MicroBiology MCQs with Answer and Explnation | Chapter 53

Answer: Immuno capture ELISA

Immuno capture ELISA is the most suitable method for analyzing TORCH infections. This technique involves capturing the target antigen using a specific antibody immobilized on a solid surface. It provides high sensitivity and specificity, making it effective for detecting antibodies or antigens associated with TORCH pathogens.

The other options are incorrect:

• Capture ELISA: Capture ELISA involves immobilizing the target antigen directly on the solid surface and then detecting it using a labeled antibody. It is not the best choice for TORCH infections as it may have lower sensitivity.
• Competitive ELISA: Competitive ELISA relies on the competition between a labeled and an unlabeled antigen for binding to a limited number of antibody binding sites. This method is not ideal for TORCH infections as it may not provide the required sensitivity for detecting low concentrations of specific antibodies or antigens.

Answer: Human

LD bodies are observed in the Reticuloendothelial system of humans. These structures, also known as Leishman-Donovan bodies, are intracellular parasites associated with the protozoan parasites causing leishmaniasis.

The other options are incorrect:

• Ruduvid bug: There is no known association between LD bodies and Ruduvid bugs. Leishmaniasis is not typically transmitted by Ruduvid bugs.
• Tsetse fly: LD bodies are not associated with the Reticuloendothelial system of Tsetse flies. Tsetse flies are known for transmitting trypanosomes, causing diseases such as sleeping sickness.
• Sand fly: Sand flies are the vectors responsible for transmitting Leishmania parasites to humans. However, LD bodies are found in the Reticuloendothelial system of humans, not in sand flies. Sand flies play a role in the transmission of leishmaniasis but do not harbor LD bodies.

Answer: Plasmodium falciparum

Plasmodium falciparum is the deadliest malaria parasite and the only one known to develop multiple Merozoite-Containing Stages (MCS) within a single red blood cell. These MCS are also referred to as “multiple mng forms.”

The other options are incorrect:

• Plasmodium malariae: This parasite typically produces only one merozoite per infected red blood cell.
• Plasmodium vivax: Similar to P. malariae, P. vivax generally results in only one merozoite per infected red blood cell.
• Plasmodium ovale: Though less common than P. vivax and P. malariae, P. ovale also typically produces only one merozoite per infected red blood cell.

Answer: Trichomonas vaginalis

Trichomonas vaginalis is a flagellate parasite and unlike the other options listed, it does not have a cyst stage. It exists only in its trophozoite form, which is the motile, feeding stage.

The other options are incorrect:

• Giardia lamblia: This parasite forms a resistant cyst stage that allows it to survive outside the host in the environment and facilitates transmission.
• Balantidium coli: This parasite has a complex life cycle that includes both a trophozoite stage (active feeding) and a cyst stage (dormant and resistant).
• E. histolytica: Entamoeba histolytica also has a cyst stage that helps it survive outside the host and spread through contaminated water or food.

Answer: Robert Koch

Robert Koch, a German physician and microbiologist, is credited with identifying the Mycobacterium tuberculosis bacteria in 1882. This discovery was a major breakthrough in understanding and combating tuberculosis, a previously devastating disease.

The other options are incorrect:

• Louis Pasteur: Louis Pasteur, a French microbiologist and chemist, made significant contributions to the development of vaccines and germ theory. However, he is not associated with identifying the tubercle bacillus.
• Edward Jenner: Edward Jenner, a British physician, pioneered the smallpox vaccine. While his work paved the way for future vaccines, he did not identify the tubercle bacillus.
• Joseph Lister: Joseph Lister, a British surgeon, is known for his work in promoting antiseptic practices. His contributions to hygiene helped reduce infections but were not directly related to identifying the tubercle bacillus.

Answer: Mycobacteria

The Ziehl-Neelsen stain is a specific staining technique used in microbiology to identify acid-fast bacteria, particularly those belonging to the Mycobacterium genus. This genus includes Mycobacterium tuberculosis, the causative agent of tuberculosis. The Ziehl-Neelsen stain differentiates acid-fast bacteria from other types of bacteria due to their unique cell wall composition.

The other options are incorrect:

• Streptococci: Streptococci are Gram-positive bacteria and do not require the Ziehl-Neelsen stain for identification. They are typically stained using Gram stain.
• Anthrax Bacilli: While Bacillus anthracis, the bacteria causing anthrax, can form spores that are resistant to staining, the Ziehl-Neelsen stain is not the primary method for identifying it. In most cases, Gram stain or other techniques are used.
• Spirochaetes: Spirochaetes are a group of bacteria with a spiral-shaped morphology. They are not acid-fast and are not identified using the Ziehl-Neelsen stain. Darkfield microscopy or specialized staining techniques are often used for visualizing spirochaetes.

Answer: Glucose, Lactose, Sucrose

Triple Sugar Iron Agar (TSI Agar) is a differential medium used to identify and differentiate bacteria based on their ability to ferment three specific sugars: glucose (dextrose), lactose, and sucrose. The presence of these sugars along with a pH indicator and a sulfur-reduction indicator allows for the observation of various fermentation patterns that help distinguish between different bacterial species.

The other options are incorrect:

• Glucose, Maltose, Sucrose: While maltose is sometimes used in other differential media, TSI Agar specifically uses lactose instead of maltose for its discriminatory power.
• Glucose, Mannitol, Sucrose: Mannitol is another sugar sometimes used in bacterial identification, but it’s not included in TSI Agar.
• Glucose, Lactose, Maltose: As mentioned earlier, TSI Agar uses lactose for differentiation, not maltose.

Answer: Enriched media

Blood agar is an enriched medium because it provides a rich and complex nutritional environment that supports the growth of a wide variety of fastidious (difficult to grow) bacteria. It contains a base medium like Tryptic Soy Agar (TSA) supplemented with sheep or horse blood. The blood provides additional nutrients essential for the growth of many bacteria, particularly those that require factors like hemin and vitamins present in red blood cells.

The other options are incorrect:

• Enrichment Media: Enrichment media are similar to enriched media but with the addition of specific factors to promote the growth of a particular type of bacteria. While blood agar can sometimes be enriched further for specific pathogens, it’s generally not considered a primary enrichment medium itself.
• Selective Media: Selective media contain ingredients that inhibit the growth of certain types of bacteria while allowing the growth of others. Blood agar itself doesn’t inhibit any particular bacteria and allows growth of a wide range, so it’s not selective.
• Differential Media: Differential media allow for the differentiation of various bacterial colonies based on their characteristics like hemolysis (red blood cell destruction) patterns on blood agar. Blood agar helps differentiate bacteria based on hemolysis but doesn’t inhibit any specific types, making it a differential medium, not a selective one.

Answer: Clostridium Perfringens

Reinforced Clostridial Medium (RCM) is a selective and enriched medium designed for the primary cultivation of anaerobic bacteria, particularly Clostridium perfringens. It provides nutrients to support the growth of these bacteria and contains certain components that inhibit the growth of aerobic and facultative anaerobic bacteria.

The other options are incorrect:

• Tubercle bacilli: RCM is not specifically designed for the primary cultivation of tubercle bacilli (Mycobacterium tuberculosis). Specialized media, such as Lowenstein-Jensen medium, are used for the cultivation of Mycobacteria.
• Vibrio cholerae: RCM is not designed for the primary cultivation of Vibrio cholerae, which is the causative agent of cholera. Selective media like thiosulfate-citrate-bile salts-sucrose (TCBS) agar are more suitable for Vibrio cholerae isolation.
• Haemophilus: RCM is not commonly used for the primary cultivation of Haemophilus species. Specialized media like chocolate agar or Haemophilus agar are more suitable for the growth of Haemophilus bacteria.

Answer: Bacitracin

Bacitracin susceptibility testing is a rapid and reliable method for identifying Group A Streptococcus (GAS) isolates. GAS are typically sensitive to bacitracin, while other beta-hemolytic streptococci (those that cause a clear zone of hemolysis around colonies on blood agar) are often resistant. This test, along with the Lancefield antigen A test, is used for definitive identification of GAS.

The other options are incorrect:

• Novobiosin: Novobiosin is an antibiotic with a broader spectrum of activity and is not specific for diagnosing GAS.
• Optochin: Optochin sensitivity testing is used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci (those causing a greenish zone of hemolysis). It’s not typically used for GAS identification.
• Polymixin: Polymixin is another broad-spectrum antibiotic and doesn’t specifically target GAS for diagnostic purposes.

Answer: Coryne bacteria diphthreriae

Elek’s test is a specialized in vitro test used to determine the toxigenicity (toxin production) of Corynebacterium diphtheriae, the bacteria that causes diphtheria. The test detects the presence of diphtheria toxin, a major virulence factor responsible for the severe symptoms of the disease.

The other options are incorrect:

• Haemophilus influenzae: Haemophilus influenzae can cause various infections but doesn’t produce diphtheria toxin. Different tests are used to assess its virulence factors.
• Group B Streptococcus: Group B Streptococcus can cause neonatal infections but doesn’t produce diphtheria toxin. Specific tests for this bacteria target its unique virulence factors.
• Clostridium Perfringens: Clostridium Perfringens produces different toxins associated with its pathogenic mechanisms. Different tests are used to detect these toxins depending on the specific type of C. Perfringens infection.

Answer: Proteus

Proteus species are known for exhibiting swarming growth on agar plates. This swarming motility is a unique characteristic mediated by flagella and allows them to rapidly colonize surfaces.

The other options are incorrect:

• Salmonella: Salmonella species typically show a smooth, circular colony morphology on agar plates without swarming.
• Shigella: Similar to Salmonella, Shigella species also generally exhibit a non-swarming colony morphology.
• Pseudomonas: While some Pseudomonas species can exhibit twitching motility, they don’t typically display the expansive swarming growth pattern characteristic of Proteus.

Answer: Capsule

Negative staining is a microscopy technique used to visualize the capsule surrounding some bacteria. The negatively charged stain particles repel the negatively charged bacterial cell wall, leaving a clear halo around the cell, highlighting the capsule (if present).

The other options are incorrect:

• Flagella: Flagella are too thin to be effectively visualized with negative staining alone. They often require techniques like transmission electron microscopy for clear observation.
• Cell wall: Negative staining doesn’t directly stain the cell wall. It relies on the repulsion between the stain and the cell, creating a contrasting background to highlight structures outside the cell, like the capsule.
• None of the above: While negative staining can’t directly visualize the cell wall itself, it is a valuable tool for demonstrating the presence of a capsule, making it a relevant answer choice.

Answer: Casteneda method

The Castenededa method is a manual blood culture technique for detecting bacteria in blood samples. It involves inoculating blood into broth media and visually observing for growth over several days.

The other options are incorrect:

• BACTEC: BACTEC is a brand name for a widely used automated blood culture system that continuously monitors cultures for bacterial growth using radiometric detection methods.
• BaCT/ALERT: Similar to BACTEC, BaCT/ALERT is another brand name for an automated blood culture system that employs a colorimetric detection method to identify bacterial growth.
• ESP system: ESP (Electrochemical Luminescence) system is another type of automated blood culture system that utilizes a bioluminescent detection method to monitor cultures for bacterial growth.

Answer: Selenite F broth

Selenite F broth is an enrichment medium specifically designed to promote the growth of Salmonella species from stool samples. It contains selenite and other inhibitory substances that suppress the growth of most commensal gut bacteria, allowing for the preferential enrichment of Salmonella, which are more resistant to these inhibitory factors.

The other options are incorrect:

• MacConkey agar: MacConkey agar is a differential medium that differentiates lactose-fermenting from non-lactose-fermenting bacteria based on colony color. It doesn’t contain specific enrichment factors for any particular organism.
• Nutrient Agar: Nutrient Agar is a basic growth medium that provides essential nutrients for many non-fastidious bacteria. It’s not specifically enriched for any particular organism.
• Peptone water: Peptone water is another basic enrichment medium that can support the growth of a wide variety of bacteria. While it may provide some enrichment compared to minimal media, it doesn’t target a specific organism like Selenite F broth does for Salmonella.

Answer: Aerobic bacteria

Aerobic bacteria are those that require oxygen (O2) for their growth and survival. They utilize oxygen in their metabolic processes, particularly cellular respiration, to generate energy.

The other options are incorrect:

• Carboxyphilic bacteria: This term is not commonly used in microbiology. Bacteria are typically classified based on their oxygen requirement, not their specific carbon source.
• Anaerobic bacteria: Anaerobic bacteria are the opposite of aerobic bacteria. They do not require oxygen for growth and can even be harmed by its presence. They have alternative metabolic pathways for energy production that don’t involve oxygen.
• Obligate anaerobes: This is a specific type of anaerobic bacteria that cannot survive in the presence of oxygen. They have evolved mechanisms to deal with the toxic effects of oxygen.

Answer: Zeihl-Neelsen’s staining

Ziehl-Neelsen staining is a specific staining technique used in microbiology to identify acid-fast bacteria, particularly Mycobacterium tuberculosis, the causative agent of tuberculosis. This staining method relies on the unique cell wall composition of Mycobacterium species, which allows them to retain carbol fuchsin stain even after strong acid-alcohol treatment.

The other options are incorrect:

• Leishman’s staining: Leishman’s stain is used to visualize blood parasites, protozoa, and some bacteria with a specific focus on their internal structures. It’s not typically used for Mycobacterium tuberculosis.
• Simple staining: Simple staining involves applying a single stain to visualize the overall morphology of bacterial cells. While it can be used for some bacteria, it wouldn’t be sufficient to differentiate Mycobacterium tuberculosis due to its specific cell wall properties.
• Gram’s staining: Gram staining is a widely used differential staining technique that classifies bacteria into two broad groups based on their cell wall structure: Gram-positive and Gram-negative. Mycobacterium tuberculosis falls outside this classification due to its unique cell wall and wouldn’t be effectively identified using Gram’s stain.

Answer: Agr

Agar is the most commonly used solidifying agent for preparing solid culture media in microbiology. It’s a complex polysaccharide derived from red algae. Agar gels at around 42°C after being boiled in liquid media and solidifies at this temperature, creating a solid matrix suitable for bacterial growth and observation of colony morphology.

The other options are incorrect:

• DPX: DPX (Distrene, Plasticizer, Xylene) is a mounting medium used in microscopy to preserve and protect stained specimens on slides. It’s not used for solidifying culture media.
• Meat extract: Meat extract is a source of nutrients for bacteria in culture media. While it can be a component of the media, it doesn’t function as a solidifying agent.
• Peptone: Similar to meat extract, peptone is another source of nutrients for bacteria in culture media. It doesn’t have solidifying properties.

Answer: cocci

Cocci (singular: coccus) are round or spherical shaped bacteria. They come in various arrangements, such as pairs (diplococci), chains (streptococci), or clusters (staphylococci).

The other options are incorrect:

• Bacilli: Bacilli (singular: bacillus) are rod-shaped bacteria.
• Vibrios: Vibrios are curved or comma-shaped bacteria.
• Actinomycetes: Actinomycetes are a group of filamentous bacteria that may appear branched or have a mold-like appearance. They are not typically round or spherical.

Answer: Citrate test

IMViC is a set of four biochemical tests used to identify members of the Enterobacteriaceae family based on their metabolic characteristics. The “C” in IMViC stands for the Citrate test. This test determines the ability of a bacterium to use citrate as the sole carbon source for growth, indicating its ability to utilize citrate in the medium.

The other options are incorrect:

• Nitrate test: While the nitrate test can be used for identification purposes, it’s not part of the traditional IMViC set.
• Coagulase test: The coagulase test is used to differentiate Staphylococcus species, particularly Staphylococcus aureus, based on their ability to clot plasma. It’s not specific to Enterobacteriaceae and not included in IMViC.
• Catalase test: The catalase test is a rapid test to detect the presence of the enzyme catalase, which breaks down hydrogen peroxide. While some Enterobacteriaceae may be catalase-positive, this test is not specific to the IMViC series.

Answer: Tubercular meningitis

Cobweb appearance in the cerebrospinal fluid (CSF) refers to a characteristic finding observed during the visual examination of CSF after it has been left undisturbed for some time. This finding is most commonly associated with tubercular meningitis, an infection of the meninges (membranes surrounding the brain and spinal cord) caused by Mycobacterium tuberculosis bacteria.

The other options are incorrect:

• Syphilis: Syphilis can cause inflammation of the meninges (meningitis), but the cobweb appearance in CSF is not a typical finding in syphilitic meningitis.
• Malaria: Malaria is a parasitic infection affecting red blood cells. It doesn’t typically cause meningitis or the cobweb appearance in CSF.
• Hepatitis: Hepatitis refers to inflammation of the liver. It primarily affects the liver and doesn’t involve the meninges or CSF.

Answer: Pandy’s test

Pandy’s test is a simple and rapid bedside test used to detect elevated protein levels in cerebrospinal fluid (CSF). It relies on the observation of agglutination (clumping) of red blood cells in the presence of CSF with high protein content.

The other options are incorrect:

• Hay’s test: Hay’s test is not a commonly used test for CSF protein. There’s limited information available about this specific test in modern medical practice.
• Smith’s test: Similar to Hay’s test, Smith’s test is not a widely used method for CSF protein detection.
• Hart’s test: Hart’s test is typically used to assess blood vessel integrity and permeability, not specifically for CSF protein levels.

Answer: Moist heat method

An autoclave uses high-pressure steam (moist heat) to kill microorganisms, spores, and viruses. This is the most common method of sterilization used in healthcare settings for instruments, dressings, and other medical supplies.

The other options are incorrect:

• Dry heat method: This method uses high temperatures (above 160°C) in the absence of moisture to sterilize materials. However, it can damage some heat-sensitive items and is not as effective against spores as moist heat.
• Filtration method: This method involves passing liquids or gases through a filter with pores small enough to exclude microorganisms. It’s commonly used for sterilizing heat-sensitive solutions and air in biological safety cabinets.
• Radiation method: This method uses ultraviolet (UV) light or ionizing radiation to kill microorganisms. It’s effective for surface sterilization but doesn’t penetrate materials deeply and can damage some plastics.

Answer: Pink

MacConkey Agar is a differential and selective culture medium used to differentiate lactose-fermenting from non-lactose-fermenting bacteria. Lactose fermenting bacteria are able to break down lactose, a sugar present in the media, producing acidic byproducts. This acidic environment causes the pH indicator in the media, neutral red, to change color, resulting in pink colonies.

The other options are incorrect:

• Colourless: Colourless colonies on MacConkey agar typically indicate non-lactose-fermenting bacteria. These bacteria cannot break down lactose and therefore don’t produce the acidic byproducts that cause the pink color.
• Green: Green colonies on MacConkey agar are uncommon but can sometimes occur with specific lactose-fermenting bacteria or due to other factors like oxidation of media components. However, pink is the most expected color for lactose fermenters.
• Black: Black colonies on MacConkey agar are very unlikely and would suggest significant contamination or growth of atypical organisms.

Answer: Louis Pasteur

Louis Pasteur is often referred to as the Father of Microbiology. His groundbreaking work in the 19th century laid the foundation for many principles in microbiology and medicine. Pasteur made significant contributions to the understanding of microbial fermentation, the development of vaccines, and the process of pasteurization, which involves heating liquids to kill bacteria and other microorganisms.

The other options are incorrect:

Robert Koch: Robert Koch is a prominent figure in microbiology known for Koch’s postulates and his contributions to the understanding of infectious diseases. However, he is not considered the Father of Microbiology.
Joseph Lister: Joseph Lister is known for his work in antiseptic surgery, introducing practices to reduce infection during surgical procedures. While a significant figure in medicine, he is not the Father of Microbiology.
Edward Jenner: Edward Jenner is credited with developing the smallpox vaccine, a pivotal achievement in immunization. However, he is not considered the Father of Microbiology, which is more closely associated with the work of Louis Pasteur.

Answer: Streptococcus

The ASO test, which stands for Antistreptolysin O titer, is used to detect antibodies against a specific toxin produced by Streptococcus bacteria, particularly Group A Streptococcus (GAS). The presence of these antibodies indicates a past or current Streptococcus infection.

The other options are incorrect:

• Staphylococcus: While Staphylococcus aureus and other Staphylococcus species can cause various infections, the ASO test is not specific for them. It detects antibodies against a toxin unique to Streptococcus.
• Pneumococcus: Pneumococcus, also known as Streptococcus pneumoniae, is a type of Streptococcus bacteria. However, the ASO test is not specific for differentiating between different Streptococcus species. It primarily indicates a past or current infection with any strain that produces the targeted toxin.
• Salmonella: Salmonella is a type of bacteria belonging to a different genus altogether. The ASO test wouldn’t be used to detect Salmonella infections. They require different diagnostic methods.

Answer: Candida Albicans

The germ tube test is a rapid and specific test used to diagnose infections caused by Candida albicans. This fungus forms germ tubes, which are elongated hyphal structures, when incubated in a serum-supplemented medium at 37°C. These germ tubes are readily detectable by microscopy and confirm the presence of C. albicans.

The other options are incorrect:

• Cryptococcus neoformans: This is a yeast-like fungus that does not form germ tubes. It is identified through different methods like India ink staining and capsule visualization.
• Trichophyton: Trichophyton is a dermatophyte fungus that grows as molds and doesn’t produce germ tubes. Fungal cultures and microscopic examination of fungal elements are used for diagnosis.
• Microsporum: Similar to Trichophyton, Microsporum is another dermatophyte fungus and doesn’t form germ tubes. Diagnosis relies on fungal culture and microscopic examination.

Answer: H. influenzae

The satellite test is used to identify Haemophilus influenzae. This bacterium requires certain growth factors, particularly X factor (hemin) and V factor (NAD), that it cannot synthesize itself. In the satellite test, a strain known to produce these factors, such as Staphylococcus aureus, is streaked across a blood agar plate. Haemophilus influenzae, if present in the sample, will grow along the streak of the Staphylococcus aureus colony where the required factors are available by diffusion. This results in satellite colonies of Haemophilus influenzae surrounding the S. aureus streak.

The other options are incorrect:

• S. aureus: Staphylococcus aureus is a common bacterium that grows well on blood agar without requiring additional factors. The satellite test is used to identify bacteria that depend on factors produced by S. aureus.
• S. typhi: Salmonella typhi, the cause of typhoid fever, is another bacterium that grows well on blood agar and doesn’t require the satellite test for identification.
• Cl. perfringens: Clostridium perfringens is an anaerobic bacterium that grows best in special media lacking oxygen. The satellite test is not used for its identification.

Answer: 160°C for 1 hr

The recommended holding time for dry heat sterilization using a hot air oven for most applications is 160°C for 1 hour. This temperature and time combination effectively kill a wide range of microorganisms, including bacteria, fungi, and some viruses.

The other options are incorrect:

• 121°C for 30 minutes: This is the standard time and temperature for steam sterilization using an autoclave, a method that utilizes moist heat under pressure. It’s not typically used for hot air ovens.
• 160°C for 30 minutes: While this temperature might be effective against some bacteria, a longer holding time of 1 hour at 160°C is generally recommended for ensuring complete sterilization and destroying spores, which are more resistant.
• 121°C for 1 hour: Similar to the previous point, this falls under the realm of moist heat sterilization parameters for autoclaves and wouldn’t be applicable for hot air ovens.

Answer: Enteric fever

The Widal test is a serological test used to diagnose enteric fever, also known as typhoid fever and paratyphoid fever. It detects the presence of antibodies produced by the body in response to infection with Salmonella Typhi (typhoid fever) or Salmonella Paratyphi (paratyphoid fever) bacteria.

The other options are incorrect:

• Jaundice: Jaundice is a symptom characterized by yellowing of the skin and eyes due to excess bilirubin in the blood. The Widal test is not used to diagnose jaundice, which has various causes.
• Diphtheria: Diphtheria is a respiratory illness caused by Corynebacterium diphtheriae bacteria. Diagnosis of diphtheria involves isolating the bacteria from throat cultures and sometimes toxin detection tests. The Widal test is not specific for diphtheria.
• Tetanus: Tetanus is a neurological disease caused by the toxin produced by Clostridium tetani bacteria. Diagnosis of tetanus is based on clinical symptoms and history, not the Widal test. The Widal test is not specific for tetanus.

Answer: Alberts stain

Albert’s stain is a specific staining technique used to visualize the metachromatic granules present in Corynebacterium diphtheriae, the bacteria that causes diphtheria. These granules are composed of polyphosphate and appear as dark blue or purple inclusions within the red-stained bacterial cells.

The other options are incorrect:

• Tuberculin test: The tuberculin skin test is used to diagnose tuberculosis infection and doesn’t involve staining bacteria.
• Gram’s stain: Gram’s stain is a widely used differential stain that classifies bacteria into two groups based on their cell wall structure: Gram-positive and Gram-negative. While it can provide some morphological information, it wouldn’t be sufficient to visualize the specific metachromatic granules of C. diphtheriae.
• India ink: India ink stain is typically used for identifying encapsulated fungi or certain parasites in body fluids. It wouldn’t be suitable for highlighting metachromatic granules in C. diphtheriae.

Answer: Corneybacterium diphtheriae

The Elek’s gel precipitation test is a specific in vitro test used to determine the toxigenicity (toxin production) of Corynebacterium diphtheriae. This test detects the presence of diphtheria toxin, a major virulence factor responsible for the severe symptoms of the disease.

The other options are incorrect:

• S. aureus: Staphylococcus aureus can produce a variety of toxins, but the Elek’s test is not used for their detection. Different tests are employed to identify specific toxins associated with S. aureus infections.
• Cl. tetani: Clostridium tetani produces a neurotoxin called tetanospasmin, but the Elek’s test is not applicable for its detection. Diagnosis of tetanus relies on clinical symptoms and history, not toxin detection in routine diagnostic procedures.
• Bacillus: Bacillus is a genus that encompasses various species with diverse characteristics. The Elek’s test is specific for Corynebacterium diphtheriae and wouldn’t be used for identifying toxin production in other Bacillus species.

Answer: Inspissation

L.J. medium (Lowenstein-Jensen medium) and Loeffler’s serum slope are sterilized by inspissation. This is a specific sterilization technique used for media containing heat-sensitive components like serum or eggs.

The other options are incorrect:

• Tyndallization: This method involves repeated cycles of heating to a sub-boiling temperature (usually 80°C) followed by incubation, allowing heat-resistant spores to germinate. The process is repeated several times to inactivate the newly germinated spores. While effective for some media, it’s not commonly used for L.J. medium or Loeffler’s serum slope due to the potential for denaturing heat-sensitive components.
• Autoclave: Autoclaves utilize high-pressure steam (moist heat) for sterilization, which is too harsh for media containing serum or eggs. These components would coagulate or denature at the high temperatures used in autoclaves.
• Hot air oven: Dry heat sterilization using a hot air oven is not suitable for L.J. medium or Loeffler’s serum slope because the high temperatures needed for effective sterilization (around 160°C) would again denature the heat-sensitive components.

Answer: UV rays

UV (ultraviolet) rays are a common method used for disinfection of hospital wards and operation rooms. They act by disrupting the DNA or RNA of microorganisms, rendering them unable to replicate and cause infection.

The other options are incorrect:

• X-rays: X-rays are a type of ionizing radiation used for medical imaging and don’t have a significant germicidal effect. They can penetrate deeply but wouldn’t be practical for routine disinfection due to safety concerns.
• Gamma rays: Similar to X-rays, gamma rays are a form of ionizing radiation with high penetration power. While they can be effective for sterilization, their use is limited due to safety hazards and the potential for damaging materials in the environment.
• Visible light: Visible light lacks the germicidal properties of UV rays. It doesn’t have enough energy to effectively inactivate most microorganisms.

Answer: Cary blair media

Cary Blair medium is a type of transport medium used to preserve and transport clinical specimens, particularly stool samples, for the isolation of enteric pathogens. It contains a semi-solid gel that helps maintain the viability of microorganisms during transit while minimizing overgrowth of contaminating bacteria.

The other options are incorrect:

• Wilson and Blairs media: There is no widely recognized medium called “Wilson and Blairs media.” It may be a combination or reference to different media, but it is not a specific transport medium.
• TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar: TCBS agar is a selective medium used for the isolation of Vibrio species, particularly Vibrio cholerae. It is not a transport medium but rather a culture medium.
• Tellurite agar: Tellurite agar is a selective medium used for the isolation of Corynebacterium diphtheriae. It is not a transport medium but a medium used for the cultivation and identification of specific bacteria.

Answer: V. cholerae

The string test is a diagnostic test used to identify Vibrio cholerae, the bacterium responsible for cholera. In this test, a bacterial colony is touched with a loop or needle, and if the bacteria produce a mucoid string (mucin clot) that is pulled out upon withdrawal, it indicates the presence of Vibrio cholerae.

The other options are incorrect:

• S. aureus: The string test is not used to identify Staphylococcus aureus. Staphylococcus aureus is a different bacterium, and its identification involves other methods, such as culture and biochemical tests.
• Proteus: The string test is not used to identify Proteus species. Identification of Proteus involves cultural and biochemical characteristics, but the string test is not applicable.
• Pseudomonas: The string test is not used to identify Pseudomonas species. Identification of Pseudomonas involves various laboratory methods, but the string test is specific to Vibrio cholerae.

Answer: Tuberculous meningitis

The cobweb appearance in the cerebrospinal fluid (CSF) is a characteristic finding associated with tuberculous meningitis. This is an infection of the meninges (membranes surrounding the brain and spinal cord) caused by Mycobacterium tuberculosis bacteria.

The other options are incorrect:

• Xanthochromia: This refers to a yellowish discoloration of the CSF due to the presence of bilirubin breakdown products. It can occur in various conditions, including subarachnoid hemorrhage (bleeding in the space around the brain), but is not typically associated with a cobweb clot.
• Poliomyelitis: Poliovirus infection primarily affects the nervous system, but the cobweb appearance in CSF is not a common finding in polio.
• All the above: While the other options can affect the CSF, they don’t typically cause the specific cobweb clot appearance seen in tuberculous meningitis.

Answer: 0.3% tricresol

Pelikan India ink, used for negative staining in microbiology, typically contains a preservative to prevent bacterial or fungal contamination. In this case, the preservative used is 0.3% tricresol, which helps maintain the stability and sterility of the ink.

The other options are incorrect:

• Lactic acid: Lactic acid is not typically used as a preservative in Pelikan India ink. It might have some antimicrobial properties, but it’s not strong enough for long-term preservation and can affect the ink properties.
• Phenol: Phenol can be a preservative, but 0.3% tricresol is the more common choice for Pelikan India ink. Tricresol is a mixture of three isomeric cresols and is generally considered more effective and less toxic than phenol for this purpose.
• None of the above: While other preservatives might exist, 0.3% tricresol is the most widely documented and referenced preservative used in Pelikan India ink.

Answer: all the above

Lawn culture, also known as carpet culture, is a microbiological technique used for several purposes:

• Obtain large amount of growth: This is the primary advantage of lawn culture. By flooding the surface of a culture plate with a liquid bacterial suspension and allowing for even distribution, a large area of the plate is inoculated, promoting abundant bacterial growth.
• Preparation of antigens: Antigens are molecules that the immune system can recognize. In vaccine development, antigens from pathogens are often used to stimulate the immune response. Lawn cultures can be used to grow large quantities of bacteria for the subsequent extraction and purification of antigens.
• Antibiotic Susceptibility Testing (ABST): This is a series of tests used to determine the effectiveness of different antibiotics against a specific bacterial isolate. While there are various methods for ABST, some techniques, like the Kirby-Bauer disk diffusion method, can utilize a lawn culture to create a confluent (evenly growing) layer of bacteria on the agar plate. Antibiotic discs are then placed on the lawn, and zones of inhibition (areas where bacteria fail to grow) around the discs indicate the bacteria’s susceptibility or resistance to the specific antibiotics.

Answer: Nagler reaction

The Nagler reaction is a specific test used to identify Clostridium perfringens. This test detects the presence of an enzyme called alpha-toxin produced by some strains of C. perfringens. The alpha-toxin lyses (breaks down) red blood cells in the presence of lecithin, resulting in a positive Nagler reaction.

The other options are incorrect:

• Satellite test: This test is used to identify bacteria that require specific growth factors produced by other organisms. It wouldn’t be suitable for identifying C. perfringens, which is not dependent on such factors.
• Coagulase test: This test is used to differentiate Staphylococcus species, particularly Staphylococcus aureus, based on their ability to clot plasma. It’s not specific for C. perfringens.
• String test: As mentioned previously, the string test has different applications depending on the context. Neither version is typically used for identifying C. perfringens.

Answer: Staphylococcus

Staphylococcus bacteria are cocci (spherical-shaped) and characteristically arrange themselves in irregular, grape-like clusters. This clustering pattern is due to cell division in multiple planes without complete separation of the daughter cells.

The other options are incorrect:

• Streptococcus: Streptococcus bacteria are also cocci, but they typically form chains of various lengths.
• Corynebacterium: These bacteria are rod-shaped (bacilli) and don’t form grape-like clusters.
• Cryptococcus: This is a yeast and not a bacterium. Yeasts are fungal organisms and can appear as single round or oval cells or budding yeast forms, but they don’t form grape-like clusters.

Answer: E. coli

The urease test is a diagnostic test that detects the ability of microorganisms to produce the enzyme urease, which catalyzes the hydrolysis of urea to ammonia and carbon dioxide. E. coli is not urease-positive, as it lacks the enzyme urease.

The other options are incorrect:

• Proteus: Proteus species, including Proteus mirabilis, are urease-positive. They produce urease, leading to the alkaline pH and the characteristic swarming growth on urea agar.
• Cryptococcus: Cryptococcus neoformans is urease-positive. This property is often used in the laboratory to differentiate it from other yeast-like fungi.
• Citrobacter: Citrobacter species are urease-positive. They produce urease, contributing to the hydrolysis of urea and the release of ammonia.

Answer: Giemsa staining

Giemsa stain is a widely used Romanowsky stain specifically designed to differentiate various blood cells, including intracellular haemoprotozoans. It effectively highlights the morphology of these parasites within host cells due to its ability to stain both nucleic acids (DNA and RNA) and cytoplasmic components.

The other options are incorrect:

• Leishman’s staining: This stain is similar to Giemsa but less versatile. While it can stain some haemoprotozoans, it might not provide the same level of detail and clarity as Giemsa.
• Wright’s staining: Primarily used for peripheral blood smears, Wright’s stain is another Romanowsky stain but optimized for visualizing different types of white blood cells. It might not provide enough contrast to clearly identify intracellular haemoprotozoans.
• Gram’s staining: This differential stain is specifically designed to classify bacteria based on their cell wall composition. It is not effective for staining haemoprotozoans.

Answer: Dark field microscope

Dark field microscopy is the preferred method for detecting Leptospira in blood samples due to its ability to visualize live, motile bacteria against a dark background. Leptospires are very thin and transparent, making them difficult to see under brightfield illumination. Dark field microscopy overcomes this by illuminating the sample at an angle, causing light to scatter around the bacteria and making their characteristic flexing and spinning movements visible.

The other options are incorrect:

• Phase contrast microscope: While phase contrast microscopy can enhance the contrast of transparent objects compared to brightfield, it doesn’t provide the same level of visualization for highly transparent and motile bacteria like Leptospira as dark field microscopy.
• Stereo zoom microscope: Primarily used for low-magnification observation and manipulation of specimens, stereo zoom microscopes are not ideal for visualizing details of single bacteria like Leptospira. They are better suited for examining larger structures or manipulating tissues.
• Fluorescence microscope: Fluorescence microscopy requires the specific labeling of the target organism with fluorescent markers. While it can be a powerful tool for detecting specific pathogens, it’s not typically used for routine diagnosis of Leptospirosis in blood samples because it requires additional preparation steps and specific antibodies.

Answer: 1.5% -2%

Agar is a gelatinous substance derived from seaweed and is commonly used as a solidifying agent in bacteriological media. The standard percentage of agar in most common bacteriological media falls within the range of 1.5% to 2%. This concentration provides the appropriate consistency for the solid medium, allowing the cultivation and isolation of various microorganisms.

The other options are incorrect:

• 0.26% — 0.5%: Too low of an agar concentration can result in a runny medium that’s difficult to streak and might not solidify properly. This could lead to challenges in isolating and differentiating bacterial colonies.
• 5%-10%: An excessively high agar concentration can create a very firm medium. While it would solidify well, it might limit the diffusion of nutrients and oxygen, potentially hindering bacterial growth.
• 10% – 20%: This is an extremely high agar concentration that would make streaking and isolating bacteria very difficult. It would also significantly restrict nutrient and gas diffusion, severely inhibiting bacterial growth.

Answer: Fluid thioglycollate medium

Fluid Thioglycollate Medium is a liquid medium designed for the cultivation of anaerobic bacteria. It contains thioglycollate, which reduces oxygen to create an anaerobic environment. The medium has a gradient of oxygen concentration, with the top being more aerobic and the bottom being more anaerobic, allowing for the growth of a variety of microorganisms with different oxygen requirements.

The other options are incorrect:

• Nutrient broth: This is a general-purpose broth that can support the growth of many bacteria, including aerobes, facultative anaerobes, and some microaerophilic bacteria (requiring low oxygen levels). However, it’s not specifically designed for strict anaerobes.
• MacConkey broth: This is a selective and differential broth medium used to differentiate lactose-fermenting from non-lactose-fermenting bacteria. It’s not formulated for anaerobic growth.
• Peptone water: This is a simple broth medium containing peptone, a source of nutrients. While some facultative anaerobes might grow in peptone water if there’s limited oxygen exposure, it’s not suitable for strict anaerobes and lacks the reducing agents needed for anaerobic growth.

Answer: Fleming’s modification

Fleming’s modification is a separate staining technique used for identifying Treponema pallidum, the bacteria causing syphilis. It’s not a modification of the Gram stain which differentiates bacteria based on cell wall structure.

The other options are incorrect:

• Kopeloff and Bearman’s modification: This modifies the Gram stain by using a carbol fuchsin solution instead of crystal violet for a more intense red color in Gram-positive bacteria.
• Burke’s modification: This modification uses a different solvent for the crystal violet stain, making it more suitable for smears containing blood or mucus.
• Weigert’s modification: This modification uses aniline water instead of ethanol as a decolorizer, making it gentler on certain bacteria with fragile cell walls.

Answer: Stain the fungus

Cotton blue is a crucial dye in lactophenol cotton blue stain. It selectively binds to the chitin in the fungal cell wall, making the fungus appear blue under a microscope. This allows for clear visualization of fungal structures for identification.

The other options are incorrect:

• Kill the fungus: Lactophenol cotton blue stain is a diagnostic stain and does not possess antifungal properties. It primarily serves to preserve and visualize the fungus.
• Clear the fungus: While the solution might slightly clarify the background, cotton blue’s main function is to stain the fungus, not to remove it from the sample entirely.
• Fix the fungus: Lactophenol cotton blue stain does not permanently fix the fungus like some histological fixatives. However, it does preserve the morphological characteristics of the fungus for microscopic examination.

Answer: Heat fixing

Heat fixing is the most common method for fixing bacterial smears in laboratories working with Biosafety Level 1 (BSL-1) organisms. It uses heat to denature proteins in the bacteria, causing them to adhere firmly to the slide for subsequent staining procedures.

The other options are incorrect:

• Phenol fixing: While historically used, phenol is rarely used today due to its toxicity and potential carcinogenic properties. Safer alternatives like heat or methanol are preferred.
• Acetic acid fixing: Acetic acid is not typically used for fixing bacterial smears. It can act as a fixative for some tissues but might distort bacterial morphology.
• Methanol fixing: Methanol fixation is another common method, but it’s generally preferred for BSL-2 organisms because it minimizes aerosol generation compared to heat fixing.

Answer: Umbonate

Umbonate describes bacterial colonies with a raised or bulging center, often resembling a small hill. This is a characteristic observed during colonial morphology examination, which helps in bacterial identification.

The other options are incorrect:

• Umbilicate: Umbilicate colonies have a central depression, like an “innie” belly button, opposite of umbonate.
• Flat: Flat colonies have a uniform, even surface with no elevation.
• Pinpoint: Pinpoint describes very small, circular colonies, often less than 1mm in diameter, and doesn’t refer to the elevation of the colony.