MicroBiology MCQs with Answer and Explnation | Chapter 22

Answer: Spread plate

The spread plate technique involves spreading a previously diluted sample evenly over the surface of an agar plate using a spreading tool, such as a hockey stick or spreader. This method allows for the isolation and enumeration of individual microbial colonies on the agar surface.

The other options are incorrect:

• Streak plate: The streak plate technique involves spreading a small amount of a concentrated microbial sample over the surface of an agar plate in a streak pattern. It is used for isolating individual colonies and achieving colony purification.
• Pour plate: In the pour plate technique, the microbial sample is mixed with melted agar and poured into a petri dish. Colonies grow both on the surface and within the agar, allowing for the enumeration of viable microorganisms.
• Stab tube: The stab tube technique involves inoculating a solid medium by inserting a needle or inoculating loop directly into the medium, typically in a tube. It is often used for cultivating anaerobic bacteria or for determining motility.

Answer: Streptococcus

Streptococcus is a genus of bacteria that is Gram-positive, spherical in shape, and tends to form chains. It is characterized by its chain-like arrangement of cells under a microscope.

The other options are incorrect:

• Staphylococcus: Staphylococcus is Gram-positive but is characterized by clusters of spherical cells rather than chains.
• Bacillus: Bacillus is a genus of bacteria that is Gram-positive and rod-shaped, not spherical or chain-forming.
• Diplococcus: Diplococcus refers to a genus of bacteria that typically occurs in pairs of cells, not chains. It is not specifically associated with Gram-positive bacteria.

Answer: Rabies virus

Negri bodies are characteristic inclusion bodies observed in the cytoplasm of nerve cells infected by the Rabies virus. They are considered a diagnostic feature of rabies infection.

The other options are incorrect:

• Cytomegalovirus: Cytomegalovirus does not produce Negri bodies. It belongs to the herpesvirus family and is associated with a different set of clinical manifestations.
• Herpes simplex virus: Herpes simplex virus also does not produce Negri bodies. It is a herpesvirus but is not associated with the characteristic inclusion bodies seen in rabies virus infection.
• Pox virus: Pox viruses, such as variola virus (causing smallpox), do not produce Negri bodies. They have distinct characteristics and clinical presentations different from rabies virus infection.

Answer: 121oC

The temperature of an autoclave at 15 psi (pounds per square inch) is typically around 121°C. This combination of pressure and temperature is commonly used for sterilization in laboratories and medical settings.

The other options are incorrect:

• 110°C: This temperature is lower than the standard autoclave temperature used for sterilization.
• 115°C: This temperature is also lower than the standard autoclave temperature. The standard temperature is 121°C.
• 125°C: While 125°C is a higher temperature, the standard temperature for autoclaving at 15 psi is 121°C. Using a higher temperature may not necessarily provide better sterilization and can have adverse effects on certain materials.

Answer: Broth

Broth is a culture medium with a liquid consistency. It is used to grow bacteria, fungi, or other microorganisms in liquid suspension.

The other options are incorrect:

• Agar: Agar is a solidifying agent used to solidify liquid media, such as agar plates or slants. It is not a liquid medium itself.
• Soft agar: Soft agar is a term used to describe agar with a lower concentration, making it softer than standard agar. It is still a solid medium, not a liquid one.

Answer: Mycobacterium

Mycobacterium is an example of an acid-fast bacterium. Acid-fast bacteria, including the species within the genus Mycobacterium, have a unique cell wall structure that makes them resistant to decolorization by acid-alcohol during staining procedures.

The other options are incorrect:

• Vibrio: Vibrio is a genus of bacteria, but it is not considered acid-fast. Vibrio species are typically Gram-negative and do not have the characteristic acid-fast staining property.
• Staphylococcus: Staphylococcus is a genus of bacteria that is Gram-positive but not acid-fast. Acid-fast bacteria have a specific waxy layer in their cell walls that differentiates them from non-acid-fast bacteria like Staphylococcus.
• Leptospira: Leptospira is a genus of spirochete bacteria and is not classified as acid-fast. Acid-fast bacteria are more commonly found in the genera Mycobacterium and Nocardia.

Answer: Staphyloxanthin

Staphyloxanthin is a pigment that gives a golden-yellow color to colonies of certain bacteria, particularly Staphylococcus aureus. It is a carotenoid pigment produced by the bacteria.

The other options are incorrect:

• Catalase: Catalase is an enzyme that catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is not responsible for the golden color of bacterial colonies.
• Coagulase: Coagulase is an enzyme produced by some bacteria, including Staphylococcus aureus, but it is not responsible for the coloration of colonies.
• Hemolysin: Hemolysin is a toxin that can cause the lysis of red blood cells, but it does not contribute to the golden coloration of bacterial colonies. Staphyloxanthin is the pigment associated with the characteristic color.

Answer: Oxygen

Streptolysin O is inactivated by oxygen. Streptolysin O is a toxin produced by some streptococcal bacteria, and exposure to oxygen causes its inactivation.

The other options are incorrect:

• CO2 (Carbon Dioxide): CO2 is not involved in the inactivation of Streptolysin O. Oxygen is the relevant factor for its inactivation.
• Nitrogen: Nitrogen is not responsible for the inactivation of Streptolysin O. Oxygen plays a key role in this process.
• Serum: While serum can neutralize some toxins, the inactivation of Streptolysin O specifically involves exposure to oxygen rather than serum.

Answer: ssRNA

The influenza virus genome is single-stranded RNA (ssRNA). It is segmented, meaning the genome is divided into eight separate RNA segments. Each segment encodes one or two proteins.

The other options are incorrect:

dsRNA: Double-stranded RNA (dsRNA) is not found in influenza viruses.
dsDNA: Double-stranded DNA (dsDNA) is the genetic material of most living organisms, but not viruses. Influenza viruses are RNA viruses.
ssDNA: Single-stranded DNA (ssDNA) is rare in nature and not found in influenza viruses.

Answer: Tat

Tat is not an accessory protein of HIV; it is a regulatory protein. Tat plays a crucial role in transcriptional activation of the viral genome.

The other options are incorrect:

• Vif: Vif (Viral infectivity factor) is an accessory protein of HIV that is involved in counteracting the host antiviral factor APOBEC3G.
• Vpu: Vpu (Viral protein U) is an accessory protein of HIV that plays a role in the release of virions from the host cell by downregulating CD4 receptors on the cell surface.
• Vpr: Vpr (Viral protein R) is an accessory protein of HIV that has various functions, including promoting nuclear import of the viral pre-integration complex and inducing cell cycle arrest in the G2 phase.

Answer: Whiff test

The whiff test involves adding a drop of potassium hydroxide (KOH) to a vaginal discharge sample. If trichomonosis is present, the KOH will cause the release of volatile amines from the parasite, resulting in a fishy odor. This is a quick and simple bedside test that can be helpful in diagnosing trichomoniasis.

The other options are incorrect:

• Demonstration of cysts by fecal examination: Trichomonas vaginalis does not form cysts and lives in the urogenital tract, not the intestines. Therefore, a fecal examination would not be useful for diagnosing trichomoniasis.
• Acidic vaginal pH: While a normal vaginal pH is slightly acidic, it can be affected by various factors and is not a reliable indicator of trichomoniasis.
• Demonstration of oocysts by fecal examination: Oocysts are the eggs of parasitic protozoans, and they are not related to trichomonads. Therefore, examining feces for oocysts would not be helpful in diagnosing trichomoniasis.

Answer: Endotoxin

All Gram-negative bacilli have endotoxin. Endotoxin is a component of the outer membrane of Gram-negative bacteria, specifically lipopolysaccharide (LPS). It is released when the bacteria die and their cell walls break down.

The other options are incorrect:

• Capsule: Not all Gram-negative bacilli have a capsule. Capsules are structures that may be present in some bacteria to provide protection or aid in adherence, but their presence varies among species.
• Exotoxin: Exotoxins are toxic proteins produced and released by bacteria. While some Gram-negative bacteria produce exotoxins, it is not a universal characteristic shared by all of them.
• DNase: DNase is an enzyme that breaks down DNA. The presence of DNase is not a universal characteristic of all Gram-negative bacilli; it varies among bacterial species.

Answer: Staphylococcus aureus

Staphylococcus aureus is a leading cause of food intoxication, also known as food poisoning. It produces a heat-stable enterotoxin that can survive cooking temperatures and cause symptoms like nausea, vomiting, diarrhea, and abdominal cramps within hours of ingesting contaminated food.

The other options are incorrect:

• Streptococcus pyogenes: While Streptococcus pyogenes can cause foodborne illness, it primarily causes infections like strep throat and impetigo. It is not a common cause of food intoxication.
• E. coli: Some strains of Escherichia coli (E. coli) can cause foodborne illness, but they typically cause symptoms like diarrhea and abdominal cramps, not intoxication.
• Salmonella: Salmonella is a major cause of foodborne illness, but it usually causes symptoms like fever, diarrhea, and abdominal cramps, not the rapid onset and specific symptoms associated with Staphylococcus aureus intoxication.

Answer: Magnification

The enhancement of size using ocular (eyepiece) and objective lenses in a microscope is called magnification. Magnification refers to the increase in apparent size of an object compared to its actual size.

The other options are incorrect:

Resolution: Resolution refers to the ability of a microscope to distinguish two distinct points as separate entities. It is related to the clarity or sharpness of the image but is not specifically about size enhancement.
Contrast: Contrast is the difference in color or brightness between different parts of an image. It is important for visualizing details but is not directly related to the size enhancement of an object.

Answer: Enrichment

An enrichment medium is designed to promote the growth of a particular organism by providing specific nutrients that encourage its growth while inhibiting the growth of other organisms. It contains substances that favor the targeted organism’s growth, allowing it to become more predominant in the culture.

The other options are incorrect:

• Selective: A selective medium contains components that inhibit the growth of certain organisms while allowing others to grow. It selects for the growth of specific types of microorganisms.
• Differential: A differential medium is designed to differentiate between different groups of microorganisms based on their biochemical characteristics. It often contains indicators or specific substrates that produce observable differences in the appearance of colonies.
• General purpose: A general-purpose or non-selective medium supports the growth of a wide range of microorganisms without favoring any particular type. It does not contain specific substances to selectively encourage the growth of a specific organism.

Answer: Nutrient agar

Nutrient agar is an example of a general-purpose medium. It contains a mix of nutrients that support the growth of a wide variety of microorganisms. It is commonly used for the cultivation and maintenance of non-fastidious bacteria and fungi.

The other options are incorrect:

• Selenite F broth: Selenite F broth is a selective enrichment medium used for the isolation of Salmonella and Shigella species from clinical and food samples. It is not a general-purpose medium.
• Mannitol salt agar: Mannitol salt agar is a selective and differential medium used for the isolation and identification of Staphylococcus aureus. It is not a general-purpose medium.
• MacConkey agar: MacConkey agar is a selective and differential medium used for the isolation of Gram-negative bacteria, particularly members of the Enterobacteriaceae family. It is not a general-purpose medium.

Answer: lactose

MacConkey agar is a differential medium specifically designed to differentiate between lactose-fermenting and non-lactose-fermenting bacteria. It contains lactose as a fermentable sugar, and bacteria that can ferment lactose will produce acid as a byproduct. This acid causes the pH of the medium to drop, which in turn triggers a color change. Lactose-fermenting bacteria will appear pink or red on MacConkey agar, while non-lactose-fermenting bacteria will remain colorless.

The other options are incorrect:

• Glucose: While some bacteria can also ferment glucose, MacConkey agar specifically uses lactose as the differentiating factor. This allows for easier identification of lactose-utilizing bacteria, which are important indicators of certain bacterial groups.
• Mannitol: Mannitol salt agar is another differential medium, but it specifically differentiates between bacteria that ferment mannitol and those that do not. It is not used to test for lactose fermentation.
• Sucrose: Similar to glucose, some bacteria can ferment sucrose, but MacConkey agar is not designed to differentiate based on this sugar. Lactose remains the primary indicator of fermentation used in this medium.

Answer: Tuberculosis

The BCG vaccine (Bacillus Calmette-Guérin) is used for the prevention of tuberculosis (TB). It is an attenuated strain of Mycobacterium bovis, a bacterium closely related to the Mycobacterium tuberculosis, which causes TB. BCG vaccination is commonly administered in many parts of the world, especially in areas with a high prevalence of tuberculosis.

The other options are incorrect:

• Botulism: The BCG vaccine is not used for the prevention of botulism. Botulism is caused by the toxin produced by Clostridium botulinum bacteria, and there is a specific antitoxin for botulism prevention.
• Cholera: BCG vaccine is not used for the prevention of cholera. Cholera vaccines are different and are specifically designed to protect against Vibrio cholerae, the bacterium that causes cholera.
• Anthrax: The BCG vaccine does not provide protection against anthrax. Anthrax is caused by the bacterium Bacillus anthracis, and specific vaccines are available for anthrax prevention.

Answer: Humans

The only known reservoir for Treponema pallidum, the bacterium that causes syphilis, is humans. This means that the bacterium can only survive and reproduce in the human body and cannot be transmitted to humans from other animals, the environment, or soil.

The other options are incorrect:

• Wild rodents: While some related Treponema species can cause disease in animals, Treponema pallidum is specific to humans and cannot infect rodents.
• Soil: Treponema pallidum cannot survive outside the human body for long periods and does not persist in soil or other environmental sources.
• Domestic Animals: Similar to wild rodents, domestic animals do not harbor Treponema pallidum and cannot transmit syphilis to humans.

Answer: Trophozoite

The trophozoite is the vegetative stage of all protozoan parasites. It is the active, feeding, and reproducing form of the parasite. Trophozoites are responsible for causing the symptoms of disease in the host.

The other options are incorrect:

• Cyst: Cysts are dormant stages of protozoan parasites that are resistant to environmental stress. They can survive outside the host for extended periods and are responsible for transmission of the parasite to new hosts. Not all protozoan parasites have a cyst stage.
• Sexual: The sexual stage of protozoan parasites involves the fusion of two gametes to produce a zygote. This stage is not present in all protozoan parasites and is not essential for their survival.
• Latent: The latent stage of protozoan parasites is a period of dormancy within the host. During this stage, the parasite does not cause any symptoms. Not all protozoan parasites have a latent stage.

Answer: It has only trophozoite stage.

Giardia lamblia has two stages in its life cycle: trophozoite and cyst. The trophozoite is the active, motile form that inhabits the small intestine and is responsible for causing symptoms. The cyst is the dormant and infectious form, allowing the protozoan to survive outside the host and facilitating transmission.

The other options are incorrect:

• It is a heart shape protozoan: Giardia lamblia does not have a heart shape. It is characterized by an asymmetrical, pear-shaped trophozoite and a round or oval cyst.
• It is transmitted by the fecal-oral route: This statement is correct. Giardia lamblia is commonly transmitted through the ingestion of contaminated water or food, often via the fecal-oral route.
• It can be diagnosed by the string test: This statement is correct. The string test is a method used to collect duodenal fluid for the diagnosis of giardiasis, allowing the detection of trophozoites or cysts.

Answer: Exotoxin

Cholera toxin is an exotoxin. Exotoxins are toxic proteins produced and released by certain bacteria into their environment. Cholera toxin is produced by Vibrio cholerae, the bacterium responsible for cholera, and it plays a key role in the pathogenesis of the disease.

The other options are incorrect:

• Endotoxin: Cholera toxin is not an endotoxin. Endotoxins are lipopolysaccharides found in the outer membrane of Gram-negative bacteria and are released when the bacteria are lysed.
• Neurotoxin: Cholera toxin is not a neurotoxin. Neurotoxins target the nervous system and affect nerve cells, whereas cholera toxin primarily affects the cells lining the small intestine.
• Hemolytic toxin: Cholera toxin is not a hemolytic toxin. Hemolytic toxins cause the destruction of red blood cells, while cholera toxin primarily affects the regulation of ion transport in the intestine.

Answer: Lysogenic cycle

The lysogenic cycle is a viral life cycle where the viral DNA integrates into the host cell’s chromosome and remains dormant. This integrated viral DNA is called a provirus. The provirus replicates along with the host cell’s DNA and is passed on to daughter cells. No progeny virions are produced during this stage.

The other options are incorrect:

• Lytic cycle: In the lytic cycle, the viral DNA replicates inside the host cell, leading to the production of numerous progeny virions. The host cell eventually bursts, releasing the virions to infect new cells.
• Replicative cycle: While “replicative” is a general term that could apply to both lytic and lysogenic cycles, it doesn’t specifically distinguish the integration into host chromosomes.
• Translational cycle: This term doesn’t accurately describe any viral lifecycle. Translation refers to the process of converting mRNA into protein, not the stages of viral replication.

Answer: T-lymphocytes

Cell-mediated immune response is mediated by T-lymphocytes (T cells). These cells play a central role in the immune system by recognizing and directly interacting with infected or abnormal cells. T cells are crucial for defense against intracellular pathogens, including viruses and certain bacteria, and are involved in various immune responses, such as cytotoxic T cell activity and helper T cell functions.

The other options are incorrect:

• B-lymphocytes: B-lymphocytes (B cells) are primarily involved in humoral immune responses, including the production of antibodies. They are not the main mediators of cell-mediated immune responses.
• Endothelial cells: Endothelial cells line the inner surface of blood vessels and play a role in vascular function but are not the primary mediators of cell-mediated immune responses.
• Granulocytes: Granulocytes, such as neutrophils, eosinophils, and basophils, are involved in innate immune responses and inflammation. They are not the main mediators of the cell-mediated immune response.

Answer: About 10-days

Passive immunity, whether acquired through the transfer of antibodies from mother to infant, administration of immune globulins, or other means, is temporary. The duration of passive immunity typically lasts for a relatively short period, approximately 10 days. This is because the transferred antibodies gradually decline in the recipient’s circulation over time.

The other options are incorrect:

• 6-9 months: This duration is more commonly associated with the passive immunity provided by maternal antibodies transferred to the infant during pregnancy and through breastfeeding.
• 9-10 years: This time frame is not characteristic of passive immunity. Passive immunity, in most cases, is of shorter duration, lasting for weeks to a few months.

Answer: Bacillus anthracis

Bacillus anthracis is an aerobic endospore-forming bacterium, known for causing the disease anthrax. It is well-studied for its ability to form spores that are highly resistant to harsh environmental conditions.

The other options are incorrect:

• Clostridium botulinum: While Clostridium botulinum is also an endospore-forming bacterium, it is an obligate anaerobe, meaning it cannot survive in the presence of oxygen.
• Bacillus and Clostridium: This option is too broad. While both Bacillus and Clostridium include endospore-forming bacteria, not all members of these genera are aerobic.
• Staphylococcus aureus: Staphylococcus aureus is not an endospore-forming bacterium. It is a Gram-positive coccus that can be aerobic or facultatively anaerobic.

Answer: Preservation

Sporulation is a process of preservation by certain bacteria, especially those in the genera Bacillus and Clostridium. During unfavorable conditions, these bacteria form resistant structures called endospores, allowing them to survive harsh environments. Sporulation is a strategy for preserving genetic material and essential cellular components, ensuring the bacterium’s survival until more favorable conditions arise.

The other options are incorrect:

• Reproduction: While sporulation involves the formation of spores, it is not a reproductive process in the sense of producing offspring. Sporulation is a survival mechanism, not a means of reproduction.
• Replication: Sporulation is not a process of replication. Replication typically refers to the process of copying DNA during cell division, whereas sporulation involves the formation of a specialized and resistant structure (the endospore).

Answer: Spore coat

The spore coat is the outermost layer of the spore envelope and is the thickest and most complex layer. It is composed of various proteins and carbohydrates that provide protection against harsh environmental conditions such as heat, radiation, and chemicals. It also plays a role in spore adhesion and germination.

The other options are incorrect:

• Core: The core is the innermost layer of the spore envelope and contains the bacterium’s cytoplasm, DNA, and ribosomes. It is surrounded by the spore coat, cortex, and spore wall.
• Cortex: The cortex is the layer between the spore coat and the inner spore wall. It is composed of peptidoglycan and plays a role in spore resistance and germination.
• Spore wall: The spore wall is the second layer of the spore envelope and is composed of peptidoglycan. It provides structural support and helps to maintain the spore’s shape.

Answer: Oil immersion lens

A 100x objective lens is also known as an oil immersion lens because it requires the use of immersion oil between the lens and the specimen to achieve optimal resolution and clarity. The oil immersion technique increases the refractive index, allowing for more light to be focused onto the specimen and reducing spherical aberration, which can distort the image.

The other options are incorrect:

• Dry lens: Dry lenses are objectives used without any immersion medium. They are typically used with lower magnifications (e.g., 4x, 10x) where resolution and clarity are less critical.
• High dry lens: This term is sometimes used interchangeably with dry lens, but it specifically refers to dry objectives with high magnification (e.g., 40x).
• Scanning lens: Scanning lenses are typically low magnification objectives (e.g., 4x) used for overview scanning of specimens. They are not used for high-resolution visualization.

Answer: Bacilli

Rod-shaped bacteria are known as bacilli. These bacteria have a cylindrical or rod-like shape and may vary in size. The term “bacillus” is commonly used to describe individual rod-shaped bacterial cells, and “bacilli” is the plural form.

The other options are incorrect:

• Cocci: Cocci are spherical or round-shaped bacteria. This term is used to describe individual spherical bacterial cells, and “cocci” is the plural form.
• Vibrio: Vibrio bacteria have a curved or comma-shaped morphology. They are characterized by a distinctive curved rod shape and are not classified as straight rod-shaped bacteria.
• Coco-bacilli: “Coco-bacilli” is not a standard term in microbiology. It seems to combine the terms “cocci” and “bacilli,” but it is not used to describe a specific group of bacteria.

Answer: Gram’s iodine

Gram’s iodine is the mordant used in Gram’s staining. It plays a crucial role in the staining process by forming a complex with crystal violet, which helps trap the dye within Gram-positive bacteria. This complex makes it difficult for the decolorization step to remove the crystal violet, resulting in the characteristic purple color of Gram-positive cells.

The other options are incorrect:

• Acid alcohol: Acid alcohol is a decolorizing agent used in Gram’s staining. It removes crystal violet from Gram-negative bacteria, causing them to appear pink when counterstained with safranin.
• Safranin: Safranin is a counterstain used in Gram’s staining. It stains all decolorized cells (Gram-negative) pink, providing contrast to Gram-positive cells which remain purple.
• Crystal violet: Crystal violet is the primary stain used in Gram’s staining. It binds to the cell wall of both Gram-positive and Gram-negative bacteria. However, it alone is not sufficient to differentiate the two types.

Answer: Hot air oven

A hot air oven is a sterilizing instrument that uses dry heat to kill microorganisms. It is effective for sterilizing metals, glassware, powders, oils, and waxes because these materials can withstand high temperatures without damage.

The other options are incorrect:

• Autoclave: An autoclave uses steam under pressure to sterilize objects. It is not suitable for sterilizing oils and waxes, as they can melt or vaporize at high temperatures.
• Water bath: A water bath uses boiling water to sterilize objects. It is not as effective as a hot air oven for killing bacteria and other microorganisms, especially heat-resistant spores.
• Seitz filter: A Seitz filter is used to remove bacteria and other particles from liquids. It is not a sterilization method and cannot be used to sterilize solids.

Answer: Both “Deep agar tube” and “Anaerobic jar”

Bacteria that can only be grown in the absence of oxygen, known as obligate anaerobes, are commonly cultivated using both deep agar tubes and anaerobic jars. Deep agar tubes are filled with a solid medium that becomes anaerobic in the depths due to limited oxygen penetration. Anaerobic jars create an anaerobic environment by removing oxygen and replacing it with a mixture of gases like nitrogen and carbon dioxide.

The other options are incorrect:

• Deep agar tube: This is the correct answer. Deep agar tubes provide an anaerobic environment in the depths of the medium, allowing the cultivation of obligate anaerobes.
• Anaerobic jar: This is the correct answer. Anaerobic jars create an oxygen-free environment, facilitating the growth of obligate anaerobes.
• Broth culture: Broth cultures can be used for growing bacteria, but they do not inherently provide an anaerobic environment. The presence of oxygen in broth cultures may not be suitable for obligate anaerobes unless additional measures are taken to create anaerobic conditions.

Answer: colony

A colony is a visible mass of bacteria on a solid medium. It arises from the growth and division of a single bacterial cell, creating a cluster of genetically identical cells. Colonies can vary in size, shape, color, and other characteristics depending on the bacterial species and the growth conditions.

The other options are incorrect:

• Turbidity: Turbidity refers to the cloudiness or haziness of a liquid culture due to the presence of suspended bacteria. It is not a specific term for visible growth on solid media.
• Sediment: Sediment refers to the material that settles at the bottom of a liquid culture. While some bacterial growth might contribute to sediment formation, it is not a specific term for visible growth on solid media.

Answer: Herd immunity

Herd immunity refers to the indirect protection individuals receive from a population where a high proportion of individuals are immune to a particular disease. When a large number of individuals are immune, it becomes difficult for the disease to spread, even among those who are not immune. This is because the chain of transmission is broken, with fewer opportunities for the pathogen to find susceptible hosts.

The other options are incorrect:

• Innate immunity: Innate immunity is the body’s first line of defense against pathogens. It provides nonspecific protection against a wide range of threats but does not contribute directly to herd immunity.
• Cell-mediated immunity: Cell-mediated immunity is a type of adaptive immunity that involves T lymphocytes and other immune cells directly attacking infected cells. While it plays a crucial role in fighting off infections, it does not directly contribute to herd immunity.
• Autoimmunity: Autoimmunity is a condition where the immune system attacks the body’s own tissues. It is not relevant to the concept of herd immunity, which focuses on the protection provided by a population’s collective immunity against a specific pathogen.

Answer: Filtration

Sugar solutions are commonly sterilized by filtration. Filtration is a method of sterilization that involves passing a liquid through a filter with small pores to physically remove microorganisms. This is particularly useful for heat-sensitive substances, such as some solutions containing sugars, which may be adversely affected by autoclaving or hot air oven sterilization.

The other options are incorrect:

• Autoclave: Autoclaving involves the use of high-pressure steam to achieve sterilization. While effective for many materials, it may not be suitable for heat-sensitive substances like sugar solutions.
• Hot air oven: Hot air ovens use dry heat for sterilization. Like autoclaving, this method may not be suitable for heat-sensitive solutions, as it can lead to caramelization or other undesirable changes.
• X-rays: X-rays are a form of ionizing radiation and are not commonly used for the sterilization of solutions. Filtration is a more practical method for liquids like sugar solutions.

Answer: Pure culture

A pure culture is a culture containing only one type of microorganism. This means that all the cells in the culture are genetically identical descendants of a single original cell. Pure cultures are essential for many microbiological studies, experiments, and applications.

The other options are incorrect:

• Colony culture: A colony culture refers to a visible mass of bacteria on a solid medium, but it doesn’t necessarily mean it only contains one type of microorganism. It could be a mixed culture containing different bacterial species.
• Liquid culture: A liquid culture simply refers to a culture of microorganisms grown in a liquid nutrient medium. It doesn’t specify whether the culture contains one type of microorganism or a mixture of different types.
• Mixed culture: A mixed culture, as the name suggests, contains two or more different types of microorganisms. This is the opposite of a pure culture.

Answer: IgG

IgG (Immunoglobulin G) is the major immunoglobulin present in human serum. It is the most abundant antibody in the blood and provides naturally acquired immunity to newborns through the placenta. IgG plays a crucial role in immune responses against bacteria, viruses, and toxins. It can also pass from the mother to the fetus during pregnancy, providing passive immunity to the newborn.

The other options are incorrect:

• IgA: IgA is found in bodily fluids such as saliva, tears, and breast milk. While it plays a role in mucosal immunity, it is not the major immunoglobulin in human serum.
• IgE: IgE is involved in allergic responses and is found in very low concentrations in the blood. It is not the major immunoglobulin in human serum.
• IgM: IgM is the first antibody produced during an immune response. It is present in the blood and can also provide naturally acquired immunity, but IgG is more significant in this regard.

Answer: Autoimmunity

Failure to eliminate self-reactive cells can lead to autoimmunity. Negative selection, which occurs during T-cell development in the thymus, is a process that eliminates self-reactive T cells to prevent autoimmunity. If this negative selection process is not effective, self-reactive T cells or B cells may escape into the peripheral circulation, leading to an immune response against the body’s own tissues and causing autoimmune diseases.

The other options are incorrect:

• Negative selection: While negative selection is a process that eliminates self-reactive cells to prevent autoimmunity, the failure to eliminate self-reactive cells is associated with autoimmunity.
• Positive selection: Positive selection is a process during T-cell development that ensures T cells can recognize self-MHC molecules. It is not directly related to the failure to eliminate self-reactive cells.
• Tolerance: Tolerance refers to the immune system’s ability to tolerate self-antigens and avoid attacking the body’s own tissues. The failure to establish tolerance can contribute to autoimmunity, but the question is specifically about the consequence of failure to eliminate self-reactive cells.

Answer: Infection

Active immunity is induced by exposure to a pathogen, which triggers the immune system to produce its own antibodies. This can occur through natural infection with the pathogen, or through vaccination, which introduces a weakened or inactive form of the pathogen to stimulate the immune response.

The other options are incorrect:

• Injection of γ-globulins: This is a form of passive immunity, where pre-made antibodies are injected into the body to provide immediate protection against a specific disease.
• Placental transfer of Abs: Antibodies from the mother can cross the placenta and provide some passive immunity to the newborn, but this is not considered active immunity.
• Injection of antibodies: Similar to injecting γ-globulins, this is a form of passive immunity and does not stimulate the body’s own immune response.

Answer: Thymus

T-cells, or T lymphocytes, are produced from the thymus. The thymus is a primary lymphoid organ located in the chest, and it plays a crucial role in the maturation and development of T-cells. T-cells undergo a process of maturation and education within the thymus, where they learn to distinguish self from non-self antigens.

The other options are incorrect:

• Bone marrow: While the bone marrow is a primary lymphoid organ responsible for the production of B-cells (B lymphocytes) and the maturation of some immune cells, it is not the site of T-cell production.
• Spleen: The spleen is a secondary lymphoid organ involved in filtering blood and removing damaged blood cells. It does not play a direct role in the production of T-cells.
• Thyroid gland: The thyroid gland is not involved in the production of T-cells. It is primarily responsible for the production and regulation of thyroid hormones.

Answer: Plasma Cells

Antibodies, also known as immunoglobulins (Ig), are produced by plasma cells. Plasma cells are specialized white blood cells derived from B lymphocytes (B cells). When B cells encounter an antigen, they differentiate into plasma cells, which then produce and release antibodies. Antibodies play a crucial role in the immune system by recognizing and binding to specific antigens, such as those on pathogens, leading to their neutralization or elimination.

The other options are incorrect:

• T lymphocytes (T cells): T lymphocytes are involved in cell-mediated immunity and do not directly produce antibodies. They recognize antigens presented on the surface of infected or abnormal cells and coordinate immune responses.
• NK cells (Natural Killer cells): NK cells are part of the innate immune system and are responsible for recognizing and destroying infected or abnormal cells. They are not involved in the direct production of antibodies.
• Eosinophils: Eosinophils are a type of white blood cell involved in the immune response against parasitic infections and allergic reactions. They do not produce antibodies.

Answer: B cells and T cells

Immunological memory, which allows the immune system to “remember” previous encounters with specific pathogens, is provided by both B lymphocytes (B cells) and T lymphocytes (T cells). Memory B cells are formed during the immune response to antigens, and they can produce a rapid and robust antibody response upon re-exposure to the same antigen. Memory T cells, similarly, can recognize and mount a quicker response to previously encountered antigens.

The other options are incorrect:

• B lymphocytes: This is the correct answer. B cells, specifically memory B cells, contribute to immunological memory by producing antibodies upon re-exposure to a familiar antigen.
• T lymphocytes: This is the correct answer. Memory T cells contribute to immunological memory by recognizing and responding more rapidly to previously encountered antigens.
• Phagocytes: Phagocytes, such as macrophages and neutrophils, are involved in the innate immune response and phagocytosis of pathogens. However, they do not provide immunological memory. Memory is a specific feature of the adaptive immune response involving B and T cells.

Answer: Reticuloendothelial system

In the human body, Brucella bacteria primarily reside in the reticuloendothelial system, which includes organs like the spleen and liver. Brucella species are intracellular bacteria that have an affinity for cells of the reticuloendothelial system. They can persist within macrophages, allowing them to evade the host’s immune response and establish chronic infections.

The other options are incorrect:

• Respiratory tract: Brucella is not typically associated with the respiratory tract. Its primary target is the reticuloendothelial system.
• Genital tract: While Brucella can be excreted in genital secretions, its primary site of infection and persistence is in the reticuloendothelial system.
• Gastrointestinal tract: Brucella is not primarily associated with the gastrointestinal tract. Its preferred site is within phagocytic cells of the reticuloendothelial system.

Answer: diversity

The light chain of an antibody does not originate from the diversity (D) region. During the development of B cells, the genes encoding the antibody heavy and light chains undergo rearrangement, involving the variable (V), diversity (D), and joining (J) gene segments. However, in the case of light chains, the diversity (D) region is not present. The rearrangement involves the V and J gene segments, contributing to the variability of the antibody’s antigen-binding site.

The other options are incorrect:

• Variable: The variable (V) region contributes to the diversity of the antigen-binding site in both heavy and light chains.
• Joining: The joining (J) region is involved in the rearrangement of both heavy and light chains, contributing to the diversity of the antibody repertoire.
• Constant: The constant (C) region is not involved in the gene rearrangement process. It determines the antibody’s class (e.g., IgM, IgG, IgA) after successful rearrangement of the V, D, and J segments.

Answer: Mumps vaccine

The mumps vaccine is a vaccine used against viral infection. It is designed to provide immunity against the mumps virus, which can cause symptoms such as swelling of the salivary glands, fever, and headache. The mumps vaccine is typically administered as part of the measles, mumps, and rubella (MMR) vaccine, which helps protect against three different viral infections.

The other options are incorrect:

• Cholera vaccine: The cholera vaccine is used to prevent infection with Vibrio cholerae, the bacterium that causes cholera. It is not a vaccine against a viral infection.
• Sub-cellular vaccine: “Sub-cellular vaccine” is a broad term that may refer to vaccines containing components of a pathogen, such as viral proteins or subunits. It does not specifically indicate a vaccine against a viral infection.
• Typhoid vaccine: The typhoid vaccine is used to prevent infection with the bacterium Salmonella typhi, which causes typhoid fever. It is not a vaccine against a viral infection.

Answer: Eosinophils

Eosinophils are immune cells that play a crucial role in the immune response against helminth worms. They are equipped with specific granules containing various toxic proteins and enzymes that can damage and kill the worms. Additionally, eosinophils can release inflammatory mediators that attract other immune cells to the site of infection and contribute to the expulsion of the worms.

The other options are incorrect:

• Lymphocytes: While lymphocytes are essential for the immune system, they primarily target viruses and bacteria, not helminth worms.
• Basophils: Basophils are involved in allergic reactions and inflammatory responses but have limited activity against helminth worms.
• Neutrophils: Neutrophils are the first line of defense against bacterial infections but are not effective against helminth worms.

Answer: Epitope

Monoclonal antibodies recognize a single epitope. An epitope, also known as an antigenic determinant, is a specific region on an antigen to which an antibody binds. Monoclonal antibodies are produced by identical immune cells that are clones of a single parent cell. These antibodies are designed to target and bind specifically to a particular epitope on an antigen.

The other options are incorrect:

• Antigen: While antibodies recognize antigens, monoclonal antibodies specifically bind to a single epitope on an antigen, not the entire antigen.
• Bacterium: Monoclonal antibodies can be designed to recognize specific epitopes on the surface of bacteria, but they do not recognize the entire bacterium.
• B cell: Monoclonal antibodies are produced outside of B cells. B cells are responsible for producing a diverse array of antibodies, each recognizing a different epitope. Monoclonal antibodies, on the other hand, are identical antibodies targeting a single epitope.

Answer: Cytotoxic T cells

MHC class II molecules are primarily found on antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells. These cells present processed antigens to CD4+ T helper cells, which play a central role in the immune response.

The other options are incorrect:

• Ig producing plasma cells: Plasma cells are differentiated B cells that produce large amounts of antibodies. While B cells do have MHC class II molecules, plasma cells lose them during differentiation.
• Dendritic cells: Dendritic cells are professional APCs and have high levels of MHC class II molecules on their surface.
• Macrophages: Macrophages are another type of APC and also express MHC class II molecules.

Answer: Epitope

Small chemical groups on the antigen molecule that can react with antibodies are called epitopes. Also known as antigenic determinants, epitopes are specific regions of an antigen that are recognized by and bind to the variable regions of antibodies. The interaction between epitopes and antibodies is crucial for the immune system’s ability to recognize and mount a response against foreign substances.

The other options are incorrect:

• Paratope: The paratope is the specific binding site on an antibody that interacts with the epitope of an antigen. It is the complementary region to the epitope.
• Isotope: An isotope refers to variants of a chemical element with the same number of protons but a different number of neutrons. It is not related to antigen-antibody interactions.
• Allotope: “Allotope” is not a recognized term in immunology. The correct term for the small chemical groups on the antigen that interact with antibodies is epitope.