff can cause non-bloody diarrhea.
• Salmonella enteritidis: Salmonella can also cause non-bloody diarrhea as part of foodborne illness.
• Shigella dysenteriae: While Shigella is more commonly associated with bloody diarrhea, it can cause non-bloody diarrhea as well.

Answer: Enterococcus faecalis

Enterococcus faecalis is a commensal bacterium found in the human gut. While it can sometimes cause opportunistic infections, it’s not typically considered a major cause of diarrhea in healthy individuals.

The other options are incorrect:

• Escherichia coli (E. coli): Certain strains of E. coli, particularly enterohemorrhagic E. coli (EHEC), can cause severe diarrhea and illness.
• Clostridium perfringens: This bacterium can cause foodborne illness, including diarrhea.
• Vibrio cholerae: This bacterium is the main culprit behind cholera, a severe diarrheal illness.

Answer: Salmonella typhi

This is the causative agent of typhoid fever, a systemic infection that can spread from the gastrointestinal tract to the bloodstream, causing bacteremia (bacteria in the blood).

The other options are incorrect:

• Pyrazinamide: Pyrazinamide is an antituberculosis medication, not an organism. It is used in the treatment of tuberculosis and is not a cause of bacteremia.
• Streptomycin: Streptomycin is an antibiotic used in the treatment of various bacterial infections, including tuberculosis. It is not an organism that causes bacteremia.
• Rifampicin: Rifampicin is an antibiotic commonly used in the treatment of bacterial infections, including tuberculosis and certain other bacterial diseases. It is not an organism and does not cause bacteremia.

Answer: Shigella sonnei

Shigella sonnei generally has the lowest 50% infectious dose (ID50) among the listed bacteria. This means it typically requires the fewest number of ingested bacteria to cause infection in 50% of exposed individuals.pen_spark

The other options are incorrect:

• Campylobacter jejuni: Campylobacter jejuni is known to have a moderate infectious dose. It is a common cause of bacterial gastroenteritis.
• Salmonella typhi: Salmonella typhi, which causes typhoid fever, generally has a higher infectious dose compared to Shigella sonnei. Typhoid fever is often associated with ingestion of a larger number of bacteria.
• Vibrio cholerae: Vibrio cholerae, the causative agent of cholera, typically has a higher infectious dose than Shigella sonnei. Cholera is associated with the consumption of contaminated water or food containing a significant number of bacteria.

Answer: Vibrio parahaemolyticus

Vibrio parahaemolyticus is the most likely cause of diarrhea after eating raw shellfish in this scenario. It is an oxidase-positive, gram-negative curved bacillus that thrives in marine environments. Vibrio parahaemolyticus is associated with seafood, particularly raw or undercooked shellfish, and can cause gastroenteritis.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus is Gram-positive, oxidase-negative, and wouldn’t be a typical cause of diarrhea from shellfish.
• Clostridium botulinum: Clostridium botulinum is an anaerobic spore-forming bacterium. It wouldn’t be associated with raw shellfish and causes a different type of illness (botulism) with distinct symptoms.
• Escherichia coli: While some E. coli strains can cause foodborne illness, they don’t typically require special media and wouldn’t be the most likely culprit for shellfish-related diarrhea.

Answer: Shigella flexneri

The most likely cause of painful bloody diarrhea in a patient with AIDS, with the described characteristics (oxidase-negative, lactose-negative, H2S-negative, and non-motile) is Shigella flexneri. Shigella species are gram-negative facultative anaerobic rods known for causing dysentery, which is characterized by bloody diarrhea and abdominal pain. Shigella flexneri, in particular, is associated with human-to-human transmission and is a common cause of Shigellosis.

The other options are incorrect:

• Vibrio cholerae: While this bacterium can cause diarrhea, it wouldn’t typically cause bloody diarrhea like Shigella. Vibrio cholerae is also typically positive for oxidase and motility.
• Enterobacter aerogenes: This bacterium is a lactose fermenter on MacConkey agar and wouldn’t be a good fit for the overall picture.
• Enterotoxigenic Escherichia coli (ETEC): While ETEC can cause diarrhea, it wouldn’t typically cause bloody diarrhea like Shigella. Additionally, some ETEC strains can be lactose fermenters.

Answer: Escherichia coli ETEC (enterotoxingenic).

Escherichia coli ETEC (enterotoxigenic). This specific strain of E. coli is a leading cause of traveler’s diarrhea. It produces toxins that disrupt fluid balance in the intestines, leading to watery diarrhea.

The other options are incorrect:

• Shigella sonnei: While Shigella can cause diarrhea, it’s less commonly associated with traveler’s diarrhea compared to ETEC. Shigella infections are more frequent in areas with poor sanitation.
• Salmonella typhimurium: Salmonella can cause foodborne illness, but it’s not the most common culprit in traveler’s diarrhea.
• Yersinia enterocolitica: This bacterium can cause traveler’s diarrhea, but it’s less frequent than ETEC.

Answer: Clostridium difficile

Clostridium difficile (C. diff). This bacterium is the most frequent cause of diarrhea associated with prolonged antibiotic use, including pseudomembranous colitis. It thrives when the normal gut flora is disrupted by antibiotics, allowing it to overgrow and produce toxins that cause inflammation and diarrhea.

The other options are incorrect:

• Yersinia enterocolitica: While Yersinia can cause diarrhea, it’s not typically associated with antibiotic-induced diarrhea.
• Salmonella enteritidis: Salmonella is a foodborne pathogen and wouldn’t be linked to antibiotic use in this context.
• Shigella dysenteriae: Shigella can cause bacterial dysentery, but it’s not a common cause of antibiotic-associated diarrhea.

Answer: Bacillus cereus food poisoning

The most likely cause of the symptoms described (abdominal cramps, nausea, and vomiting) following the consumption of food from an Indian restaurant, with the presence of gram-positive aerobic rods, is Bacillus cereus food poisoning. Bacillus cereus is known to produce emetic and diarrheal toxins. The emetic toxin causes symptoms such as nausea and vomiting within a few hours of ingestion.

The other options are incorrect:

• Staphylococcal food poisoning: Staphylococcal food poisoning is caused by Staphylococcus aureus and is characterized by rapid-onset symptoms such as nausea, vomiting, abdominal cramps, and diarrhea. However, the organism in the scenario is described as gram-positive aerobic rods, making Bacillus cereus a more likely cause.
• Clostridium perfringens food poisoning: Clostridium perfringens is an anaerobic bacterium associated with food poisoning. However, it typically causes symptoms of abdominal cramps and diarrhea rather than nausea and vomiting.
• Clostridium botulinum food poisoning: Clostridium botulinum produces a potent neurotoxin that leads to botulism, a condition characterized by symptoms such as paralysis and respiratory failure. It is not a common cause of food poisoning with symptoms like abdominal cramps, nausea, and vomiting.

Answer: Escherichia coli

The diagnostic results described (Gram-negative rod, colorless/pale colonies on MacConkey agar) suggest the absence of lactose fermentation. Escherichia coli (E. coli) is a member of the Enterobacteriaceae family but is known for fermenting lactose, resulting in pink/red colonies on MacConkey agar. Therefore, the presence of colorless/pale colonies on MacConkey agar would exclude Escherichia coli from the diagnosis.

The other options are incorrect:

• Shigella spp: Shigella spp are non-lactose fermenters, and their colonies on MacConkey agar would appear colorless/pale. They are included in the diagnosis of the specimen.
• Salmonella spp: Salmonella spp are also non-lactose fermenters and would yield colorless/pale colonies on MacConkey agar. They are included in the diagnosis of the specimen.
• Both Salmonella and Shigella: Both Salmonella and Shigella are non-lactose fermenters and would show colorless/pale colonies on MacConkey agar. They are included in the diagnosis of the specimen.

Answer: Bacillus cereus

The symptoms of nausea, vomiting, and diarrhea occurring after consuming reheated rice are indicative of Bacillus cereus food poisoning. Bacillus cereus produces heat-stable toxins that can survive cooking, and when rice is left at room temperature for a period and then reheated, it provides an ideal environment for toxin production.

The other options are incorrect:

• Salmonella typhi: Salmonella typhi is associated with typhoid fever and is not commonly linked to symptoms such as rapid-onset nausea, vomiting, and diarrhea seen in this case.
• Clostridium perfringens: Clostridium perfringens is known to cause food poisoning, but it usually involves the consumption of large amounts of contaminated food. It is not commonly associated with reheated rice.
• Enterotoxigenic Escherichia coli: Enterotoxigenic Escherichia coli (ETEC) is a cause of traveler’s diarrhea and is characterized by watery diarrhea. It is not typically associated with the rapid-onset symptoms described in the scenario.