Bacteriology Practice Questions

Q: Which of the following mycobacteria are non-pathogenic to humans?

M. phlei. **Explanation:** The genus *Mycobacterium* is broadly classified into the *M. tuberculosis* complex, *M. leprae*, and Non-Tuberculous Mycobacteria (NTM). NTM are further categorized by the **Runyon Classification** based on growth rate and pigment production. **M. phlei** (Option D) is a **saprophytic**, rapidly growing mycobacterium (Runyon Group IV). It is commonly found in soil and dust and is considered **non-pathogenic to humans**. It is frequently used in laboratory settings for biochemical testing (e.g., as a control in the urease test) because it does not cause disease. **Analysis of Incorrect Options:** * **M. bovis (Option A):** A member of the *M. tuberculosis* complex. It causes bovine tuberculosis but is highly pathogenic to humans, typically transmitted via unpasteurized milk, leading to extrapulmonary TB. * **M. kansasii (Option B):** A photochromogen (Runyon Group I). It is a significant human pathogen that causes a chronic pulmonary disease clinically indistinguishable from tuberculosis. * **M. avium-intracellulare (Option C):** Also known as MAC (Runyon Group III). It is a major opportunistic pathogen, especially in HIV/AIDS patients with low CD4 counts, causing disseminated infection. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group IV (Rapid Growers):** Includes *M. phlei* (saprophyte), *M. smegmatis* (commensal), and *M. fortuitum/chelonae* (potential pathogens causing skin/soft tissue abscesses). * **M. smegmatis:** Found in smegma; must be differentiated from *M. tuberculosis* in urine samples. * **Buruli Ulcer:** Caused by *M. ulcerans* (produces mycolactone toxin). * **Swimming Pool Granuloma:** Caused by *M. marinum*.

Q: Which one of the following enteric organisms is anaerogenic and non-motile?

Shigella sonnei. ### Explanation The correct answer is **Shigella sonnei**. In microbiology, **anaerogenic** refers to an organism that does not produce gas (CO₂ and H₂) during the fermentation of carbohydrates. **Non-motile** refers to the absence of flagella. **Why Shigella sonnei is correct:** All species of the genus *Shigella* are characteristically **non-motile** and **anaerogenic** (with the rare exception of certain *S. flexneri* serotypes). They are Gram-negative, non-lactose fermenting (NLF) bacilli (though *S. sonnei* is a late-lactose fermenter). Their lack of motility is a key diagnostic feature used to differentiate them from other Enterobacteriaceae. **Analysis of Incorrect Options:** * **Salmonella typhi:** While it is anaerogenic (it produces acid but no gas from glucose), it is **motile** via peritrichous flagella. * **Proteus mirabilis:** This organism is highly **motile** (exhibiting characteristic "swarming" on agar) and is **aerogenic** (produces gas during fermentation). * **Klebsiella pneumoniae:** While it is **non-motile** (due to a thick polysaccharide capsule), it is characteristically **aerogenic**, producing abundant gas during the fermentation of lactose and other sugars. **NEET-PG High-Yield Pearls:** * **The "Non-Motile" Rule:** Remember the mnemonic **"K-S-Y"** for non-motile Enterobacteriaceae: *Klebsiella*, *Shigella*, and *Yersinia* (at 37°C; *Yersinia* is motile at 25°C). * **Gas Production:** *Shigella* and *S. typhi* are the two major enteric pathogens that do **not** produce gas from glucose. * **S. sonnei specific:** It is the only *Shigella* species that is a **late-lactose fermenter** (forming pale pink colonies on MacConkey agar after 48 hours) and is Indole negative.

Q: Which of the following statements is true about Bacteroides?

It may cause peritonitis.. **Explanation:** **Bacteroides** (specifically *Bacteroides fragilis*) are the most common anaerobic bacteria found in the human colon. 1. **Why Option C is Correct:** *Bacteroides* species are commensals of the gastrointestinal tract. However, when the mucosal barrier is breached—due to trauma, surgery, or a ruptured appendix—they escape into the sterile peritoneal cavity. Their **capsular polysaccharide** is a key virulence factor that induces abscess formation, leading to **secondary peritonitis**. 2. **Why Other Options are Incorrect:** * **Option A:** *Bacteroides* are **Gram-negative bacilli**. They are often pleomorphic and pale-staining. * **Option B:** They are **obligate (strict) anaerobes**. They lack catalase and superoxide dismutase, making them unable to survive in high-oxygen environments. * **Option D:** Since *Bacteroides* are part of the **normal resident flora** of the colon (outnumbering *E. coli* 1000:1), their presence in a stool culture is a normal finding and does not indicate infection or a need for treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Bile Resistance:** *B. fragilis* is unique among anaerobes because it can grow in **20% bile**, a feature used for its laboratory identification (Bile Esculin Agar). * **Antibiotic Profile:** They are characteristically resistant to penicillin due to **beta-lactamase production**. The drug of choice is typically **Metronidazole**. * **LPS Activity:** Unlike most Gram-negative bacteria, *Bacteroides* LPS has low toxicity and does not typically cause classic endotoxic shock.

Q: A 44-year-old man develops intra-abdominal sepsis after undergoing difficult bowel resection and anastomosis. He is initially given ceftizoxime sodium, which is ineffective because of overgrowth of which of the following?

Pseudomonas. **Explanation:** The correct answer is **Pseudomonas (Option A)**. **Why Pseudomonas is correct:** Ceftizoxime is a **third-generation cephalosporin**. While third-generation cephalosporins have expanded Gram-negative coverage compared to earlier generations, they are not all created equal. Ceftizoxime, specifically, has excellent activity against many Enterobacteriaceae and anaerobes (like *Bacteroides*), but it lacks significant activity against **Pseudomonas aeruginosa**. In a clinical setting of intra-abdominal sepsis, if a patient is treated with a drug that covers most flora but misses *Pseudomonas*, that organism can undergo "selective overgrowth," leading to treatment failure. **Why the other options are incorrect:** * **Staphylococcus aureus (B):** While third-generation cephalosporins have less Gram-positive activity than first-generation ones, they still possess some efficacy. However, the specific "overgrowth" scenario in post-surgical bowel sepsis typically points toward a resistant Gram-negative rod. * **Neisseria gonorrhoeae (C):** This is a primary pathogen of the urogenital tract, not a common cause of post-surgical intra-abdominal sepsis. * **Bacteroides fragilis (D):** Ceftizoxime is unique among many third-generation cephalosporins because it actually has **good anaerobic activity**, including against *Bacteroides*. Therefore, *Bacteroides* would likely be covered, not overgrow. **NEET-PG High-Yield Pearls:** * **Anti-Pseudomonal Cephalosporins:** Only **Ceftazidime** (3rd gen) and **Cefepime** (4th gen) have reliable activity against *Pseudomonas*. * **Ceftizoxime vs. Cefotaxime/Ceftriaxone:** Ceftizoxime is often noted in exams for its better anaerobic profile compared to its peers. * **Intra-abdominal Sepsis:** Usually polymicrobial (Gram-negative rods + Anaerobes). If a drug covers anaerobes but lacks anti-pseudomonal activity, *Pseudomonas* is the most likely culprit for persistent infection.

Q: A female presents with leaking and meconium-stained liquor at 32 weeks gestation. Which organism is she most likely infected with?

Listeria monocytogenes. **Explanation:** The presence of **meconium-stained liquor (MSL)** in a preterm pregnancy (especially before 34 weeks) is a classic clinical red flag for **Listeria monocytogenes** infection. **Why Listeria monocytogenes is correct:** Listeria is a gram-positive motile bacillus typically transmitted via contaminated food (unpasteurized cheese, cold meats). In pregnancy, it has a unique tropism for the placenta. It causes "granulomatosis infantiseptica" and is notorious for inducing **amnionitis**. A hallmark of Listeria infection is the passage of meconium in utero by a preterm fetus, which is otherwise rare, as the fetal gastrointestinal tract is usually too immature to pass meconium before 34 weeks unless severely stressed by this specific pathogen. **Why other options are incorrect:** * **CMV (Option A):** The most common congenital infection; typically presents with periventricular calcifications, microcephaly, and IUGR, but not acute MSL. * **Toxoplasma gondii (Option C):** Presents with the classic triad of hydrocephalus, chorioretinitis, and intracranial calcifications. It does not typically cause preterm leaking with MSL. * **HSV (Option D):** Usually transmitted during delivery through the birth canal, leading to skin-eye-mouth (SEM) lesions or encephalitis in the neonate. It is not associated with preterm meconium passage. **Clinical Pearls for NEET-PG:** * **Listeria** is the only Gram-positive bacteria that produces an **endotoxin**. * It shows **"tumbling motility"** at 25°C. * **Drug of Choice:** Ampicillin (Listeria is inherently resistant to cephalosporins). * **High-Yield Association:** Preterm labor + Fever + Meconium-stained liquor = Think **Listeria**.

Q: A 70-year-old female patient was readmitted to a local hospital with fever and chills following cardiac surgery. Blood cultures were taken and a gram-positive coccus grew from the blood cultures within 24 hours. Initial tests indicated that this isolate was resistant to penicillin. Which of the following is the most likely identification?

Enterococcus. **Explanation:** The clinical presentation of a post-surgical patient with fever and chills, combined with the laboratory finding of **Gram-positive cocci (GPC)** resistant to penicillin, strongly points toward **Enterococcus**. 1. **Why Enterococcus is correct:** Enterococci (formerly Group D Streptococci) are notorious for their **intrinsic resistance** to many antibiotics, including most cephalosporins and, frequently, penicillins (due to low-affinity Penicillin-Binding Proteins). They are common causes of healthcare-associated infections, including endocarditis and bacteremia following invasive procedures or surgeries. Their ability to grow in 6.5% NaCl and hydrolyze bile esculin are key biochemical markers. 2. **Why the other options are incorrect:** * **Group A (S. pyogenes) and Group B (S. agalactiae) Streptococcus:** These remain **exquisitely sensitive to Penicillin G**. If a GPC is penicillin-resistant, these species are effectively ruled out in a standard exam context. * **Neisseria species:** These are **Gram-negative diplococci**, which contradicts the Gram stain finding (Gram-positive cocci) provided in the question. **High-Yield Clinical Pearls for NEET-PG:** * **Enterococcus faecalis** is the most common species isolated, but **Enterococcus faecium** is more likely to be multidrug-resistant (including Vancomycin resistance - VRE). * **Treatment:** For serious infections (like endocarditis), a combination of a cell-wall active agent (Penicillin/Ampicillin/Vancomycin) plus an **Aminoglycoside** is required for synergistic bactericidal action. * **Culture Characteristics:** They show **Gamma-hemolysis** (usually) on blood agar and can grow in harsh conditions (40% bile and 6.5% NaCl).

Q: Which of the following is known to cause Pedal Botryomycosis?

Staphylococcus aureus. **Explanation:** **Botryomycosis** is a rare, chronic granulomatous bacterial infection that clinically mimics deep fungal infections (mycetomas). Despite its name (which implies a fungal origin), it is a **bacterial infection** characterized by the formation of "grains" or "sulfur granules" within the tissues. 1. **Why Staphylococcus aureus is correct:** * **Staphylococcus aureus** is the most common causative agent of Botryomycosis (responsible for ~40% of cases). Other common bacteria include *Pseudomonas aeruginosa* and *E. coli*. * The pathogenesis involves a low-virulence strain of bacteria or a compromised host immune response, leading to a "Splendore-Hoeppli phenomenon"—an eosinophilic material surrounding the bacterial colonies, forming the characteristic grains. * **Pedal Botryomycosis** specifically refers to involvement of the foot, presenting with nodules, sinuses, and discharge containing grains, resembling a Madura foot. 2. **Analysis of Incorrect Options:** * **A & B (Actinomyces & Nocardia):** These are causes of **Actinomycetoma**. While they also produce grains and sinus tracts, they are filamentous bacteria (higher bacteria) and are distinct from the true bacterial botryomycosis. * **D (Candida glabrata):** This is a yeast. Botryomycosis is strictly a bacterial process; fungal causes of similar lesions are classified as **Eumycetoma**. **High-Yield Clinical Pearls for NEET-PG:** * **Key Histology:** Look for the **Splendore-Hoeppli phenomenon** (intense eosinophilic material around bacterial clusters). * **Differential Diagnosis:** Always differentiate from **Mycetoma**. In Botryomycosis, grains contain **cocci or bacilli**, whereas in Mycetoma, grains contain **fungal hyphae or filamentous bacteria**. * **Treatment:** Long-term antibiotics based on culture (usually targeting *S. aureus*).

Question 1

Which of the following mycobacteria are non-pathogenic to humans?

Accepted Answer

M. phlei

Answer

M. bovis

Answer

M. kansii

Answer

M. avium intracellular

Question 2

Regarding Proteus, which statement is true?

Accepted Answer

PPA test positive

Answer

Musty odor

Answer

Most common infection caused is respiratory tract infection

Answer

Esthiomene is a complication in females

Question 3

Which one of the following enteric organisms is anaerogenic and non-motile?

Accepted Answer

Shigella sonnei

Answer

Salmonella typhi

Answer

Proteus mirabilis

Answer

Klebsiella pneumonia

Question 4

Which of the following statements is true about Bacteroides?

Accepted Answer

It may cause peritonitis.

Answer

It is gram-positive bacilli.

Answer

It is strictly aerobic.

Answer

Presence in stool culture indicates a need for treatment.

Question 5

A 44-year-old man develops intra-abdominal sepsis after undergoing difficult bowel resection and anastomosis. He is initially given ceftizoxime sodium, which is ineffective because of overgrowth of which of the following?

Accepted Answer

Pseudomonas

Answer

Staphylococcus aureus

Answer

Neisseria gonorrhoeae

Answer

Bacteroides fragilis

Question 6

Which of the following is NOT true for Bordetella pertussis?

Accepted Answer

Infection can be prevented by an acellular vaccine.

Answer

It is a strict human pathogen.

Answer

It can be cultured from the patient during the catarrhal stage.

Answer

It leads to invasion of the respiratory mucosa.

Question 7

A female presents with leaking and meconium-stained liquor at 32 weeks gestation. Which organism is she most likely infected with?

Accepted Answer

Listeria monocytogenes

Answer

Cytomegalovirus (CMV)

Answer

Toxoplasma gondii

Answer

Herpes simplex virus (HSV)

Question 8

Which of the following statements about tetanus are true?

Accepted Answer

Vegetative cell will die with tetracycline

Answer

Gram-negative spore-forming aerobe

Answer

Caused by tetanospasmin

Answer

Muscle enzymes are normal

Question 9

A 70-year-old female patient was readmitted to a local hospital with fever and chills following cardiac surgery. Blood cultures were taken and a gram-positive coccus grew from the blood cultures within 24 hours. Initial tests indicated that this isolate was resistant to penicillin. Which of the following is the most likely identification?

Accepted Answer

Enterococcus

Answer

Group A streptococcus

Answer

Group B streptococcus

Answer

Neisseria species

Question 10

Which of the following is known to cause Pedal Botryomycosis?

Accepted Answer

Staphylococcus aureus

Answer

Actinomyces somaliensis

Answer

Nocardia asteroides

Answer

Candida glabrata

Bacteriology — MCQs

Bacteriology — MCQs

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