Bacteriology Practice Questions

Q: Moraxella catarrhalis is resistant to which antibiotic?

Amoxicillin. **Explanation:** The correct answer is **Amoxicillin**. **1. Why Amoxicillin is correct:** *Moraxella catarrhalis* is a Gram-negative diplococcus that is a common cause of respiratory tract infections. The primary mechanism of resistance in *M. catarrhalis* is the production of **beta-lactamase enzymes** (specifically BRO-1 and BRO-2 types). Over **90–95% of clinical isolates** produce these enzymes, which hydrolyze the beta-lactam ring of narrow-spectrum penicillins. Therefore, amoxicillin alone is ineffective as it is easily degraded by these enzymes. **2. Why the other options are incorrect:** * **B & C (Clarithromycin and Azithromycin):** These are macrolides. *M. catarrhalis* is generally susceptible to macrolides, making them common alternative treatments for patients with penicillin allergies. * **D (Amoxicillin and Clavulanic acid):** While *M. catarrhalis* is resistant to amoxicillin alone, it is **highly susceptible** to this combination. Clavulanic acid is a beta-lactamase inhibitor that "suicidally" binds to the enzymes produced by the bacteria, protecting the amoxicillin and allowing it to exert its bactericidal effect. **3. NEET-PG High-Yield Pearls:** * **Morphology:** Gram-negative, kidney-shaped diplococci (resembles *Neisseria*). * **Key Identification Test:** It is **DNase positive** and **Butyrate esterase positive** (the "M. cat" disk test), which differentiates it from *Neisseria* species. * **Clinical Spectrum:** It is a leading cause of **Otitis media** in children and **Acute Exacerbations of COPD (AECOPD)** in adults. * **Hockey Puck Sign:** On agar, colonies can be pushed across the surface with a loop without breaking, similar to a hockey puck.

Q: The capsule of Streptococcus pyogenes is made up of?

Hyaluronic acid. **Explanation:** **Correct Answer: A. Hyaluronic acid** *Streptococcus pyogenes* (Group A Streptococcus) is unique among most pathogenic bacteria because its capsule is composed of **hyaluronic acid**. This is a critical virulence factor because hyaluronic acid is chemically identical to the ground substance found in human connective tissue. This "molecular mimicry" allows the bacterium to evade the host’s immune system, as the capsule is not recognized as foreign (non-immunogenic), thereby preventing opsonization and phagocytosis. **Analysis of Incorrect Options:** * **B. Polysaccharide:** This is the most common capsule material for the majority of bacteria (e.g., *Streptococcus pneumoniae*, *Neisseria meningitidis*, and *Haemophilus influenzae*). While *S. pyogenes* has a cell wall polysaccharide (Group A antigen), its **capsule** is specifically hyaluronic acid. * **C. Polypeptide:** This is a classic "distractor." Only *Bacillus anthracis* is known for having a polypeptide capsule (made of D-glutamic acid). * **D. Glycopeptides:** This term refers to the structural components of the cell wall (peptidoglycan) or a class of antibiotics (like Vancomycin), but it does not constitute the capsule of any major human pathogen. **High-Yield Clinical Pearls for NEET-PG:** * **M-Protein:** The major virulence factor of *S. pyogenes* is the M-protein (anchored in the cell wall, not the capsule), which is also anti-phagocytic and responsible for molecular mimicry leading to Rheumatic Fever. * **Quellung Reaction:** Since the hyaluronic acid capsule is non-immunogenic, *S. pyogenes* does **not** show a positive Quellung reaction (unlike *S. pneumoniae*). * **Non-Antigenic:** Because it mimics human tissue, the capsule does not induce protective antibodies; immunity to *S. pyogenes* is instead directed against the M-protein.

Q: A 25-year-old heterosexual male presents with dysuria, noted pus-like urethral discharge, and penile tip tenderness. He has had multiple sexual partners in the past 6 months. Gram staining of the urethral discharge shows Gram-negative cocci. Based on the Gram stain findings, what is the probable causative organism?

Gonococci. ### Explanation The clinical presentation of acute dysuria, purulent (pus-like) urethral discharge, and penile tenderness in a sexually active male is classic for **Urethritis**. The definitive clue in this question lies in the **Gram stain findings**. **Why Gonococci is correct:** * **Neisseria gonorrhoeae (Gonococci)** is a **Gram-negative diplococcus** (kidney-bean shaped) typically found within polymorphonuclear leukocytes (intracellular). * It is the primary cause of "Gonococcal Urethritis," characterized by a short incubation period (2–7 days) and thick, creamy, purulent discharge. **Why the other options are incorrect:** * **Chlamydia trachomatis:** This is the most common cause of Non-Gonococcal Urethritis (NGU). However, *Chlamydia* is an obligate intracellular bacterium that **does not show up on a standard Gram stain** because it lacks a traditional peptidoglycan cell wall and is too small to be seen under a light microscope. * **Mycoplasma hominis & Ureaplasma urealyticum:** These are also causes of NGU. Like *Chlamydia*, they **lack a cell wall** entirely, meaning they cannot be visualized by Gram staining and are inherently resistant to beta-lactam antibiotics. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (selective) or Chocolate agar. However, **NAAT** (Nucleic Acid Amplification Test) is now the investigation of choice for screening. * **Co-infection:** Patients with Gonorrhea are frequently co-infected with *Chlamydia*. Therefore, empirical treatment often covers both (e.g., Ceftriaxone for Gonococci + Azithromycin/Doxycycline for Chlamydia). * **Gram Stain Sensitivity:** In symptomatic males, a Gram stain of urethral discharge showing intracellular Gram-negative diplococci is >95% sensitive and specific.

Q: A 30-year-old pregnant woman develops severe sepsis with headache and neck stiffness after eating coleslaw, soft cheese, and cantaloupes at a potluck dinner. Two other women who attended the event develop self-limiting abdominal cramps, nausea, and vomiting. Which infectious agent is most likely responsible?

Listeria monocytogenes. The clinical presentation is classic for **Listeriosis**, caused by *Listeria monocytogenes*. This gram-positive, facultative intracellular bacillus is a common cause of foodborne illness associated with contaminated **deli meats, soft cheeses (brie, camembert), coleslaw, and unwashed produce (cantaloupes).** 1. **Why Listeria is correct:** * **Vulnerable Populations:** It primarily affects pregnant women (due to altered cell-mediated immunity), neonates, and the elderly. * **Clinical Spectrum:** In healthy individuals, it causes self-limiting febrile gastroenteritis (as seen in the two other women) [1]. However, in pregnant women, it can lead to severe sepsis and has a high tropism for the CNS, causing **meningitis** (headache and neck stiffness). It is a leading cause of neonatal sepsis and meningitis via transplacental transmission. 2. **Why other options are incorrect:** * **HSV & EBV:** These viruses do not typically present as foodborne outbreaks. While HSV can cause encephalitis, it is not associated with the consumption of specific foods or cluster outbreaks. * **Parvovirus B19:** This causes Erythema Infectiosum (Fifth disease) and can lead to hydrops fetalis in pregnancy, but it does not cause acute sepsis or meningitis following food ingestion. **High-Yield Clinical Pearls for NEET-PG:** * **Microscopy:** Exhibits characteristic **"Tumbling Motility"** at 25°C. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (refrigeration doesn't prevent growth). * **Treatment of Choice:** **Ampicillin** (Listeria is inherently resistant to all cephalosporins). * **Pathogenesis:** Uses **Actin tails** (actin rockets) for cell-to-cell spread, avoiding the host's humoral immune response.

Q: Several weeks following a severe streptococcal sore throat, a 7-year-old boy develops acute glomerulonephritis with edema, hypertension, and hematuria. Which of the following best describes the properties of the most likely causative organism?

Beta hemolytic, inhibited by bacitracin. ### Explanation **1. Why the Correct Answer is Right:** The clinical presentation of edema, hypertension, and hematuria (nephritic syndrome) following a sore throat is diagnostic of **Post-Streptococcal Acute Glomerulonephritis (PSAGN)**. This is a Type III hypersensitivity reaction caused by nephritogenic strains of **Group A Beta-hemolytic Streptococcus (GABS)**, also known as *Streptococcus pyogenes*. In the microbiology laboratory, *S. pyogenes* is characterized by: * **Beta-hemolysis:** Complete clearing of RBCs on blood agar. * **Bacitracin Sensitivity:** It is uniquely inhibited by low concentrations of bacitracin (Taxo A disc), which distinguishes it from other beta-hemolytic streptococci like Group B Strep. **2. Why Incorrect Options are Wrong:** * **Option A:** Describes *Streptococcus pneumoniae*. While it causes respiratory infections, it is alpha-hemolytic and does not cause PSAGN. * **Option B:** Describes Viridans group streptococci (e.g., *S. mutans*). These are common causes of subacute bacterial endocarditis, not glomerulonephritis. * **Option C:** Describes *Enterococcus* species (formerly Group D Strep). These are typically gamma or alpha-hemolytic and are associated with UTIs and biliary tract infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Latent Period:** PSAGN occurs 1–3 weeks after pharyngitis or 3–6 weeks after skin infections (impetigo). Note that Rheumatic Fever *only* follows pharyngitis. * **Diagnosis:** Look for elevated **ASO titers** (after pharyngitis) or **Anti-DNase B** (after skin infections). Low C3 complement levels are a classic finding. * **M Protein:** The major virulence factor of *S. pyogenes* that is anti-phagocytic and responsible for molecular mimicry. * **PYR Test:** *S. pyogenes* is PYR positive, another rapid diagnostic marker.

Q: An enterococcus (E. faecium) was isolated from a urine specimen (100,000 cfu/mL). Treatment of the patient with ampicillin and gentamicin failed. What is the most clinically appropriate action?

Consider vancomycin as an alternative drug. ### Explanation **1. Why Option D is Correct:** The patient has a significant urinary tract infection (UTI) caused by *Enterococcus faecium* (colony count $\geq 10^5$ CFU/mL). The failure of ampicillin and gentamicin suggests **High-Level Aminoglycoside Resistance (HLAR)** or intrinsic resistance common in *E. faecium*. Unlike *E. faecalis*, *E. faecium* is frequently resistant to ampicillin due to altered penicillin-binding proteins (PBP5). When first-line synergistic therapy (Beta-lactam + Aminoglycoside) fails, **Vancomycin** is the drug of choice, provided the strain is not Vancomycin-Resistant Enterococci (VRE). **2. Analysis of Incorrect Options:** * **Option A:** Incorrect. A colony count of 100,000 CFU/mL in a symptomatic patient is clinically significant and requires treatment modification if the initial therapy fails. * **Option B:** Incorrect. 100,000 CFU/mL is the standard threshold for "significant bacteriuria." Suggesting low colony counts is irrelevant here as the count is already high. * **Option C:** Incorrect. Actinomycosis is caused by *Actinomyces* species (anaerobic Gram-positive branching bacilli), not *Enterococcus*. It typically presents with sulfur granules and chronic abscesses, not acute UTIs. **3. NEET-PG High-Yield Pearls:** * **Enterococci Characteristics:** Catalase negative, PYR positive, grow in 6.5% NaCl and 40% bile (Bile Esculin Agar). * **Resistance Patterns:** *E. faecium* is generally more antibiotic-resistant than *E. faecalis*. * **Synergy:** Aminoglycosides alone cannot penetrate the cell wall of Enterococci; they must be combined with a cell-wall active agent (Ampicillin/Vancomycin) for bactericidal effect. * **VRE Treatment:** If Vancomycin resistance occurs (VanA/VanB genes), the next-line drugs are **Linezolid, Daptomycin, or Quinupristin-Dalfopristin.**

Q: What is the incubation period for Bacillus cereus food poisoning?

Bacillus cereus. **Explanation:** *Bacillus cereus* is a Gram-positive, spore-forming aerobic rod known for causing two distinct types of food poisoning based on the production of different toxins. The question focuses on the **Emetic type**, which is characterized by a very short incubation period (1–6 hours), similar to *Staphylococcus aureus*. **1. Why Bacillus cereus is correct:** The emetic syndrome is caused by a preformed, heat-stable toxin called **Cereulide**. Because the toxin is already present in the food (classically **reheated rice**), symptoms like nausea and vomiting manifest rapidly after ingestion. There is also a **Diarrheal type** caused by a heat-labile enterotoxin produced in the intestine, which has a longer incubation period (8–16 hours). **2. Why the other options are incorrect:** * **Staphylococcus aureus:** While it also has a short incubation period (1–6 hours) due to preformed enterotoxins, it is typically associated with protein-rich foods (custard, mayonnaise, processed meats) rather than rice. * **Salmonellosis:** This is an invasive infection or toxico-infection with a much longer incubation period, typically **12–72 hours**, presenting with fever, abdominal cramps, and inflammatory diarrhea. * **Vibrio cholerae:** The incubation period is usually **1–3 days**. It produces a potent enterotoxin (Choleragen) in the gut, leading to profuse "rice-water" stools and severe dehydration. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic form:** Heat-stable toxin, 1–6 hours incubation, associated with **Fried Rice**. * **Diarrheal form:** Heat-labile toxin, 8–16 hours incubation, associated with **Meat/Vegetables**. * *Bacillus cereus* is also a significant cause of post-traumatic **endophthalmitis**. * **Nagler Reaction:** Used to identify *C. perfringens*, but *B. cereus* also produces lecithinase, showing a positive reaction on Egg Yolk Agar.

Q: Which of the following infections is caused by anaerobic gram-positive cocci?

Puerperal infection. ### Explanation **1. Why Puerperal Infection is Correct:** Anaerobic Gram-positive cocci (AGPC), primarily **Peptostreptococcus** species, are significant members of the normal flora of the female genitourinary tract. They are frequently implicated in polymicrobial infections following childbirth or abortion, such as **puerperal sepsis**, pelvic inflammatory disease (PID), and tubo-ovarian abscesses. Their ability to thrive in the necrotic, anaerobic environment of the postpartum uterus makes them a leading cause of these infections. **2. Why Other Options are Incorrect:** * **Food Poisoning:** This is typically caused by Gram-positive bacilli (e.g., *Clostridium perfringens*, *Bacillus cereus*) or Gram-positive cocci that are **aerobic/facultative anaerobes** (e.g., *Staphylococcus aureus*). * **Endocarditis:** The most common causative agents are aerobic or facultative anaerobic cocci, such as *Viridans group Streptococci*, *Staphylococcus aureus*, or *Enterococci*. Anaerobic cocci are extremely rare causes of endocarditis. * **Septicemia:** While AGPC can cause bacteremia, "Septicemia" as a general clinical entity is most commonly associated with aerobic Gram-negative bacilli (like *E. coli*) or aerobic Gram-positive cocci (like *S. aureus*). **3. Clinical Pearls for NEET-PG:** * **Peptostreptococcus** is the most clinically relevant genus of AGPC. * **Specimen Collection:** Since these are anaerobes, samples must be collected via aspiration (not swabs) and transported in specialized anaerobic transport media. * **Synergy:** AGPC often act synergistically with other anaerobes (like *Bacteroides fragilis*) or facultative anaerobes, leading to foul-smelling discharges—a hallmark of anaerobic infection. * **Treatment:** They are generally sensitive to Penicillin, Metronidazole, and Clindamycin.

Question 1

Moraxella catarrhalis is resistant to which antibiotic?

Accepted Answer

Amoxicillin

Answer

Clarithromycin

Answer

Azithromycin

Answer

Amoxicillin and clavulanic acid combination

Question 2

The capsule of Streptococcus pyogenes is made up of?

Accepted Answer

Hyaluronic acid

Answer

Polysaccharide

Answer

Polypeptide

Answer

Glycopeptides

Question 3

A 25-year-old heterosexual male presents with dysuria, noted pus-like urethral discharge, and penile tip tenderness. He has had multiple sexual partners in the past 6 months. Gram staining of the urethral discharge shows Gram-negative cocci. Based on the Gram stain findings, what is the probable causative organism?

Accepted Answer

Gonococci

Answer

Mycoplasma hominis

Answer

Ureaplasma urealyticum

Answer

Chlamydia trachomatis

Question 4

A 30-year-old pregnant woman develops severe sepsis with headache and neck stiffness after eating coleslaw, soft cheese, and cantaloupes at a potluck dinner. Two other women who attended the event develop self-limiting abdominal cramps, nausea, and vomiting. Which infectious agent is most likely responsible?

Accepted Answer

Listeria monocytogenes

Answer

Herpes simplex virus

Answer

Epstein-Barr virus

Answer

Parvovirus B19

Question 5

Several weeks following a severe streptococcal sore throat, a 7-year-old boy develops acute glomerulonephritis with edema, hypertension, and hematuria. Which of the following best describes the properties of the most likely causative organism?

Accepted Answer

Beta hemolytic, inhibited by bacitracin

Answer

Alpha hemolytic, lysed by optochin

Answer

Alpha hemolytic, not lysed by optochin

Answer

Beta hemolytic, can grow in 40% bile and 6.5% sodium chloride

Question 6

An enterococcus (E. faecium) was isolated from a urine specimen (100,000 cfu/mL). Treatment of the patient with ampicillin and gentamicin failed. What is the most clinically appropriate action?

Accepted Answer

Consider vancomycin as an alternative drug

Answer

Do no further clinical workup

Answer

Suggest to the laboratory that low colony counts may reflect infection

Answer

Determine if fluorescent microscopy is available for the diagnosis of actinomycosis

Question 7

Donovan bodies are seen in which of the following?

Accepted Answer

Calymmatobacterium granulomatis

Answer

Leishmania donovani

Answer

Chlamydia trachomatis

Answer

Klebsiella

Question 8

What is the incubation period for Bacillus cereus food poisoning?

Accepted Answer

Bacillus cereus

Answer

Staphylococcus aureus

Answer

Salmonellosis

Answer

Vibrio cholera

Question 9

The 7th pandemic of cholera is primarily attributed to which of the following?

Accepted Answer

El Tor biotype

Answer

O139 Vibrio cholerae

Answer

Classical Vibrio cholerae

Answer

Vibrio mimicus

Question 10

Which of the following infections is caused by anaerobic gram-positive cocci?

Accepted Answer

Puerperal infection

Answer

Food poisoning

Answer

Endocarditis

Answer

Septicemia

Bacteriology — MCQs

Bacteriology — MCQs

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