MicroBiology MCQs with Answer and Explnation | Chapter 47

Answer: Haemagglutination test

Fimbriae are demonstrated by the Haemagglutination test. This test is used to detect the presence of fimbriae, which are hair-like appendages on the surface of some bacteria that facilitate attachment to host cells. The Haemagglutination test involves the agglutination (clumping) of red blood cells, indicating the presence of fimbriae.

The other options are incorrect:

• Culture: Culture is a general method for growing and maintaining microorganisms but does not specifically demonstrate fimbriae.
• Gram stain: The Gram stain is a technique used to differentiate bacteria into Gram-positive and Gram-negative based on their cell wall structure. It does not specifically demonstrate fimbriae.
• Biochemical reaction: Biochemical reactions are used to identify the metabolic characteristics of microorganisms but do not directly demonstrate fimbriae.

Answer: Transduction

Transduction is a process where bacterial DNA is packaged into a phage capsid and transferred to another bacterium by the phage during its lytic or lysogenic cycle. This allows for horizontal gene transfer, meaning genes can be exchanged between non-related bacteria.

The other options are incorrect:

• Translation: This is the process by which ribosomes use mRNA to synthesize proteins. It does not involve the transfer of genetic material between cells.
• Lysogeny: This is a phage life cycle where the phage integrates its DNA into the host bacterium’s chromosome. While it involves phage DNA entering the host, it doesn’t necessarily transfer new genes between bacteria.
• Conjugation: This is a mechanism where bacteria directly transfer DNA plasmids between each other via a physical connection. It does not involve phages.

Answer: Group B

The cAMP reaction, also known as the Christie-Atkins-Munch-Peterson test, is a diagnostic test used to identify Group B Streptococcus agalactiae. This test relies on the synergistic interaction between a protein called CAMP factor produced by Group B streptococci and the beta-hemolysin produced by Staphylococcus aureus. When both bacteria are grown on the same plate close together, a characteristic “arrowhead” of enhanced hemolysis is observed where the two streaks meet. This positive reaction indicates the presence of Group B streptococci.

The other options are incorrect:

• Group A: Group A streptococci (Streptococcus pyogenes) do not produce CAMP factor and are typically CAMP negative.
• Group C and D: While some Group C and D streptococci might show weak or atypical CAMP reactions, they are not reliable indicators and should not be interpreted as positive for Group B.

Answer: Pertussis

The cough plate method involves collecting droplets from a patient’s cough onto a special agar plate designed to support the growth of Bordetella pertussis. These bacteria are fastidious and require specific conditions for growth, making other culture methods less effective. If present, Bordetella pertussis will grow on the plate, allowing for identification and diagnosis of whooping cough.

The other options are incorrect:

• Y. pestis: Yersinia pestis, the causative agent of plague, can be identified using different methods like culture from blood or lymph nodes, but not the cough plate method.
• Mycoplasma: Mycoplasma pneumoniae, a common cause of atypical pneumonia, cannot be reliably detected using the cough plate method. Specific culture or molecular tests are required.
• M. Tuberculosis: Mycobacterium tuberculosis, the causative agent of tuberculosis, also grows slowly and requires specialized media, making the cough plate unsuitable for its identification.

Answer: Gram negative organisms

While all types of microorganisms can contribute to nosocomial pneumonia (NP) in the ICU, Gram-negative bacteria dominate as the leading cause. These bacteria thrive in hospital environments due to antibiotic resistance and the vulnerability of critically ill patients with weakened immune systems.

The other options are incorrect:

• Gram-positive organisms: While less prevalent, Staphylococcus aureus (including MRSA) and Streptococcus pneumoniae can also contribute to NP, accounting for about 20-30% of cases.
• Mycoplasma: Mycoplasma pneumoniae, a common cause of community-acquired pneumonia, rarely plays a significant role in NP.
• Virus infections: Viral respiratory infections can occur in ICUs, but they’re not as common as bacterial NP. Influenza, rhinovirus, and cytomegalovirus are potential causative agents.

Answer: Bacterial capsule

Negative staining utilizes negatively charged dyes like India ink or nigrosin to stain the background surrounding the bacteria rather than the bacteria themselves. This creates a contrasting dark background against which the unstained, transparent capsules appear as clear halos around the bacterial cells.

The other options are incorrect:

• Bacterial spores: Spores are highly resistant structures produced by certain bacteria for survival in harsh conditions. They are typically too dense and refractive to be easily visualized with negative staining.
• Bacterial flagella: Flagella are hair-like appendages that enable bacterial motility. They are too thin and delicate to be effectively visualized with negative staining, which lacks the high resolution needed to resolve such fine structures.
• Bacterial fimbriae: Similar to flagella, fimbriae are also thin appendages but serve for adhesion rather than motility. They often require higher resolution techniques like electron microscopy for proper visualization.

Answer: Fusiform bacilli

Borrelia vincentii is always associated with fusiform bacilli. This bacterium is commonly associated with Vincent’s angina, a condition characterized by severe ulcerative gingivitis and pharyngitis. The fusiform bacilli (also known as Fusobacterium) are often found in association with Borrelia vincentii in the lesions of Vincent’s angina.

The other options are incorrect:

Spirochaetes: Borrelia vincentii is not associated with spirochaetes. Spirochaetes are a different group of bacteria, and Borrelia species themselves are spirochaetes.
Treponema vincenti: There is no species known as Treponema vincenti. The correct term associated with fusiform bacilli in Vincent’s angina is Borrelia vincentii.

Answer: Incorporating part of host DNA

Bacteria acquire characteristics through various mechanisms, including plasmids, bacteriophages, and conjugation. However, incorporating part of the host DNA is not a typical mechanism for bacteria to acquire characteristics. Bacteria primarily acquire traits through horizontal gene transfer, where genetic material is exchanged between different bacteria.

The other options are incorrect:

• Through plasmids: Bacteria can acquire characteristics through plasmids, which are small, circular DNA molecules that can carry additional genes. Plasmids can be transferred between bacteria, allowing the spread of specific traits.
• Through bacteriophages: Bacteriophages, or viruses that infect bacteria, can transfer genetic material between bacteria during the process of transduction. This is a mechanism by which bacteria acquire new traits.
• Through conjugation: Conjugation is a process of horizontal gene transfer where genetic material is directly transferred from one bacterium to another through a conjugative pilus. This is another mechanism by which bacteria acquire characteristics.

Answer: Streptococcus pyogenes

Streptococcus pyogenes is not known to survive intracellularly. It is primarily an extracellular pathogen that causes diseases such as strep throat, skin infections, and invasive conditions like necrotizing fasciitis. Unlike the other listed organisms, Streptococcus pyogenes does not have a significant intracellular survival component in its pathogenesis.

The other options are incorrect:

• Neisseria meningitidis: Neisseria meningitidis, the causative agent of meningococcal meningitis, is known to survive intracellularly within certain host cells.
• Salmonella typhi: Salmonella typhi, responsible for causing typhoid fever, is known to survive and replicate inside host cells, particularly in macrophages.
• Legionella pneumophila: Legionella pneumophila, the bacterium responsible for Legionnaires’ disease, is an intracellular pathogen that survives and replicates within host cells, particularly in alveolar macrophages.

Answer: Parotitis

Chlamydia does not typically cause parotitis. Parotitis is inflammation of the parotid glands, and it is more commonly associated with viral infections, such as mumps, rather than Chlamydia.

The other options are incorrect:

• Trachoma: Chlamydia trachomatis is a common cause of trachoma, an eye infection that can lead to blindness if left untreated.
• Non-gonococcal urethritis: Chlamydia trachomatis is a leading cause of non-gonococcal urethritis (NGU), a sexually transmitted infection affecting the urethra in males.
• Pneumonia: Chlamydia pneumoniae is a species of Chlamydia that can cause respiratory infections, including pneumonia. It is distinct from the other species causing sexually transmitted infections.

Answer: Polysaccharide C antigen

Lancefield classification of B hemolytic streptococci is based on the presence and specific types of polysaccharide C antigens located in their cell wall. These polysaccharides are complex carbohydrate molecules that react differently with specific antisera, allowing for identification of various groups within B hemolytic streptococci.

The other options are incorrect:

• M Protein: While M protein is a significant virulence factor in some B hemolytic streptococci (especially Group A), it’s not directly used for Lancefield classification, which focuses solely on surface polysaccharides.
• Type of lipid in cell membrane: The lipid composition of bacterial cell membranes can vary, but it’s not the basis for Lancefield classification, which specifically targets carbohydrate antigens.
• Physiological properties: Certain physiological properties like growth requirements or fermentation patterns can differentiate bacteria, but they are not used in Lancefield classification, which relies on distinct antigenic markers.

Answer: Through intact skin

Leprosy bacilli (Mycobacterium leprae) are not typically transmitted through intact skin. The primary mode of transmission is through prolonged and close contact with respiratory droplets from infected individuals. The bacilli are not highly infectious, and transmission usually requires prolonged exposure.

The other options are incorrect:

• Discharge from nasal mucosa: Leprosy bacilli can be present in respiratory secretions, including nasal discharge, and can be a source of transmission.
• From open sore: Open sores or lesions in individuals with leprosy can contain a high concentration of Mycobacterium leprae, making it a potential source of transmission.
• Through breast milk: While rare, transmission of leprosy through breast milk has been reported in some cases, although it is not a common mode of transmission.

Answer: Niacin test

Mycobacterium tuberculosis is differentiated from other atypical mycobacteria by the Niacin test. This test is based on the ability of M. tuberculosis to produce niacin (nicotinic acid), while many other atypical mycobacteria do not. The presence of niacin is detected by chemical methods, helping to identify M. tuberculosis.

The other options are incorrect:

• AFB staining (Acid-Fast Bacilli staining): AFB staining is a common method to detect mycobacteria, including M. tuberculosis. However, it does not specifically differentiate M. tuberculosis from other atypical mycobacteria.
• PAS staining (Periodic Acid-Schiff staining): PAS staining is used for visualizing the cell walls of certain fungi and bacteria. While it may be used in the laboratory, it is not a specific method for differentiating M. tuberculosis from other atypical mycobacteria.

Answer: CD 4*

In tuberculosis (TB), cell-mediated immunity is primarily mediated by CD4-positive T cells (CD4+ T cells). These cells, also known as helper T cells, play a crucial role in coordinating the immune response against Mycobacterium tuberculosis, the bacterium causing TB. CD4+ T cells activate other immune cells, such as macrophages, to help eliminate the intracellular bacteria.

The other options are incorrect:

• CD8: CD8-positive T cells (CD8+ T cells) are cytotoxic T cells that primarily target and kill infected cells. While they play a role in cell-mediated immunity, CD4+ T cells are more central in coordinating the immune response against M. tuberculosis.
• B-lymphocytes: B-lymphocytes are involved in humoral immunity, which is mediated by antibodies. While antibodies can contribute to the immune response against extracellular pathogens, they are not the primary mediators of cell-mediated immunity against intracellular bacteria like M. tuberculosis.

Answer: Cl. tetani

Drumstick appearance refers to the characteristic shape of Clostridium tetani spores seen under a microscope. They appear swollen at one end, resembling a drumstick. C. tetanomorphum and C. sphenoids don’t exhibit this morphology.

The other options are incorrect:

• Cl. tetanomorphum: These bacteria resemble C. tetani but lack the drumstick spores. They often occur alongside C. tetani but are not associated with tetanus.
• Cl. sphenoids: These bacteria have a different spore morphology not resembling drumsticks. They are associated with infections in the intestinal tract.
• All of the above: This is incorrect as only C. tetani exhibits the drumstick appearance.

Answer: C. burnetti

Q fever is an infectious disease caused by the bacterium Coxiella burnetii. While it technically belongs to the family Rickettsiaceae, it is placed in a separate genus due to its unique characteristics. Rickettsiae are typically obligate intracellular parasites, whereas C. burnetii can survive and replicate outside of host cells. Additionally, Q fever often has different symptoms and transmission modes compared to other rickettsial diseases.

The other options are incorrect:

• Rickettsia tsutsugamushi: This bacterium causes scrub typhus, which is transmitted through the bite of chigger mites. Symptoms include fever, headache, and a characteristic eschar at the bite site.
• R. prowozekii: This bacterium causes epidemic typhus, which is transmitted through infected lice and body lice. Symptoms include fever, headache, and a rash.
• R. quintana: This bacterium causes trench fever, which is transmitted through lice and body lice. Symptoms include fever, headache, and shin pain.

Answer: Gram positive bacteria

Actinomycetes are a diverse group of filamentous, Gram-positive bacteria belonging to the phylum Actinomycetota. They possess a high G+C content in their DNA and form branching, thread-like colonies reminiscent of fungal hyphae. This often leads to confusion, but their cell structure and biochemical characteristics definitively classify them as bacteria.

The other options are incorrect:

• Gram-negative bacteria: These bacteria have a different cell wall structure compared to Actinomycetes, characterized by an outer membrane, and stain pink with Gram’s stain. Examples include E. coli and Salmonella.
• Fungus: Fungi are eukaryotic organisms with distinct cellular structures like a nucleus and membrane-bound organelles. They differ from Actinomycetes in their sexual reproduction, cell wall composition, and metabolic pathways.
• A yeast-like form: While some Actinomycetes can fragment into yeast-like forms, this is not their primary growth form and doesn’t define their classification. True yeasts are unicellular fungi that reproduce through budding or fission.

Answer: All of the above

The VDRL test is indeed characterized by all the options listed:

• Non-specific: While the VDRL test primarily detects antibodies indicative of syphilis, it can also react to other conditions like autoimmune diseases, certain infections, and even vaccinations. This non-specificity necessitates further confirmatory tests for definitive diagnosis.
• Slide flocculation test: The VDRL test involves mixing blood serum with a cardiolipin antigen on a microscope slide. If antibodies to syphilis are present, clumps (floccules) form, observed under the microscope.
• Best followed for drug therapy: Monitoring VDRL test results over time helps assess the effectiveness of syphilis treatment. A decreasing titer (antibody level) indicates successful treatment, while a persistent or rising titer suggests possible treatment failure or re-infection.

Answer: Scarlet fever

While Staphylococcus aureus (staph) is a common cause of many infections, it does not cause scarlet fever. Scarlet fever is primarily caused by Streptococcus pyogenes (strep) bacteria and is characterized by a rash, fever, and sore throat. Staphylococcus aureus typically causes skin infections, abscesses, pneumonia, and other localized infections.

The other options are incorrect:

• Osteomyelitis: Staphylococcus aureus can indeed cause osteomyelitis, an infection of the bone, especially in cases of penetrating trauma or surgery.
• Abscess: Staphylococcus aureus is a frequent culprit in abscess formation, both superficial and deep within the body.
• Pneumonia: Staphylococcus aureus can cause pneumonia, particularly in healthcare settings or individuals with weakened immune systems.

Answer: Koch

Although John Snow conducted crucial epidemiological research on cholera transmission, the bacterium responsible, Vibrio cholerae, was actually discovered by Robert Koch in 1883. Using his innovative staining techniques, Koch isolated and identified the bacterium as the causative agent of cholera, providing strong evidence for the germ theory of disease.

The other options are incorrect:

• Medintoff: This name seems misspelt. Perhaps you meant Joseph LeBaron Medin, who made significant contributions to cholera research but didn’t discover the bacterium itself.
• John Snow: As mentioned, Snow’s work focused on identifying cholera’s transmission through contaminated water, not discovering the actual bacteria.
• Virchow: Rudolf Virchow was a prominent German pathologist who focused on cellular pathology and didn’t specifically research cholera or discover the causative agent.

Answer: Pili

The virulence of Neisseria gonorrhoeae, the bacterium that causes gonorrhea, is primarily attributed to the presence of pili (also known as fimbriae). Pili are hair-like appendages on the surface of the bacterial cell that facilitate adherence to host cells. In the case of N. gonorrhoeae, pili play a crucial role in the initial attachment to mucosal surfaces of the genital tract, allowing the bacterium to establish infection.

The other options are incorrect:

• Cell Membrane: While the cell membrane is essential for the integrity and function of the bacterial cell, the specific virulence factor for N. gonorrhoeae is the presence of pili.
• Its intracellular location: The intracellular location of Neisseria gonorrhoeae is not a major factor contributing to its virulence. It is primarily an extracellular pathogen that infects mucosal surfaces.
• Cytolytic enzymes: Cytolytic enzymes, which can cause cell lysis, are not the primary virulence factor for Neisseria gonorrhoeae. The bacterium’s ability to adhere to host cells through pili is more critical in the context of gonococcal infection.

Answer: Loefflers serum slope

Löffler’s serum slope is the preferred culture medium for isolating and identifying Corynebacterium diphtheriae, the bacterium responsible for diphtheria. It is a semi-solid medium composed of solidified serum, usually bovine or horse, with added nutrients like dextrose and peptone. This rich and moist environment provides the ideal conditions for C. diphtheriae growth, allowing for easy visualization of its characteristic colonies and morphology.

The other options are incorrect:

• McConkey agar: This medium is selective for Gram-negative bacteria and inhibits the growth of most Gram-positive bacteria, including C. diphtheriae. It’s commonly used for isolating enteric pathogens like E. coli and Salmonella.
• Saboraud’s agar: This medium is formulated for growing fungi and yeasts, as it contains an acidic pH and lacks essential nutrients for bacterial growth. C. diphtheriae wouldn’t thrive on this medium.
• Löwenstein-Jensen medium: This medium is used for isolating Mycobacterium tuberculosis, another slow-growing bacterium with specific nutritional requirements. It’s not suitable for C. diphtheriae, as it lacks the necessary nutrients and has an acidic pH.

Answer: Bacillus anthracis

The “thumbprint appearance” in a culture film smear is characteristic of Bacillus anthracis, the bacterium responsible for anthrax. This unique morphology arises from the square-ended, boxy arrangement of the bacilli in chains, resembling the pattern of an inked thumbprint. It’s one of the key features used for rapid identification of B. anthracis in clinical settings.

The other options are incorrect:

• Brucella species: While Brucella species can also form chains, they typically appear shorter and more irregular than B. anthracis, lacking the characteristic boxy ends and thumbprint-like arrangement.
• Bordetella pertussis: This bacterium usually appears as coccobacilli (short, plump rods) arranged singly or in pairs, not the long chains with square ends seen in B. anthracis.
• Clostridium weichii: This bacterium is less commonly encountered in clinical settings and exhibits a varied morphology, not including the distinct thumbprint pattern associated with B. anthracis.

Answer: Typhoid

The Widal test, while no longer considered a definitive diagnostic tool due to its lack of specificity and sensitivity, was historically used for identifying typhoid fever. It detects antibodies against lipopolysaccharide (LPS) antigens on the surface of Salmonella Typhi, the bacterium responsible for the disease.

The other options are incorrect:

• Plague: Plague is caused by Yersinia pestis, a bacterium different from Salmonella Typhi. The Widal test wouldn’t detect antibodies specific to Y. pestis.
• Leprosy: Leprosy is caused by Mycobacterium leprae, another distinct bacterium. The Widal test wouldn’t be effective in identifying this pathogen.
• Pemhigoid: Pemhigoid is an autoimmune skin disease, not an infectious disease caused by bacteria. The Widal test wouldn’t be helpful in diagnosing this condition.

Answer: Cell mediated immunity

The tuberculin test screens for cell-mediated immunity (CMI) against Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). It works by injecting a small amount of tuberculin, a purified protein derivative from M. tuberculosis, under the skin. If someone has been exposed to TB bacteria, their immune system will trigger an inflammatory response at the injection site, resulting in a raised bump (induration).

The other options are incorrect:

• Hemoral immunity: While antibodies play a role in TB infection, the tuberculin test specifically assesses CMI, which involves T cells directly attacking infected cells.
• Complement function: The complement system is part of the innate immune response and doesn’t directly measure specific immunity to TB.
• Phagocyte dysfunction: Although phagocytes like macrophages are crucial for fighting TB, the tuberculin test doesn’t assess their specific function.

Answer: Detection of coagulase

The detection of coagulase is a crucial laboratory test for determining the virulence of staphylococci, especially Staphylococcus aureus. Coagulase is an enzyme produced by some staphylococcal strains, and its presence can lead to the coagulation of blood plasma. Staphylococcus aureus, a pathogenic species, is often coagulase-positive, distinguishing it from other less virulent staphylococci.

The other options are incorrect:

• Mannitol fermentation: Mannitol fermentation is a characteristic of Staphylococcus aureus, but it is not as specific for virulence determination as the coagulase test.
• Hemolysis of sheep erythrocytes: Hemolysis of sheep erythrocytes is a feature seen in various bacteria, including different staphylococci. While hemolysis is an important characteristic, it is not as specific for virulence as the coagulase test.
• The catalase test: The catalase test is useful for differentiating staphylococci (catalase-positive) from streptococci (catalase-negative). While it is a valuable diagnostic test, it may not be as directly related to determining the virulence of staphylococci compared to the coagulase test.

Answer: Having either DNA or RNA

All bacteria contain both DNA and RNA as part of their genetic material. DNA stores the majority of their genetic information, while RNA plays a crucial role in protein synthesis and other cellular processes. Therefore, the option stating “Having either DNA or RNA” is inaccurate.

The other options are incorrect:

• Unicellular: Bacteria are indeed single-celled organisms, lacking complex tissue organization like plants and animals.
• Free living: While some bacteria have symbiotic relationships with other organisms, the majority can exist and reproduce independently in the environment.
• Cell wall containing muramic acid: This is a defining characteristic of bacterial cell walls. Muramic acid provides rigidity and protection to the bacterial cell.

Answer: All of the above

Salmonella typhi, the causative agent of typhoid fever, can indeed be isolated from various sources at different stages of the infection. Here’s a breakdown:

• Feaces: This is the most common source for isolating Salmonella typhi, particularly during the acute phase and carrier state of the infection.
• Blood: Blood cultures can be positive during the early stages of typhoid fever, but the yield is lower compared to stool cultures.
• Urine: Although less common, Salmonella typhi can be isolated from urine, especially in individuals with complications involving the urinary tract.

Answer: 37 degrees

Mycobacterium species are mesophiles, meaning they thrive in moderate temperatures. The optimal temperature for growth for most pathogenic mycobacteria, including Mycobacterium tuberculosis and Mycobacterium leprae, is 37°C, which is roughly the same as human body temperature. This aligns with their natural niche of colonizing warm-blooded hosts.

The other options and why they are incorrect:

• 2 degrees: This is too cold for most Mycobacterium species to grow actively. They would likely enter a dormant state at such a low temperature.
• 0 degrees: Similar to 2 degrees, freezing temperatures would completely halt growth and potentially damage the bacteria.
• 27 degrees: While some non-pathogenic Mycobacterium species can grow at slightly lower temperatures like 27°C, 37°C remains the optimal and most commonly used temperature for cultivation of medically relevant strains.

Answer: Both of the above

Silver impregnation methods are highly effective techniques for visualizing two specific types of bacterial structures.

• Bacterial flagella: These fine hair-like appendages enable bacteria to move, and silver impregnation stains them intensely, making them easily visible under a microscope.
• Spirochetes: These spiral-shaped bacteria, like Treponema pallidum (causing syphilis), have a unique cell wall structure that readily binds silver and appears dark and distinct under the microscope.

Answer: Red color

Gram staining is a widely used technique to differentiate bacteria based on their cell wall composition. Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane, unlike Gram-positive bacteria with a thicker peptidoglycan layer.

The other options are incorrect:

• Blue color: This is the typical color of Gram-positive bacteria after crystal violet and safranin staining.
• Green: Green is not a typical color used in the Gram staining technique.
• Colorless: Colorless bacteria wouldn’t be visible under the microscope, which wouldn’t be helpful for identification.

Answer: Gall Bladder

In typhoid carriers, the bacteria responsible for the disease (Salmonella Typhi) can persist in various organs after the acute illness has subsided. However, the gallbladder is the most common site due to its unique environment favoring bacterial growth. The bile stored in the gallbladder concentrates bile acids, which can inhibit the growth of most bacteria. However, Salmonella Typhi has developed mechanisms to tolerate bile acids, allowing it to thrive in the gallbladder and be shed intermittently through bile into the intestine, making the carrier infectious.

The other options are incorrect:

• Spleen: While Salmonella Typhi can also be present in the spleen, it’s less common than the gallbladder and not known to be readily shed into the digestive system, making it less significant in transmission.
• Liver: Similarly, Salmonella Typhi can be present in the liver, but the bacteria are unlikely to be directly released into the intestine unless there’s significant liver damage.
• Salivary gland: Salmonella Typhi is not typically found in the salivary glands of typhoid carriers. While some types of Salmonella can cause gastroenteritis through oral-fecal transmission, Salmonella Typhi primarily spreads through contaminated food and water, not saliva.

Answer: Beta phage

Diphtheria, caused by the bacterium Corynebacterium diphtheriae, becomes pathogenic only when infected with a specific virus called the beta phage. This phage carries the gene for producing a powerful toxin called diphtheria toxin, responsible for the severe symptoms of the disease. Without the beta phage, C. diphtheriae remains harmless and cannot cause diphtheria.

The other options are incorrect:

• Alfa phage: While there are alpha phages that infect C. diphtheriae, they don’t carry the diphtheria toxin gene and therefore play no role in causing the disease.
• Lambda K12: This phage specifically infects E. coli, not C. diphtheriae, and has no bearing on diphtheria pathogenesis.
• Delta Phage: Similar to Alfa phages, delta phages infecting C. diphtheriae lack the diphtheria toxin gene and cannot contribute to disease development.

Answer: Mycobacterium bacilli

Mycobacterium bacilli are a group of rod-shaped bacteria characterized by acid-fastness, a unique property where they retain a special stain even after washing with acid. This characteristic makes them readily identifiable under a microscope and is crucial for diagnosing infections like tuberculosis and leprosy, caused by Mycobacteria.

The other options are incorrect:

• Treponema pallidum: This bacterium, responsible for syphilis, is spiral-shaped and not acid-fast. It requires different staining techniques for visualization.
• Neisseria gonorrhea: This bacterium, causing gonorrhea, is also not acid-fast and has a distinct coffee bean-shaped morphology.
• All of the above: Only Mycobacterium bacilli are acid-fast within these options. The other two mentioned bacteria have different staining properties and cannot be identified using acid-fast staining methods.

Answer: Rheumatic fever

Group A beta-hemolytic streptococcal (GABHS) infection of the pharynx, commonly known as strep throat, can be followed by rheumatic fever in some individuals. This is an autoimmune complication where the body’s immune response to the streptococcal infection mistakenly attacks its own tissues, specifically the heart, joints, skin, and nervous system.

The other options are incorrect:

• Diphtheria: This disease is caused by Corynebacterium diphtheriae, not Group A Streptococcus, and presents with different symptoms like sore throat, difficulty breathing, and characteristic grayish-white patches in the throat.
• Typhoid fever: This systemic infection is caused by Salmonella Typhi bacteria and primarily affects the gastrointestinal tract and immune system. Typical symptoms include fever, headache, abdominal pain, and diarrhea.
• Syphilis: This sexually transmitted infection caused by Treponema pallidum primarily affects the genitals and skin, with different stages and symptoms progressing over time.

Answer: Borrelia vincenti and fusobacterium fusiformis

Vincent’s angina, also known as trench mouth, is an infection of the oral cavity, particularly the gums, and is often associated with poor oral hygiene and compromised immune function. The causative agents are primarily Borrelia vincenti, a spirochete, and Fusobacterium fusiformis, a gram-negative anaerobic bacterium.

The other options are incorrect:

• Borrelia recurrents and bateroides: While Borrelia recurrents is indeed a spirochete, it causes relapsing fever, not Vincent’s angina. Bateroides are also anaerobic bacteria found in the mouth, but they’re not typically involved in this specific infection.
• Borrelia burgoloferi and lactobacillus: Borrelia burgdorferi is the causative agent of Lyme disease, while lactobacillus are common probiotic bacteria not associated with Vincent’s angina.
• Leptospira and Treponema pallidum: Leptospira cause leptospirosis, a bacterial infection affecting kidneys and liver. Treponema pallidum causes syphilis, primarily a sexually transmitted infection with different skin and genital manifestations.

Answer: Free of a cell wall and a capsule

A protoplast is a bacterial cell that has been treated to remove its cell wall, resulting in a structure composed only of the cell membrane and the cytoplasm. This process is achieved by using enzymes, such as lysozyme, to digest the cell wall. Protoplasts lack the rigid cell wall that is characteristic of most bacterial cells but retain the cell membrane and other internal components.

The other options are incorrect:

• With a cell wall but free of a capsule: This contradicts the definition of a protoplast, as the lack of a cell wall is a defining characteristic.
• Containing a cell wall and a capsule: As mentioned previously, both structures are absent in a protoplast.
• Uniquely sensitive to penicillin: Although protoplasts are indeed vulnerable to certain antibiotics due to the absence of the protective cell wall, this is not their defining feature. Other factors also contribute to antibiotic sensitivity.

Answer: Mesophiles

Pathogenic bacteria, those capable of causing disease in humans, primarily prefer moderate temperatures for growth and optimal activity. This falls within the range of mesophiles, defined as organisms thriving between 20°C and 45°C (68°F and 113°F). This temperature range aligns with the human body’s internal temperature (around 37°C, 98.6°F), providing a suitable environment for bacterial colonization and infection.

The other options are incorrect:

• Psychrophiles: These organisms favor cold environments below 20°C (68°F) and are unlikely to be significant human pathogens due to their low activity at our internal temperature.
• Cryophiles: Similar to psychrophiles, these extremely cold-loving organisms thrive below 0°C (32°F) and pose minimal threat as pathogens in our warm bodies.
• Thermophiles: While some pathogenic bacteria can tolerate or even grow at high temperatures (above 45°C, 113°F), they’re less common and often associated with specific environments like hot springs. However, the majority of human pathogens still function best within the mesophilic range.

Answer: Widal test

The Widal test is a serological test (testing blood serum) used for the diagnosis of typhoid fever caused by the bacteria Salmonella Typhi. It detects the presence of antibodies produced by the body’s immune system in response to S. Typhi infection. These antibodies agglutinate (clump together) with S. Typhi antigens used in the test, indicating a past or present infection.

The other options are incorrect:

• Eleks test: This test detects the presence of specific antibodies against different bacterial toxins and is used to diagnose diseases like diphtheria and tetanus, not typhoid fever.
• Mantoux test: This is a skin test used for the diagnosis of tuberculosis, where a purified protein derivative (PPD) from Mycobacterium tuberculosis is injected into the skin to observe the immune response.
• Wasserman test: This is a historical serological test that was used for the diagnosis of syphilis caused by Treponema pallidum. It has been replaced by more specific and sensitive tests.

Answer: Irregular clusters

Staphylococci bacteria are known for their characteristic arrangement when viewed under a microscope. While they can occasionally be found in pairs, chains, or tetrads, their most typical arrangement is in irregular clusters, often resembling grape bunches. This is due to their tendency to divide repeatedly in multiple planes without separating completely.

The other options are incorrect:

• Pairs: While staphylococci can be found in pairs, especially after initial division, it’s not their most common arrangement.
• Chains: Chain formation is more characteristic of streptococci, another group of bacteria that divide in one plane and remain attached.
• Tetrads: Tetrads, square arrangements of four cells, are rare for staphylococci and more common in some other bacterial groups like Micrococcus.

Answer: Blood culture

Blood culture: After 15 days of fever, the sensitivity of blood culture for Salmonella Typhi (the bacteria causing typhoid) starts to decline compared to earlier stages. However, it still remains the gold standard for diagnosis due to its high specificity. Multiple blood cultures (3-5) are recommended to increase the chance of detecting the bacteria.

The other options are incorrect:

• Widal test: While this serological test detects antibodies against Salmonella Typhi, its accuracy decreases after 15 days due to the rising cross-reactivity with other bacteria and the potential for persistent antibodies from previous infections. Therefore, it’s not considered the most reliable option in this scenario.
• Stool culture: While stool culture can be helpful in detecting Salmonella Typhi, its sensitivity is also lower after 15 days. Additionally, some typhoid carriers may intermittently shed the bacteria, making stool tests less consistent.
• Urine culture: Urine culture is typically not the first-line investigation for typhoid fever as Salmonella Typhi is not commonly found in urine. However, it may be considered in some cases, especially if other potential diagnoses are being ruled out.

Answer: M-Proteins

M-proteins are a virulence factor characteristic of Group A Streptococcus (GAS), not Neisseria gonorrhea. They are involved in evading the immune system and binding to host cells.

The other options are incorrect:

• Outer membrane proteins (OMPs): These proteins on the surface of the bacteria help Neisseria gonorrhea evade the immune system, adhere to host cells, and resist antibiotics.
• IgA Protease: This enzyme cleaves the antibody IgA, an important defense mechanism in mucous membranes, allowing Neisseria gonorrhea to colonize these tissues.
• Pili: These hair-like structures help Neisseria gonorrhea adhere to epithelial cells and move across mucosal surfaces, facilitating colonization and infection.

Answer: Conjugation

Conjugation is a process where bacteria directly transfer genetic material (plasmids) through a physical connection formed between two cells. In vivo, this means within a living organism, like the human body. Plasmids often carry genes for antibiotic resistance, allowing the recipient bacterium to acquire resistance from the donor.

The other options are incorrect:

• Rarefaction: This term refers to the decrease in the number of species or their abundance within a community, and it’s not a mechanism for transferring genetic material between bacteria.
• Transduction: Transduction involves bacteriophages (viruses that infect bacteria) carrying bacterial DNA fragments from one bacterium to another. While it can transfer antibiotic resistance genes, it typically occurs less frequently than conjugation in vivo.
• Transformation: Transformation is the uptake of free DNA fragments from the environment by bacteria. This can also transfer antibiotic resistance genes, but it’s less common than conjugation in vivo because environmental DNA availability is often limited.

Answer: FTAB

FTA-Abs is a highly specific treponemal test for syphilis. It detects both IgM and IgG antibodies against Treponema pallidum, the bacteria causing syphilis. This makes it useful for identifying both early and late stages of the disease.

The other options are incorrect:

• Widal test: This is a test for typhoid fever, not syphilis, and detects antibodies against Salmonella Typhi bacteria.
• Wasserman test: This is a historical serological test for syphilis, replaced by more specific and sensitive treponemal tests like FTA-Abs. It used non-treponemal antigens, leading to potential cross-reactivity with other conditions.
• Kahn test: This is another non-treponemal test for syphilis, similar to the Wasserman test. While it’s still used in some screening algorithms due to its affordability, it’s not specific and requires confirmation with a treponemal test like FTA-Abs.

Answer: Logarithmic phase

The Logarithmic phase, also known as the exponential or log phase, is the phase in bacterial growth where the population is actively dividing and multiplying. During this phase, bacterial cells are in a state of rapid growth, and their metabolic and biochemical activities are at a peak. Additionally, cells in this phase tend to have a uniform size and staining reaction, making it an optimal time for studying bacterial cultures for various purposes, including microscopy and biochemical assays.

The other options are incorrect:

• Lag phase: The lag phase is the initial phase of bacterial growth, where cells are adapting to their environment, and there is minimal cell division. Cells in this phase may vary in size and metabolic activity.
• Death phase: The death phase is the phase of bacterial growth where the population is declining due to nutrient depletion or other adverse conditions. Cells in this phase may exhibit varied morphology and reduced metabolic activity.
• Stationary phase: The stationary phase is a phase of bacterial growth where the rate of cell division equals the rate of cell death, resulting in a stable population size. Cells in this phase may vary in metabolic activity, and the growth rate has slowed compared to the logarithmic phase.

Answer: Flagella

Flagella are whip-like appendages found on the surface of some bacteria that function as the primary organ of locomotion. These structures enable bacteria to move in response to stimuli such as chemical gradients, allowing them to navigate their environment. The rotation of flagella propels bacteria through liquid media, facilitating their movement towards favorable conditions or away from harmful environments.

The other options are incorrect:

• Fimbria: Fimbriae are short, hair-like structures on the surface of some bacteria that aid in adhesion to surfaces. They are not involved in locomotion.
• Capsule: The capsule is a protective outer layer found in some bacteria, contributing to virulence and protection against the host immune system. It is not an organ of locomotion.
• Cell wall: The cell wall provides structural support and protection to bacterial cells but is not directly involved in locomotion. Flagella are responsible for the movement of bacteria.

Answer: Produced by gram positive bacteria

Endotoxins are lipopolysaccharides (LPS) found in the outer membrane of the cell wall of Gram-negative bacteria. They are not produced by Gram-positive bacteria. Endotoxins are released when the bacteria die and their cell walls break down. Some of their properties include being heat-stable, not secreted outside the cell, and the inability to be toxoided (converted into toxoids for use in vaccines).

The other options are incorrect:

• Produced by Gram-positive bacteria: Endotoxins are exclusively associated with Gram-negative bacteria. Gram-positive bacteria produce exotoxins, not endotoxins.
• Not secreted outside the cell: Endotoxins are components of the bacterial cell wall and are not actively secreted by the bacteria. Exotoxins, produced by both Gram-negative and Gram-positive bacteria, are actively secreted.
• Cannot be toxoided: Endotoxins cannot be toxoided, meaning they cannot be converted into toxoids for vaccine development. Toxoiding is a process commonly applied to exotoxins.

Answer: Pyrogenic exotoxin

Toxic shock syndrome (TSS) is primarily caused by Staphylococcus aureus, a gram-positive bacterium. The key virulence factor responsible for TSS is the pyrogenic exotoxin, also known as TSST-1. This toxin acts as a superantigen, stimulating a massive immune response that leads to the characteristic symptoms of TSS, including fever, rash, hypotension, and multi-organ failure.

The other options are incorrect:

• M protein: While M protein is a major virulence factor of Group A Streptococcus (GAS), it’s not directly associated with TSS. GAS can cause Streptococcal toxic shock syndrome (STSS), but through the action of pyrogenic exotoxins, not M protein.
• Streptolysin O: Another virulence factor of GAS, Streptolysin O, contributes to tissue damage and hemolysis but doesn’t play a primary role in TSS.
• Carbohydrate cell wall: The carbohydrate cell wall is a general component of bacterial structure and doesn’t possess specific virulence properties related to TSS.

Answer: Schick test

The Schick test is used to identify individuals who are carriers of Corynebacterium diphtheriae, the bacterium responsible for causing diphtheria. In the Schick test, a small amount of diphtheria toxin is injected intradermally. A positive reaction indicates the presence of antibodies against the toxin, suggesting previous exposure to diphtheria or carriage of the bacterium. Carriers may have developed immunity without showing clinical symptoms.

The other options are incorrect:

• Dick test: The Dick test is not used for the detection of diphtheria carriers. It is historically associated with the diagnosis of susceptibility to scarlet fever.
• Casoni’s test: Casoni’s test is used for the diagnosis of hydatid cyst disease caused by the tapeworm Echinococcus. It is not related to the detection of diphtheria carriers.
• Charles’ test: There is no widely recognized “Charles’ test” associated with the detection of diphtheria carriers. The Schick test is the more common method for this purpose.

Answer: cell wall

The primary difference between gram-positive and gram-negative bacteria lies in the structure and composition of their cell walls. This distinction affects their staining properties under the gram-staining technique.

The other options are incorrect:

• Cell membrane: While both types of bacteria have a cell membrane, it’s not the key differentiating factor. The cell wall lies outside the cell membrane and offers a more distinct structural variation.
• Nuclear material: The internal components like the nuclear material (DNA) are largely similar across various bacteria, not specific to gram-positive or negative types.
• Mesosomes: These are internal membrane folds involved in respiration and other functions, but their presence or absence doesn’t define a bacterium as gram-positive or negative.