Bacteriology Practice Questions

Q: Which bacterium, other than Vibrio cholerae, possesses flagellar motility?

Campylobacter jejuni. **Explanation:** The correct answer is **Campylobacter jejuni**. Bacterial motility is primarily mediated by flagella, and identifying specific patterns of motility is a high-yield topic for NEET-PG. **1. Why Campylobacter jejuni is correct:** *Campylobacter jejuni* is a Gram-negative, curved (comma or S-shaped) rod. It possesses a single polar flagellum at one or both ends, which gives it a characteristic **"darting motility"** (similar to *Vibrio cholerae*). This motility is a crucial virulence factor, allowing the bacteria to penetrate the thick mucus layer of the gastrointestinal tract. **2. Why the other options are incorrect:** * **Shigella:** All species of *Shigella* are **non-motile**. They lack flagella. In the laboratory, they are identified by their lack of movement in semi-solid agar. * **Pneumococcus (*Streptococcus pneumoniae*):** This is a Gram-positive coccus. Almost all medically important cocci are **non-motile**. * **Bacillus anthracis:** While many species in the *Bacillus* genus are motile (e.g., *B. cereus*), *B. anthracis* is a notable exception. It is **non-motile**, a key feature used to differentiate it from "anthracoid" bacilli. **High-Yield Clinical Pearls for NEET-PG:** * **Vibrio cholerae:** Exhibits "darting motility" (liquid) or "swarming" (solid media). * **Listeria monocytogenes:** Shows "tumbling motility" at 25°C but is non-motile at 37°C. * **Proteus species:** Famous for "swarming motility" on blood agar. * **Salmonella Typhi:** Exhibits "stately motility." * **Trichomonas vaginalis:** A parasite (not bacteria) that also shows "twitching/jerky motility."

Q: A patient with diarrhea has a fecal smear that is negative for leukocytes. The patient's diarrhea is most likely caused by what?

Enterotoxigenic Escherichia coli. **Explanation:** The presence or absence of fecal leukocytes is a critical diagnostic marker in differentiating the pathophysiology of infectious diarrhea. **1. Why Enterotoxigenic *Escherichia coli* (ETEC) is correct:** ETEC is the classic cause of **secretory (watery) diarrhea**. Its pathogenesis involves the production of enterotoxins (Heat-labile/LT and Heat-stable/ST) that stimulate intestinal secretion of water and electrolytes without causing histological damage to the mucosa. Because there is **no mucosal invasion or inflammation**, no inflammatory cells (leukocytes) are recruited into the intestinal lumen. This results in a "bland" stool smear. **2. Why the other options are incorrect:** * **Shigellosis & Campylobacter enteritis:** These are **invasive/inflammatory** pathogens. They invade the intestinal epithelium and produce cytotoxins, leading to cell death and an intense inflammatory response. This typically results in dysentery (blood and mucus) with a high count of fecal polymorphonuclear leukocytes (neutrophils). * **Typhoid fever:** While primarily a systemic illness, *Salmonella Typhi* involves the invasion of Peyer’s patches. Fecal smears in enteric fever typically show a predominance of **mononuclear cells (macrophages/lymphocytes)** rather than being negative for leukocytes. **Clinical Pearls for NEET-PG:** * **Fecal Leukocytes Present:** *Shigella, Salmonella, Campylobacter, EIEC, Clostridium difficile, Yersinia.* * **Fecal Leukocytes Absent:** *ETEC, Vibrio cholerae*, Viruses (Rotavirus, Norovirus), and Parasites (*Giardia, Cryptosporidium*). * **High-Yield Mnemonic:** ETEC = **E**laborates **T**oxins (Secretory), whereas *Shigella/Campylobacter* = **I**nvade (Inflammatory). * ETEC is the most common cause of **Traveler’s Diarrhea**.

Q: Stalactite growth is by:

Yersinia. **Explanation:** **Correct Option: A (Yersinia)** The term **"Stalactite growth"** is a classic laboratory characteristic of *Yersinia pestis* (the causative agent of Plague). When *Y. pestis* is grown in a liquid medium (nutrient broth) containing a layer of oil or ghee on the surface, the bacteria grow downwards from the oil droplets in the form of hanging, icicle-like projections. This occurs because the organism is non-motile and prefers the interface for initial attachment. **Analysis of Incorrect Options:** * **B. Mycoplasma:** These organisms are known for their **"Fried egg" appearance** on solid agar (PPLO agar) due to the central part of the colony growing into the medium. * **C. Diphtheria:** *Corynebacterium diphtheriae* shows specific growth on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite agar**, where it forms black/grey colonies. It does not exhibit stalactite growth. * **D. Clostridia:** These are obligate anaerobes. *C. perfringens* is famous for **"Stormy fermentation"** in litmus milk and **"Target hemolysis"** (double zone) on blood agar. **High-Yield Clinical Pearls for NEET-PG:** * **Yersinia pestis Morphology:** Shows characteristic **"Safety pin appearance"** (bi-polar staining) with Wayson or Giemsa stain. * **Temperature Sensitivity:** It is optimally motile at 25°C but **non-motile at 37°C** (the temperature of the human body). * **Virulence Factors:** Look for **Fraction 1 (F1) capsular antigen** and **V and W antigens**, which are essential for resisting phagocytosis. * **Public Health:** It is a potential bioterrorism agent and is transmitted by the rat flea (*Xenopsylla cheopis*).

Q: Which of the following conditions is NOT caused by Chlamydia trachomatis?

Vulvitis. **Explanation:** *Chlamydia trachomatis* (Serotypes D-K) is an obligate intracellular bacterium that specifically targets **columnar and transitional epithelium**. This tissue tropism dictates the clinical manifestations of the infection. **1. Why Vulvitis is the Correct Answer:** The vulva is covered by **keratinized stratified squamous epithelium**. *Chlamydia* cannot infect these cells; therefore, it does not cause primary vulvitis. Vulvitis is more commonly associated with fungal infections (Candidiasis), viral infections (HSV), or chemical irritants. **2. Why the other options are incorrect:** * **Cervicitis (Option C):** The endocervix is lined by columnar epithelium, making it the most common site of Chlamydial infection in women. It often presents as mucopurulent cervicitis. * **Salpingitis (Option A):** If left untreated, the infection ascends to the fallopian tubes (lined by ciliated columnar epithelium), leading to Pelvic Inflammatory Disease (PID) and salpingitis. * **Urethritis (Option D):** The urethra is lined by transitional and columnar epithelium, making it a primary site for Chlamydial infection in both males and females (Non-gonococcal urethritis). **Clinical Pearls for NEET-PG:** * **Silent Infection:** Up to 70-80% of Chlamydial infections in women are asymptomatic, leading to "silent" PID and subsequent tubal factor infertility. * **Fitz-Hugh-Curtis Syndrome:** A complication of Chlamydial PID involving peri-hepatitis ("violin-string" adhesions). * **Reiter’s Syndrome:** A triad of Urethritis, Conjunctivitis, and Arthritis (can follow Chlamydial infection). * **Drug of Choice:** Azithromycin (1g single dose) or Doxycycline (100mg BID for 7 days). In pregnancy, Azithromycin is preferred.

Q: Which Streptococcus species is primarily responsible for causing dental caries?

Streptococci mutans. **Explanation:** **Streptococcus mutans** is the primary etiologic agent of dental caries. The underlying medical concept involves its ability to metabolize dietary sucrose into **extracellular polysaccharides (glucans)** using the enzyme glucosyltransferase. These glucans allow the bacteria to adhere tenaciously to the tooth enamel, forming a biofilm known as **dental plaque**. Once attached, *S. mutans* ferments sugars to produce lactic acid; this localized acid production drops the pH below 5.5, leading to the demineralization of tooth enamel and subsequent cavity formation. **Analysis of Incorrect Options:** * **A. S. equisimilis:** A Group C or G Streptococcus that typically causes pharyngitis or skin infections, similar to *S. pyogenes*, but is not associated with oral pathology. * **C. S. pneumoniae:** A major cause of community-acquired pneumonia, meningitis, and otitis media. It is alpha-hemolytic but lacks the acidogenic properties required for dental caries. * **D. S. bovis:** A Group D Streptococcus. Its clinical significance lies in its strong association with **colonic malignancy** (specifically *S. gallolyticus*) and endocarditis, not dental health. **High-Yield Clinical Pearls for NEET-PG:** * **Viridans Group:** *S. mutans* belongs to the Viridans group Streptococci (VGS), which are normal flora of the oropharynx. * **Subacute Bacterial Endocarditis (SBE):** If *S. mutans* or *S. sanguinis* enters the bloodstream (e.g., during dental procedures), they can cause SBE, particularly in patients with pre-existing valvular heart disease. * **Dextran Production:** The synthesis of high-molecular-weight dextrans from sucrose is the key virulence factor for plaque formation.

Question 1

A boy presents with fever, throat pain, and difficulty breathing. Examination reveals cervical adenopathy and a characteristic throat finding. Despite hospitalization, he later dies of heart failure. Histopathological examination (HPE) reveals specific findings. Which other organism produces a toxin that functions similarly to the toxin produced by the organism identified in the HPE?

Accepted Answer

Pseudomonas exotoxins

Answer

Heat-labile enterotoxin produced by E. coli

Answer

Cholera toxin

Answer

Verocytotoxin produced by E. coli

Question 2

What is true about E. coli?

Accepted Answer

Urinary tract infections (UTIs) are usually caused by a single serotype at a time.

Answer

Sereny test is done to diagnose Enterotoxigenic E. coli (ETEC).

Answer

Enteropathogenic E. coli (EPEC) are also enteroinvasive.

Answer

E. coli is the most common causative organism to infect burn wounds.

Question 3

Which bacterium, other than Vibrio cholerae, possesses flagellar motility?

Accepted Answer

Campylobacter jejuni

Answer

Shigella

Answer

Pneumococcus

Answer

Bacillus anthrax

Question 4

Which of the following is true about Campylobacter jejuni?

Accepted Answer

Most often occurs several days after consumption of undercooked chicken.

Answer

Gram-positive bacillus

Answer

Symptoms may initially mimic cholecystitis

Answer

Penicillin is the drug of choice

Question 5

A patient with diarrhea has a fecal smear that is negative for leukocytes. The patient's diarrhea is most likely caused by what?

Accepted Answer

Enterotoxigenic Escherichia coli

Answer

Campylobacter enteritis

Answer

Shigellosis

Answer

Typhoid fever

Question 6

Stalactite growth is by:

Accepted Answer

Yersinia

Answer

Mycoplasma

Answer

Diphtheria

Answer

Clostridia

Question 7

Which of the following conditions is NOT caused by Chlamydia trachomatis?

Accepted Answer

Vulvitis

Answer

Salpingitis

Answer

Cervicitis

Answer

Urethritis

Question 8

Which Streptococcus species is primarily responsible for causing dental caries?

Accepted Answer

Streptococci mutans

Answer

Streptococci equisimilis

Answer

Streptococci pneumoniae

Answer

Streptococci bovis

Question 9

A 12-year-old boy with sickle cell disease presents with pain and swelling of the right lower extremity. A bone scan reveals osteomyelitis of the tibial diaphysis. Which organism is found more commonly in these cases than in the general population?

Accepted Answer

Salmonella species

Answer

Haemophilus influenzae

Answer

Klebsiella species

Answer

Bacteroides species

Question 10

What is the most common organism seen in peritonitis?

Accepted Answer

Escherichia coli

Answer

Clostridium welchii

Answer

Staphylococci

Answer

Klebsiella

Bacteriology — MCQs

Bacteriology — MCQs

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