Bacteriology Practice Questions

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.

Q: Which of the following statements about Bacillus anthracis is FALSE?

Most common transmission is by inhalation of spores. **Explanation:** The correct answer is **D** because it is a false statement regarding the epidemiology of *Bacillus anthracis*. While inhalation anthrax (Woolsorter’s disease) is the most severe and fatal form, it is **not** the most common. Approximately **95% of all anthrax cases** globally are **Cutaneous Anthrax**, which occurs through the inoculation of spores into skin abrasions while handling infected animal products (hides, wool, or bone meal). **Analysis of other options:** * **Option A (True):** *B. anthracis* is the causative agent of anthrax, a zoonotic disease primarily affecting herbivores. * **Option B (True):** The anthrax toxin is **tripartite**, consisting of three plasmid-encoded proteins: **Protective Antigen (PA)** (the binding unit), **Edema Factor (EF)** (an adenylate cyclase), and **Lethal Factor (LF)** (a metalloprotease). * **Option C (True):** Unlike most bacteria that have polysaccharide capsules, *B. anthracis* has a unique **polypeptide capsule** composed of **poly-D-glutamic acid**, which is essential for evading phagocytosis. **High-Yield NEET-PG Pearls:** * **Morphology:** Large, Gram-positive, non-motile bacilli with "square ends" (Box-car appearance). * **Culture:** Shows **"Medusa head"** colonies on agar and a **"Bamboo stick"** appearance on microscopy. * **String of Pearls Reaction:** Occurs when grown on agar containing penicillin. * **McFadyean’s Reaction:** Uses polychrome methylene blue to visualize the purple capsule. * **Biological Warfare:** Spores are highly resistant and considered a Category A bioterrorism agent.

Q: Which of the following is NOT caused by Bartonella henselae?

Oroya fever. **Explanation:** The correct answer is **Oroya fever** because it is caused by ***Bartonella bacilliformis***, not *Bartonella henselae*. *Bartonella bacilliformis* is transmitted by the sandfly (*Lutzomyia*) and is endemic to the Andes mountains. It causes two distinct phases of **Carrion’s disease**: 1. **Oroya fever:** An acute febrile illness characterized by severe hemolytic anemia. 2. **Verruga peruana:** A chronic phase presenting with eruptive vascular skin nodules. **Analysis of Incorrect Options:** * **Cat scratch disease:** This is the most common clinical manifestation of ***B. henselae***. It typically presents with regional lymphadenopathy following a cat scratch or bite. * **Bacillary angiomatosis:** Caused by both ***B. henselae*** and ***B. quintana***. It involves proliferation of small blood vessels in the skin and visceral organs, primarily seen in immunocompromised patients (e.g., HIV/AIDS). * **Subacute bacterial endocarditis (SABE):** *Bartonella* species (especially *B. henselae* and *B. quintana*) are well-known causes of **culture-negative endocarditis**, often associated with exposure to cats or body lice. **High-Yield Clinical Pearls for NEET-PG:** * **B. quintana:** Causes **Trench fever** (transmitted by body lice) and is common in homeless populations. * **Warthin-Starry stain:** The silver stain of choice used to visualize *Bartonella* species in tissue sections. * **Stellate Necrotizing Granulomas:** The characteristic histopathological finding in lymph nodes of Cat Scratch Disease. * **Treatment:** Azithromycin is the first-line treatment for Cat Scratch Disease; Doxycycline or Erythromycin is used for Bacillary Angiomatosis.

Q: Osteomyelitis in sickle cell anemia is most commonly caused by which organism?

Salmonella. **Explanation:** In the general population, *Staphylococcus aureus* is the most common cause of osteomyelitis. However, in patients with **Sickle Cell Anemia (SCA)**, **Salmonella** species are the most common causative organisms (followed by *S. aureus*). **Why Salmonella?** The predisposition to Salmonella in SCA is due to several factors: 1. **Autosplenectomy:** Chronic splenic infarction leads to functional asplenia, impairing the clearance of encapsulated and intracellular organisms [1]. 2. **Vaso-occlusive Crises:** Micro-infarctions in the intestinal mucosa allow Salmonella (normal gut flora) to enter the bloodstream. 3. **Bone Infarction:** Ischemic areas in the bone marrow serve as a nidus for bacteria to settle during transient bacteremia [1]. 4. **Complement Deficiency:** Impaired alternative complement pathway activity reduces the opsonization of Salmonella. **Analysis of Incorrect Options:** * **B. Streptococcus:** While *S. pneumoniae* is a major cause of sepsis and meningitis in SCA due to asplenia, it is not the leading cause of osteomyelitis. * **C. Haemophilus:** *H. influenzae* was a common cause of infection in asplenic children, but its incidence has significantly decreased due to widespread vaccination. * **D. Neisseria:** Patients with terminal complement deficiency are at risk for Neisserial infections, but it is not a primary pathogen in sickle cell-related osteomyelitis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of Osteomyelitis in SCA:** *Salmonella* (unique association). * **Most common cause of Osteomyelitis overall:** *Staphylococcus aureus*. * **Most common cause of death in SCA (Children):** *S. pneumoniae* sepsis [1]. * **Most common cause of death in SCA (Adults):** Acute Chest Syndrome. * **Radiology:** It is often difficult to distinguish between a bone infarct and osteomyelitis on X-ray; MRI or bone scans are preferred.

Question 1

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 2

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 3

Donovan bodies are seen in which of the following?

Accepted Answer

Calymmatobacterium granulomatis

Answer

Leishmania donovani

Answer

Chlamydia trachomatis

Answer

Klebsiella

Question 4

What is the incubation period for Bacillus cereus food poisoning?

Accepted Answer

Bacillus cereus

Answer

Staphylococcus aureus

Answer

Salmonellosis

Answer

Vibrio cholera

Question 5

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 6

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

Accepted Answer

Puerperal infection

Answer

Food poisoning

Answer

Endocarditis

Answer

Septicemia

Question 7

Which of the following statements about Bacillus anthracis is FALSE?

Accepted Answer

Most common transmission is by inhalation of spores

Answer

Causes anthrax

Answer

Toxin is tripartite

Answer

Capsule made up of polypeptide

Question 8

Pigment production by Staphylococcus aureus occurs in which condition?

Accepted Answer

Bullous impetigo

Answer

Erythrasma

Answer

Buruli ulcer

Answer

Food poisoning

Question 9

Which of the following is NOT caused by Bartonella henselae?

Accepted Answer

Oroya fever

Answer

Cat scratch disease

Answer

Bacillary angiomatosis

Answer

Subacute bacterial endocarditis (SABE)

Question 10

Osteomyelitis in sickle cell anemia is most commonly caused by which organism?

Accepted Answer

Salmonella

Answer

Streptococcus

Answer

Haemophilus

Answer

Neisseria

Bacteriology — MCQs

Bacteriology — MCQs

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