General Microbiology Practice Questions

Q: Which of the following infectious agents lacks RNA?

Prions. ### Explanation **Correct Answer: C. Prions** **1. Why Prions are the Correct Answer:** Prions (Proteinaceous Infectious Particles) are unique among infectious agents because they consist **entirely of protein** and lack any form of nucleic acid (neither DNA nor RNA). They are misfolded isoforms of a normal cellular protein ($PrP^C$) known as $PrP^{Sc}$. Because they lack a genome, they do not follow the central dogma of biology; instead, they replicate by inducing normal proteins to refold into the infectious, beta-sheet-rich conformation. **2. Why Other Options are Incorrect:** * **A. Virus:** Viruses are obligate intracellular parasites that contain a nucleic acid core. While some contain DNA, many clinically significant viruses (e.g., HIV, Influenza, SARS-CoV-2) use **RNA** as their primary genetic material. * **B. Staphylococci:** As prokaryotic bacteria, *Staphylococci* contain both **DNA** (genomic and plasmid) and **RNA** (mRNA, tRNA, and rRNA) required for protein synthesis and cellular metabolism. * **C. Cryptococcus:** This is an encapsulated fungus (eukaryote). Like all fungi, it possesses a nucleus containing **DNA** and utilizes various types of **RNA** for cellular functions. **3. NEET-PG High-Yield Pearls:** * **Resistance:** Prions are highly resistant to standard sterilization methods, including boiling, radiation, and formalin. They require **autoclaving at 134°C for 1-2 hours** or treatment with **1N NaOH**. * **Pathology:** They cause **Transmissible Spongiform Encephalopathies (TSEs)**, characterized by neuronal loss, astrocytosis, and a "spongiform" appearance of the brain without an inflammatory response. * **Key Diseases:** Creutzfeldt-Jakob Disease (CJD) in humans, Kuru (associated with cannibalism), and Bovine Spongiform Encephalopathy (Mad Cow Disease). * **Diagnosis:** Look for the **14-3-3 protein** in CSF as a biochemical marker for CJD.

Q: Which of the following bacteria does not produce spores?

Pseudomonas. **Explanation:** The ability to form spores (sporulation) is a survival mechanism used by certain bacteria to withstand adverse environmental conditions like heat, desiccation, and disinfectants. In medical microbiology, the most clinically significant spore-forming bacteria belong to two specific Gram-positive genera: **Bacillus** (aerobic) and **Clostridium** (anaerobic). * **Pseudomonas (Correct Answer):** *Pseudomonas aeruginosa* is a Gram-negative, non-spore-forming motile rod. It relies on other virulence factors, such as biofilms and exotoxins, rather than spores for survival and pathogenicity. * **Bacillus anthracis & Bacillus subtilis (Incorrect):** Both belong to the genus *Bacillus*. *B. anthracis* (the causative agent of Anthrax) produces central or subterminal spores that do not swell the mother cell. *B. subtilis* is a common laboratory contaminant known for its highly resistant spores. * **Clostridium (Incorrect):** This genus includes pathogens like *C. tetani* and *C. botulinum*. These are obligate anaerobes that produce spores, which are often used as a key morphological feature for identification (e.g., "drumstick" appearance of *C. tetani*). **High-Yield Clinical Pearls for NEET-PG:** 1. **Composition:** Bacterial spores contain a high concentration of **Calcium dipicolinate**, which provides heat resistance. 2. **Sterilization:** Spores are the standard for testing sterilization. *Geobacillus stearothermophilus* spores are used to check the efficacy of **Autoclaves**. 3. **Staining:** Spores do not take up routine Gram stains (appearing as clear areas) and require special stains like the **Modified Ziehl-Neelsen** or **Schaeffer-Fulton** stain. 4. **Non-sporing pathogens:** Most Gram-negative bacteria (like *Pseudomonas*, *E. coli*) and many Gram-positive bacteria (like *Staphylococcus*, *Streptococcus*) do not form spores.

Q: Which of the following is a normal vaginal anaerobe?

Lactobacillus. **Explanation:** The correct answer is **Lactobacillus** (specifically Döderlein’s bacilli). **1. Why Lactobacillus is correct:** Lactobacillus is the predominant commensal of the healthy adult vagina. While often discussed as a Gram-positive rod, it is physiologically an **aerotolerant anaerobe**. It plays a critical protective role by fermenting glycogen (stored in vaginal epithelial cells under estrogen influence) into **lactic acid**. This maintains an acidic vaginal pH (3.8–4.5), which inhibits the overgrowth of pathogenic bacteria. **2. Why the other options are incorrect:** * **Corynebacterium:** These are Gram-positive "diphtheroids" that can be part of the normal skin and mucosal flora, but they are generally **aerobic or facultative anaerobic**, not the primary anaerobic commensal of the vagina. * **Gardnerella vaginalis:** While often present in low numbers in some healthy women, it is a facultative anaerobe primarily associated with **Bacterial Vaginosis (BV)** when it overgrows due to a decrease in Lactobacillus. * **E. coli:** This is a Gram-negative facultative anaerobe. While it can colonize the vagina (often leading to UTIs), it is considered transient colonic flora rather than a primary normal vaginal anaerobe. **High-Yield Clinical Pearls for NEET-PG:** * **Vaginal pH:** A pH >4.5 is a diagnostic criterion for Bacterial Vaginosis and Trichomoniasis, indicating a loss of Lactobacillus dominance. * **Clue Cells:** Stippled epithelial cells seen on saline microscopy, pathognomonic for *Gardnerella vaginalis* infection. * **Nugent Scoring:** The gold standard for diagnosing Bacterial Vaginosis based on the microscopic morphotypes of Lactobacilli (large GP rods) vs. Gardnerella/Mobiluncus. * **Estrogen Link:** The presence of Lactobacillus is estrogen-dependent; therefore, the vaginal pH is higher (more basic) in pre-pubertal and post-menopausal females.

Q: What is true about endotoxin?

No enzymatic activity. **Explanation:** Endotoxins are structural components of the cell wall of Gram-negative bacteria, specifically the **Lipid A** portion of the Lipopolysaccharide (LPS) layer. **Why Option C is correct:** Unlike exotoxins, which are often enzymes (e.g., phospholipases or proteases) that actively catalyze biochemical reactions, endotoxins possess **no enzymatic activity**. They act as potent **PAMPs (Pathogen-Associated Molecular Patterns)** that trigger a systemic inflammatory response by binding to TLR-4 receptors on macrophages, leading to the release of cytokines like IL-1, IL-6, and TNF-α. **Analysis of Incorrect Options:** * **A. Protein:** Endotoxins are **Lipopolysaccharides** (LPS). Exotoxins are proteins. * **B. Highly antigenic:** Endotoxins are **poorly antigenic**. They do not induce the formation of high-titer antibodies (antitoxins) and cannot be converted into toxoids for vaccines. * **D. Produced by Gram-positive bacteria:** Endotoxins are almost exclusively produced by **Gram-negative bacteria** (integral part of the outer membrane). A notable exception is *Listeria monocytogenes* (Gram-positive), which possesses endotoxin-like activity. **High-Yield Clinical Pearls for NEET-PG:** * **Heat Stability:** Endotoxins are heat-stable (withstand 100°C for 1 hour), whereas exotoxins are heat-labile. * **Toxicity:** Endotoxins have low potency (required in large amounts to cause shock), while exotoxins are highly potent (lethal in microgram doses). * **Detection:** The **Limulus Amebocyte Lysate (LAL) test** is the gold standard for detecting endotoxins in parenteral fluids. * **Clinical Manifestation:** They are the primary mediators of **Septic Shock** (fever, hypotension, and DIC).

Q: In a bacterial growth curve, in which phase is the maximum cell size typically obtained?

End of lag phase. **Explanation:** The bacterial growth curve consists of four distinct phases: Lag, Log (Exponential), Stationary, and Decline. **Why the "End of Lag Phase" is correct:** The **Lag phase** is a period of intense metabolic activity but no cell division. During this phase, bacteria are adapting to their new environment. They synthesize enzymes, proteins, and RNA, and increase their metabolic rate. Consequently, the cells increase significantly in physical volume and mass. Since cell division has not yet started to "split" the cell mass, the **maximum cell size** is reached at the very end of the lag phase, just before the first division occurs. **Analysis of Incorrect Options:** * **A. Beginning of Lag phase:** At this point, the bacteria have just been inoculated and have not yet begun the metabolic buildup required for size increase. * **C. Beginning of stationary phase:** By this stage, nutrients are depleted and toxic metabolic byproducts have accumulated. Cell size typically decreases as the bacteria enter a survival mode. * **D. Log phase:** During this phase, cells are dividing at their maximum mathematical rate (generation time). Because they are constantly dividing, the average cell size is smaller than at the end of the lag phase. **High-Yield NEET-PG Pearls:** * **Lag Phase:** Maximum cell size; no increase in number; high metabolic activity. * **Log Phase:** Maximum growth rate; cells are most sensitive to **Beta-lactam antibiotics** (e.g., Penicillin) because they act on the cell wall during active synthesis. * **Stationary Phase:** Growth rate equals death rate; **sporulation** (spore formation) typically begins here; secondary metabolites like **exotoxins and antibiotics** are produced. * **Morphology:** Bacteria show the most "typical" morphological characteristics during the Log phase.

Q: The bacterial cell wall has all of the following properties except:

It is a rigid structure.. **Explanation:** The bacterial cell wall is a complex, semi-rigid structure that provides shape and protection to the cell. However, in the context of this question, the statement "It is a rigid structure" is considered the "except" because, while it provides structural integrity, it is **not an absolute or immutable property** across all bacteria (e.g., *Mycoplasma* lack a cell wall entirely, and L-forms lose it). More importantly, in competitive exams like NEET-PG, this question often tests the nuance that the cell wall is **porous and permeable**, rather than a solid, impermeable barrier. **Analysis of Options:** * **Option A (Incorrect):** Peptidoglycan (murein) is the backbone of the bacterial cell wall in both Gram-positive and Gram-negative bacteria. * **Option B (Incorrect):** The cell wall thickness and cross-linking determine the Gram stain. Gram-positive walls have thick peptidoglycan that retains the Crystal Violet-Iodine complex, while Gram-negative walls have thin peptidoglycan and an outer membrane that allows decolorization. * **Option D (Incorrect):** Bacterial cell walls uniquely contain **D-isomers** of amino acids (like D-alanine and D-glutamic acid). This is a high-yield fact as most biological proteins consist exclusively of L-amino acids; these D-amino acids protect the wall from degradation by most host proteases. **High-Yield Clinical Pearls for NEET-PG:** * **Mycoplasma:** The only naturally occurring bacteria that lack a cell wall (contain sterols in the membrane instead); hence, they are inherently resistant to Beta-lactams. * **Lysozyme:** An enzyme found in tears/saliva that cleaves the β-1,4 glycosidic bond between NAM and NAG in the peptidoglycan. * **Protoplasts vs. Spheroplasts:** Protoplasts are derived from Gram-positives (wall entirely removed), while Spheroplasts are from Gram-negatives (wall partially removed).

Q: Which of the following organisms does not have a non-human reservoir?

Salmonella typhi. ### Explanation The core concept tested here is the **reservoir of infection**. A reservoir is the natural habitat (human, animal, or environmental) where an infectious agent lives and multiplies. **1. Why Salmonella typhi is correct:** *Salmonella typhi* and *Salmonella paratyphi* are **obligate human pathogens**. They have no known animal or environmental reservoir. Humans are the only hosts, and transmission occurs via the feco-oral route through contaminated food or water. This unique characteristic makes typhoid fever a candidate for potential eradication, unlike zoonotic diseases. **2. Why the other options are incorrect:** * **Yersinia pestis:** This is the classic example of a **zoonotic** pathogen. Its primary reservoirs are wild rodents (like rats, squirrels, and prairie dogs), and it is transmitted to humans via flea bites. * **Escherichia coli:** While many strains are part of the human normal flora, *E. coli* (especially STEC/EHEC) has significant reservoirs in **cattle** and other ruminants. It can also survive in environmental water sources. * **Clostridium tetani:** This organism has an **environmental reservoir**. It exists as highly resilient spores in **soil** and the intestinal tracts of grazing animals (like horses and cattle), which further contaminate the soil through feces. **Clinical Pearls for NEET-PG:** * **Chronic Carriers:** In *S. typhi*, the **gallbladder** is the most common site of long-term colonization (e.g., the famous case of "Typhoid Mary"). * **Zoonosis:** Diseases with animal reservoirs (like Plague, Rabies, or Brucellosis) are significantly harder to eradicate than human-only diseases (like Smallpox or Polio). * **Vi Antigen:** This capsular polysaccharide is a key virulence factor for *S. typhi* and is used in subunit vaccines.

Q: Limulus amoebocyte lysate assay is used for the detection of which of the following?

Endotoxin. **Explanation:** The **Limulus Amoebocyte Lysate (LAL) assay** is the most sensitive and specific test used for the detection and quantification of **bacterial endotoxins** (Lipopolysaccharide/LPS), which are structural components of the cell wall of Gram-negative bacteria. **Why Endotoxin is correct:** The test utilizes the blood (hemolymph) of the Atlantic Horseshoe crab (*Limulus polyphemus*). The amoebocytes (blood cells) of this crab contain a clotting enzyme system that is triggered specifically by the presence of even minute amounts of endotoxin. When endotoxin comes into contact with the lysate, it initiates an enzymatic coagulation cascade, resulting in the formation of a detectable gel clot or turbidity. **Why other options are incorrect:** * **Exotoxins:** These are proteins secreted by both Gram-positive and Gram-negative bacteria (e.g., Tetanus or Diphtheria toxin). They do not trigger the LAL coagulation cascade; they are typically detected via assays like ELISA or specific biological activity tests (e.g., Elek’s test). * **Preformed toxins:** These are toxins produced in food before ingestion (e.g., *Staphylococcus aureus* enterotoxin or *Clostridium botulinum* toxin). These are identified via mass spectrometry or latex agglutination, not the LAL assay. **High-Yield Clinical Pearls for NEET-PG:** * **Pyrogen Testing:** The LAL assay has largely replaced the "Rabbit Pyrogen Test" for checking the sterility of intravenous fluids, injectable drugs, and medical devices. * **Sensitivity:** It can detect endotoxin levels as low as 1 picogram/ml. * **Gram-negative Sepsis:** Endotoxin (LPS) is the primary mediator of septic shock, acting through the Lipid A component. * **False Positives:** Certain (1→3)-β-D-glucans (from fungal cell walls) can sometimes cause a positive reaction, which is a known limitation of the test.

Q: A bacterium divides every 20 minutes. If the population starts with a single bacterium and grows exponentially for 3 hours, how many bacteria will be present?

512. **Explanation:** This question tests the concept of **Bacterial Growth Kinetics**, specifically the calculation of population size during the exponential (log) phase. **1. Why Option A (512) is Correct:** The growth of a bacterial population can be calculated using the formula: **$N_n = N_0 \times 2^n$** * **$N_0$ (Initial number):** 1 bacterium. * **Generation Time ($G$):** 20 minutes (the time taken for one cell to divide into two). * **Total Time ($T$):** 3 hours = 180 minutes. * **Number of Generations ($n$):** Total time / Generation time = $180 / 20 = 9$ generations. Applying the formula: $1 \times 2^9 = 512$. Thus, after 3 hours, 512 bacteria will be present. **2. Why the Other Options are Incorrect:** * **Option B (440):** This is a distractor and does not follow the geometric progression ($2^n$) of bacterial growth. * **Option C (18):** This is a common error where the student multiplies the number of generations (9) by 2, rather than using it as an exponent. * **Option D (1024):** This represents $2^{10}$, which would be the population after 3 hours and 20 minutes (10 generations). **Clinical Pearls for NEET-PG:** * **Generation Time:** Also known as "doubling time." For *E. coli*, it is ~20 minutes, whereas for *Mycobacterium tuberculosis*, it is much slower (~12–20 hours), explaining the chronic nature of TB. * **Bacterial Growth Curve:** Consists of 4 phases: **Lag** (increase in size, not number), **Log/Exponential** (maximum metabolic activity; best for antibiotic action like Penicillin), **Stationary** (growth rate equals death rate; spores form here), and **Decline**. * **Calculation Tip:** Memorizing powers of 2 (e.g., $2^5=32, 2^{10}=1024$) is highly useful for quick calculations in the exam.

Question 1

Which of the following infectious agents lacks RNA?

Accepted Answer

Prions

Answer

Virus

Answer

Staphylococci

Answer

Cryptococcus

Question 2

Which of the following bacteria does not produce spores?

Accepted Answer

Pseudomonas

Answer

Bacillus anthracis

Answer

Clostridium

Answer

Bacillus subtilis

Question 3

Which of the following is a normal vaginal anaerobe?

Accepted Answer

Lactobacillus

Answer

Corynebacterium

Answer

Gardnerella

Answer

E. coli

Question 4

What is true about endotoxin?

Accepted Answer

No enzymatic activity

Answer

Protein

Answer

Highly antigenic

Answer

Produced by Gram-positive bacteria

Question 5

In a bacterial growth curve, in which phase is the maximum cell size typically obtained?

Accepted Answer

End of lag phase

Answer

Beginning of Lag phase

Answer

Beginning of stationary phase

Answer

Log phase

Question 6

Which of the following statements regarding bacterial extracellular polysaccharide production is true?

Accepted Answer

a and e are false; b, c, and d are true

Answer

a and b are false; c, d, and e are true

Answer

b, c, and d are false; a and e are true

Answer

c, d, and e are false; a and b are true

Question 7

The bacterial cell wall has all of the following properties except:

Accepted Answer

It is a rigid structure.

Answer

It consists of a peptidoglycan polymer.

Answer

It is the structure principally responsible for the Gram staining reaction.

Answer

It contains D-isomers of amino acids.

Question 8

Which of the following organisms does not have a non-human reservoir?

Accepted Answer

Salmonella typhi

Answer

Yersinia pestis

Answer

Escherichia coli

Answer

Clostridium tetani

Question 9

Limulus amoebocyte lysate assay is used for the detection of which of the following?

Accepted Answer

Endotoxin

Answer

Exotoxin

Answer

Preformed toxin

Answer

None of the above

Question 10

A bacterium divides every 20 minutes. If the population starts with a single bacterium and grows exponentially for 3 hours, how many bacteria will be present?

Accepted Answer

512

Answer

440

Answer

18

Answer

1024

General Microbiology — MCQs

General Microbiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?