General Microbiology Practice Questions

Q: Which of the following infections can be transmitted to a baby during delivery?

Toxoplasmosis. **Explanation:** The question focuses on the **timing of vertical transmission**. Infections can be transmitted from mother to child via three routes: **In utero** (transplacental), **Intrapartum** (during delivery), or **Postpartum** (breastfeeding). **Correct Answer: A. Toxoplasmosis** *Toxoplasma gondii* is primarily transmitted **transplacentally (in utero)**. While the question asks which infection can be transmitted *during* delivery, in the context of standard microbiology textbooks (like Ananthanarayan), Toxoplasmosis is classically categorized under congenital infections acquired across the placenta. However, it is important to note that if a mother acquires a primary infection very late in the third trimester, the parasite can be present in the birth canal/blood, though transplacental passage remains the hallmark. **Analysis of Incorrect Options:** * **B. Gonococcus:** This is a classic **intrapartum** infection. *Neisseria gonorrhoeae* is transmitted as the baby passes through an infected birth canal, leading to *Ophthalmia neonatorum* (purulent conjunctivitis) within 2–5 days of birth. * **C. Herpes Simplex Type II:** Transmission is predominantly **intrapartum** (85–90% of cases) due to direct contact with herpetic lesions in the birth canal during labor. * **D. Hepatitis B:** Transmission is most commonly **intrapartum** through exposure to infected maternal blood and vaginal secretions during delivery. **NEET-PG High-Yield Pearls:** * **TORCH Complex:** Refers to *Toxoplasma*, Others (Syphilis, HIV, HBV, VZV), Rubella, CMV, and HSV. * **Transplacental (In utero):** Toxoplasmosis, Rubella, CMV, Syphilis, HIV. * **Intrapartum (During birth):** Gonococcus, Chlamydia, HSV-2, Hepatitis B, Group B Streptococcus. * **Postpartum:** HIV, CMV, and HTLV-1 (via breast milk). * **Toxoplasmosis Triad:** Chorioretinitis, Hydrocephalus, and Intracranial calcifications.

Q: 2x10^2 Streptococcus viridans are inoculated into a flask containing one liter of enriched broth. If the lag time is 30 minutes and the generation time is 20 minutes, how many bacteria will there be in the culture after two and a half hours?

1.28 x 10^4. ### Explanation To solve this problem, we must apply the principles of bacterial growth kinetics. Bacterial growth follows a predictable mathematical progression: **$N_t = N_0 \times 2^n$**, where $N_t$ is the final population, $N_0$ is the initial population, and $n$ is the number of generations. **Step-by-Step Calculation:** 1. **Total Time:** 2.5 hours = 150 minutes. 2. **Effective Growth Time:** Bacteria do not divide during the **Lag Phase**. Subtract the lag time from the total time: $150\text{ mins} - 30\text{ mins} = 120\text{ minutes}$ of active division (Log Phase). 3. **Number of Generations ($n$):** Divide the growth time by the generation time: $120 / 20 = 6\text{ generations}$. 4. **Final Count:** $N_t = (2 \times 10^2) \times 2^6$. * $2^6 = 64$. * $200 \times 64 = 12,800$, which is **$1.28 \times 10^4$**. --- ### Analysis of Options * **Option D (Correct):** Correctly accounts for the lag phase and 6 doubling cycles. * **Option A & B:** These represent significant underestimations, likely failing to apply the exponential growth formula ($2^n$). * **Option C:** This would be the result if the lag phase was ignored and the bacteria divided for the full 150 minutes ($7.5$ generations), which is biologically incorrect. --- ### NEET-PG Clinical Pearls * **Lag Phase:** Period of intense metabolic activity and enzyme synthesis, but **no increase in cell number**. * **Log (Exponential) Phase:** Period of rapid growth where generation time is determined. Bacteria are **most sensitive to antibiotics** (e.g., Beta-lactams) during this phase. * **Stationary Phase:** Growth rate equals death rate due to nutrient exhaustion and toxin accumulation. **Sporulation** typically occurs here. * **Viridans Streptococci:** Common commensals of the oral cavity; the most common cause of **Subacute Bacterial Endocarditis (SBE)** following dental procedures.

Q: All of the following statements about penicillin-binding proteins are true EXCEPT:

These are target site of vancomycin. ### Explanation **1. Why Option C is the Correct Answer (The "Except" Statement):** Penicillin-binding proteins (PBPs) are **not** the target of Vancomycin. Vancomycin is a glycopeptide antibiotic that inhibits cell wall synthesis by binding directly to the **D-Ala-D-Ala terminus** of the nascent peptidoglycan precursor (lipid II). By "capping" this substrate, it sterically hinders the PBP enzymes from performing their function. In contrast, Beta-lactams (like Penicillins, Cephalosporins, and Carbapenems) bind directly to the active site of PBPs. **2. Analysis of Other Options:** * **Option A (Present on cell surface):** PBPs are membrane-bound enzymes (transpeptidases, carboxypeptidases) located on the outer face of the cytoplasmic membrane, making them accessible to antibiotics in the periplasmic space. * **Option B (Mutation gives rise to resistance):** This is a classic mechanism of resistance. For example, **MRSA** (Methicillin-resistant *S. aureus*) produces a modified PBP called **PBP2a**, which has a low affinity for almost all beta-lactams. Similarly, *S. pneumoniae* develops resistance via mosaic PBP genes. * **Option D (Targeted by imipenem):** Imipenem is a Carbapenem. All beta-lactam antibiotics, including carbapenems, monobactams, and cephalosporins, exert their bactericidal effect by binding to and inactivating PBPs. **3. High-Yield Clinical Pearls for NEET-PG:** * **PBP Function:** Their primary role is **transpeptidation** (cross-linking of peptidoglycan chains), which provides structural rigidity to the bacterial cell wall. * **Vancomycin Resistance:** Occurs via the alteration of the target D-Ala-D-Ala to **D-Ala-D-Lac** (seen in VRSA/VRE). * **Beta-lactamase vs. PBP Mutation:** While many bacteria produce beta-lactamase enzymes to degrade the drug, organisms like MRSA and *Enterococci* primarily use PBP modification to achieve resistance.

Question 1

A 25-year-old male from a flood-affected area presents with sudden onset of profuse, painless, rice-water diarrhoea and severe dehydration. Stool is collected and inoculated onto selective media. The plate after 18 hours of incubation at 37°C is shown (Image 1). Which single biochemical or growth characteristic of the organism forming these colonies best explains why this selective medium differentiates it from non-pathogenic vibrios such as Vibrio parahaemolyticus?

Accepted Answer

A. Fermentation of sucrose, producing acid that shifts the pH indicator to yellow

Answer

B. Inability to tolerate bile salts, resulting in selective inhibition of competing enteric flora

Answer

C. Production of hydrogen sulphide, causing blackening of the colony centre

Answer

D. Fermentation of thiosulfate with release of free sulphur, yielding opaque white colonies

Question 2

Which of the following infections can be transmitted to a baby during delivery?

Accepted Answer

Toxoplasmosis

Answer

Gonococcus

Answer

Herpes simplex type II

Answer

Hepatitis-B

Question 3

2x10^2 Streptococcus viridans are inoculated into a flask containing one liter of enriched broth. If the lag time is 30 minutes and the generation time is 20 minutes, how many bacteria will there be in the culture after two and a half hours?

Accepted Answer

1.28 x 10^4

Answer

6 x 10^2

Answer

1.2 x 10^3

Answer

1.6 x 10^3

Question 4

Reversion of Neisseria gonorrhoeae from a fimbriated (fim 1) to a non-fimbriated (fim 2) state would result in which one of the following phenomena?

Accepted Answer

Inability of N. gonorrhoeae to colonize the mucosal epithelium

Answer

Reversion to a Gram-positive stain

Answer

Death of the organism

Answer

Loss of serologic specificity

Question 5

All of the following statements about penicillin-binding proteins are true EXCEPT:

Accepted Answer

These are target site of vancomycin

Answer

Present on cell surface

Answer

Mutation in PBPs gives rise to resistance

Answer

These are targeted by imipenem

General Microbiology — MCQs

General Microbiology — MCQs

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