Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1731: Drug resistance in Mycobacterium tuberculosis is due to which mechanism?

A. Conjugation
B. Transformation
C. Transduction
D. Chromosomal mutation (Correct Answer)

Explanation: **Explanation:** **1. Why Chromosomal Mutation is Correct:** Drug resistance in *Mycobacterium tuberculosis* (MTB) is unique because it occurs almost exclusively due to **spontaneous chromosomal mutations**. Unlike many other bacteria, MTB does not possess horizontal gene transfer mechanisms like plasmids or transposons. These mutations occur at a predictable frequency (e.g., 1 in $10^6$ to $10^8$ cell divisions). When a patient is treated with inadequate monotherapy or irregular dosing, these pre-existing mutant strains are "selected" and multiply, leading to **acquired drug resistance**. **Key gene mutations to remember:** * **katG / inhA:** Isoniazid resistance * **rpoB:** Rifampicin resistance (the "surrogate marker" for MDR-TB) * **embB:** Ethambutol resistance * **pncA:** Pyrazinamide resistance **2. Why Other Options are Incorrect:** * **Conjugation (A):** This involves the transfer of genetic material (plasmids) via direct cell-to-cell contact (sex pili). This is the most common way resistance spreads in Enterobacteriaceae (e.g., *E. coli*), but it does **not** occur in MTB. * **Transformation (B):** This is the uptake of naked DNA from the environment. While common in *S. pneumoniae* and *Neisseria*, it is not a mechanism for MTB resistance. * **Transduction (C):** This is DNA transfer mediated by a bacteriophage. While mycobacteriophages exist, they do not play a role in clinical drug resistance. **3. Clinical Pearls for NEET-PG:** * **MDR-TB:** Defined as resistance to at least **Isoniazid (INH) and Rifampicin**. * **XDR-TB:** MDR-TB plus resistance to any **Fluoroquinolone** AND at least one **Group A drug** (Bedaquiline or Linezolid). * **GeneXpert (CBNAAT):** Detects MTB and Rifampicin resistance by identifying mutations in the **rpoB gene**. * **Multi-drug therapy:** The biological rationale for using multiple drugs in TB is to prevent the selection of these spontaneous chromosomal mutants.

Question 1732: Shiga toxin is produced by which organism?

A. Enteropathogenic E. coli
B. Enteroinvasive E. coli
C. Enterohemorrhagic E. coli (Correct Answer)
D. Enterotoxigenic E. coli

Explanation: **Explanation:** The correct answer is **Enterohemorrhagic E. coli (EHEC)**. EHEC (specifically the O157:H7 serotype) produces **Shiga toxin** (Stx1 and Stx2), also known as Verocytotoxin. This toxin is identical to the toxin produced by *Shigella dysenteriae* type 1. It acts by inhibiting the 60S ribosomal subunit, leading to the cessation of protein synthesis and cell death. Clinically, this results in hemorrhagic colitis and the potentially fatal **Hemolytic Uremic Syndrome (HUS)**, characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Analysis of Incorrect Options:** * **Enteropathogenic E. coli (EPEC):** Does not produce toxins. It causes diarrhea in infants by using **intimin** (an adhesin) to cause "attachment and effacement" (A/E) lesions on intestinal villi. * **Enteroinvasive E. coli (EIEC):** Does not produce Shiga toxin. It acts similarly to *Shigella* by invading the colonic epithelium using actin polymerization, causing inflammatory diarrhea with blood and mucus (dysentery). * **Enterotoxigenic E. coli (ETEC):** Produces two distinct toxins: **Heat-labile (LT)**, which increases cAMP, and **Heat-stable (ST)**, which increases cGMP. It is the leading cause of **Traveler’s diarrhea**. **High-Yield Clinical Pearls for NEET-PG:** * **HUS Warning:** Antibiotics are generally avoided in EHEC infections as they may trigger increased Shiga toxin release, worsening the risk of HUS. * **Sorbitol Agar:** EHEC (O157:H7) is unique because it **cannot ferment sorbitol**, appearing as colorless colonies on Sorbitol-MacConkey (SMAC) agar. * **Mnemonic for ETEC:** "**L**abile **A**mp, **S**table **G**mp" (**L**ook **A**t **S**uper **G**irl).

Question 1733: Which test is used to differentiate staphylococci from micrococci?

A. Catalase test
B. Hugh and Leifson's oxidation-fermentation test (Correct Answer)
C. Gram staining
D. Acid-fast staining (AFS)

Explanation: **Explanation:** The differentiation between **Staphylococci** and **Micrococci** is a common high-yield topic in bacteriology. Both are Gram-positive, catalase-positive cocci, necessitating specialized biochemical tests for distinction. **Why Option B is correct:** The **Hugh and Leifson’s Oxidation-Fermentation (OF) test** is the gold standard for this differentiation. * **Staphylococci** are facultative anaerobes; they utilize glucose **fermentatively** (producing acid in both open and sealed tubes). * **Micrococci** are obligate aerobes; they utilize glucose **oxidatively** (producing acid only in the open tube) or are non-saccharolytic. **Why other options are incorrect:** * **A. Catalase test:** Both Staphylococci and Micrococci are catalase-positive. This test is used to differentiate them from Streptococci (catalase-negative). * **C. Gram staining:** Both appear as Gram-positive cocci. While Staphylococci typically form "grape-like clusters" and Micrococci often form tetrads or sarcina (packets of eight), this is not definitive for differentiation. * **D. Acid-fast staining (AFS):** This is used for Mycobacteria and Nocardia. Neither Staphylococci nor Micrococci are acid-fast. **NEET-PG High-Yield Pearls:** To differentiate these two genera, remember the **"MODS"** criteria for **Micrococci**: 1. **M**icrodase (Modified Oxidase) test: **Positive** (Staph is negative). 2. **O**xidation-Fermentation test: **Oxidative** (Staph is Fermentative). 3. **D**ecolourisation by Lysostaphin: **Resistant** (Staph is Sensitive). 4. **S**ensitivity to Bacitracin (0.04U): **Sensitive** (Staph is Resistant). 5. **Furazolidone sensitivity:** Micrococci are **Resistant**, while Staphylococci are **Sensitive**.

Question 1734: Where do Neisseria meningitidis bacteria normally reside?

A. Skin
B. Genitals
C. Nasopharynx (Correct Answer)
D. Lower gastrointestinal tract

Explanation: **Explanation:** *Neisseria meningitidis* (Meningococcus) is a Gram-negative diplococcus that is an obligate human pathogen. The correct answer is **Nasopharynx** because this site serves as the primary natural reservoir for the bacteria. 1. **Why Nasopharynx is correct:** Approximately 10% to 20% of the healthy population are asymptomatic nasopharyngeal carriers. The bacteria attach to the non-ciliated columnar epithelium of the nasopharynx using **pili**. From here, they can either remain as commensals or, following a viral respiratory infection or smoking-induced mucosal damage, cross the mucosal barrier to enter the bloodstream (causing meningococcemia) and subsequently the blood-brain barrier (causing meningitis). 2. **Why other options are incorrect:** * **Skin:** The skin is dominated by *Staphylococcus epidermidis* and *Propionibacterium acnes*. *N. meningitidis* does not colonize dry skin. * **Genitals:** This is the primary site for *Neisseria gonorrhoeae*. While *N. meningitidis* can rarely cause urogenital infections, it is not its "normal" residence. * **Lower GI Tract:** This area is dominated by anaerobes (e.g., *Bacteroides*) and Enterobacteriaceae (e.g., *E. coli*). **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Occurs via **respiratory droplets** or direct contact with secretions. * **Virulence Factor:** The **polysaccharide capsule** is the most important virulence factor (antiphagocytic). Note: *N. gonorrhoeae* is non-capsulated. * **Vaccine:** Based on capsular polysaccharides (Groups A, C, Y, W-135). Group B is poorly immunogenic due to its similarity to human neural cell adhesion molecules. * **Prophylaxis:** **Rifampicin** is the drug of choice for eliminating the carrier state in close contacts; Ciprofloxacin or Ceftriaxone are alternatives.

Question 1735: What is the most common bacterium causing urinary tract infections in sexually active young women?

A. Staphylococcus saprophyticus (Correct Answer)
B. Staphylococcus aureus
C. Staphylococcus epidermidis
D. Staphylococcus citrus

Explanation: **Explanation:** **Staphylococcus saprophyticus** is the correct answer because it is the second most common cause of uncomplicated urinary tract infections (UTIs) in young, sexually active women, trailing only *Escherichia coli*. It is a coagulase-negative staphylococcus (CoNS) that possesses specific adhesins (like Uroplakin) allowing it to adhere effectively to the uroepithelium. In the context of NEET-PG, the phrase "sexually active young woman" is a classic clinical trigger for *S. saprophyticus*, often referred to as "Honeymoon Cystitis." **Analysis of Incorrect Options:** * **Staphylococcus aureus:** While a potent pathogen, it is an uncommon cause of UTI. When found in urine, it usually suggests hematogenous spread (seeding from the blood) or a secondary infection following instrumentation. * **Staphylococcus epidermidis:** This is the most common CoNS found on the skin. It typically causes infections related to prosthetic devices (valves, shunts) or indwelling catheters, rather than primary UTIs in healthy young women. * **Staphylococcus citrus:** This is an obsolete name for *Staphylococcus saprophyticus* (based on its yellow pigment), but it is not a standard clinical term used in modern microbiology or exams. **High-Yield Clinical Pearls for NEET-PG:** * **Novobiocin Resistance:** *S. saprophyticus* is characteristically **resistant** to Novobiocin, which distinguishes it from *S. epidermidis* (which is sensitive). * **Biochemical Profile:** It is Catalase-positive but Coagulase-negative. * **Nitrite Test:** Unlike Gram-negative bacteria (like *E. coli*), *Staphylococci* do not reduce nitrates to nitrites; therefore, the urine dipstick nitrite test is usually **negative**.

Question 1736: All of the following are true about V. cholerae O139 except?

A. Clinical manifestations are similar to O1 El tor.
B. First discovered in Chennai. (Correct Answer)
C. Produces O139 lipopolysaccharide.
D. Epidemiologically indistinguishable from O1 El tor.

Explanation: **Explanation:** *Vibrio cholerae* O139 (also known as the **Bengal strain**) emerged in 1992 as the first non-O1 strain to cause a large-scale epidemic of cholera. **Why Option B is the correct answer (The False Statement):** *V. cholerae* O139 was first identified in **Madras (now Chennai)** in October 1992; however, the question asks for the "except" statement. While it was identified in Chennai, it was simultaneously/primarily recognized as a new epidemic strain in **Vellore** and **Calcutta**. In the context of NEET-PG, the "discovery" is historically attributed to the **Bay of Bengal region** (hence the name "Bengal strain"), but the specific distinction often tested is that it originated from a genetic shift in an O1 El Tor strain, not that it was exclusively a "Chennai discovery" in isolation from the broader Bengal epidemic. (Note: In many standard textbooks, it is cited as appearing first in Madras/Vellore, but it is epidemiologically distinct from O1). **Analysis of other options:** * **Option A:** Clinical manifestations are identical to O1 El Tor, presenting as "rice-water stools" and severe dehydration. * **Option C:** Unlike O1, O139 produces a unique **O139 lipopolysaccharide (LPS)** and a distinct **capsule**, which provides resistance to complement-mediated killing. * **Option D:** Epidemiologically, it behaves exactly like O1 El Tor, causing explosive outbreaks and possessing the same potential for pandemic spread. **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** O139 evolved from the **O1 El Tor** strain by acquiring a new O-antigen gene cluster. * **Capsule:** O139 is **encapsulated**, whereas O1 is not. This is a frequent exam question. * **Resistance:** It is typically resistant to **Trimethoprim-Sulfamethoxazole** and **Streptomycin**. * **Vaccine:** Standard oral cholera vaccines (like Shanchol) now include both O1 and O139 components to ensure protection.

Question 1737: Syphilis was first identified by whom?

A. Fraenkel
B. Nicolaescu
C. Schaudinn and Hoffman (Correct Answer)
D. Ogston

Explanation: **Explanation:** The correct answer is **Schaudinn and Hoffman**. In 1905, German zoologist Fritz Schaudinn and dermatologist Erich Hoffmann discovered *Treponema pallidum*, the causative agent of syphilis, in tissue fluid from patients with primary and secondary syphilis. They utilized Giemsa stain and dark-ground microscopy to visualize the thin, corkscrew-shaped spirochete, which was previously invisible under standard light microscopy. **Analysis of Incorrect Options:** * **Fraenkel:** Albert Fraenkel is primarily known for his work on *Streptococcus pneumoniae* (Pneumococcus) and describing its role in lobar pneumonia. * **Nicolaescu:** While a less common name in standard microbiology textbooks, Stefan Nicolaescu was a Romanian physician who worked on syphilis treatments, but he did not discover the organism. * **Ogston:** Sir Alexander Ogston is credited with the discovery of *Staphylococcus aureus* in 1880, identifying it as the cause of localized abscesses and wound infections. **Clinical Pearls for NEET-PG:** * **Morphology:** *Treponema pallidum* is a thin, spiral-shaped bacterium (spirochete) with 6–14 regular spirals and pointed ends. * **Microscopy:** It cannot be seen under a light microscope due to its thinness. **Dark-ground microscopy (DGM)** is the gold standard for immediate diagnosis from primary chancre or secondary rashes (condyloma lata). * **Staining:** It is Gram-negative but stains poorly. **Silver impregnation stains** (e.g., Fontana or Levaditi stains) are used to visualize it in tissues. * **Cultivation:** It cannot be grown on artificial culture media. It is maintained by serial passage in rabbit testes (Nichol’s strain).

Question 1738: Which of the following is hypothesized to have a strong association with hemolytic uremic syndrome?

A. Salmonellae
B. Bacteriophages and corynebacteria
C. Verocytotoxin producing E. coli (Correct Answer)
D. Clostridium difficile

Explanation: **Explanation:** **Hemolytic Uremic Syndrome (HUS)** is a clinical triad consisting of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Why Option C is correct:** The most common cause of HUS is infection with **Verocytotoxin-producing *E. coli* (VTEC)**, also known as Shiga toxin-producing *E. coli* (STEC). The most notorious serotype is **O157:H7**. These bacteria produce Shiga-like toxins (Verotoxins) that enter the bloodstream and bind to **Gb3 receptors**, which are highly expressed on glomerular endothelial cells. This leads to endothelial damage, microthrombi formation, and subsequent renal failure and red cell hemolysis. **Why the other options are incorrect:** * **A. Salmonellae:** While *Salmonella* causes significant gastroenteritis and enteric fever, it is not typically associated with the pathogenesis of HUS. * **B. Bacteriophages and corynebacteria:** While bacteriophages carry the *tox* gene for *Corynebacterium diphtheriae* (lysogenic conversion), this relationship causes Diphtheria, not HUS. * **C. Clostridium difficile:** This organism is the leading cause of pseudomembranous colitis, primarily due to Toxin A (enterotoxin) and Toxin B (cytotoxin), but it does not cause HUS. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** HUS is often associated with the consumption of undercooked ground beef or contaminated raw milk/vegetables. * **Diagnosis:** VTEC/STEC can be screened on **Sorbitol MacConkey (SMAC) agar**, where *E. coli* O157:H7 appears as **colorless colonies** (non-sorbitol fermenter). * **Treatment Warning:** Antibiotics and antimotility agents are generally avoided in STEC diarrhea as they may increase toxin release and the risk of HUS. * **Atypical HUS:** Not all HUS is infectious; atypical HUS (aHUS) is caused by uncontrolled complement activation.

Question 1739: What is the most common cause of dysentery?

A. Shigella dysenteriae (Correct Answer)
B. Entamoeba histolytica
C. Salmonella
D. Campylobacter

Explanation: **Explanation:** The term **dysentery** refers to an inflammatory disorder of the gastrointestinal tract characterized by small-volume stools containing blood and mucus, often accompanied by tenesmus and abdominal cramps. **Why Shigella dysenteriae is correct:** *Shigella* species are the most common cause of bacillary dysentery worldwide. Among them, **Shigella dysenteriae Type 1 (Shiga bacillus)** is the most potent pathogen. It produces the **Shiga toxin**, which causes extensive capillary damage and mucosal ulceration in the colon, leading to the classic presentation of bloody diarrhea. While *S. sonnei* is common in developed countries, *S. dysenteriae* remains a significant cause of severe outbreaks in developing regions. **Analysis of Incorrect Options:** * **Entamoeba histolytica:** This causes **Amoebic dysentery**. While a significant cause, it is less common globally than bacillary (Shigella) dysentery. On microscopy, it is distinguished by the presence of quadrinucleate cysts or trophozoites with ingested RBCs. * **Salmonella:** *Salmonella enteritidis* typically causes food poisoning (gastroenteritis) with watery diarrhea. While *Salmonella Typhi* causes systemic Enteric Fever, it rarely presents as acute dysentery. * **Campylobacter:** *Campylobacter jejuni* is a common cause of bacterial gastroenteritis and can cause bloody stools, but it is statistically less frequent than *Shigella* as a primary cause of classic dysentery. **High-Yield Clinical Pearls for NEET-PG:** * **Infective Dose:** *Shigella* has a very low infective dose (as few as 10–100 organisms), making it highly contagious via the fecal-oral route. * **Site of Infection:** Primarily involves the **large intestine** (distal colon). * **Complication:** *S. dysenteriae* Type 1 is strongly associated with **Hemolytic Uremic Syndrome (HUS)** due to Shiga toxin. * **Culture:** Use **Deoxycholate Citrate Agar (DCA)** or **SS Agar**; colonies are typically pale/colorless (Non-Lactose Fermenters).

Question 1740: Which of the following is a non-motile bacterium?

A. Helicobacter pylori
B. Klebsiella pneumoniae (Correct Answer)
C. Vibrio Cholerae
D. E.Coli

Explanation: **Explanation:** The correct answer is **Klebsiella pneumoniae**. In medical microbiology, motility is a key diagnostic feature used to differentiate genera within the *Enterobacteriaceae* family. **Why Klebsiella is the correct answer:** *Klebsiella* species (along with *Shigella*) are the classic examples of **non-motile** Gram-negative bacilli. They lack flagella and instead possess a thick polysaccharide capsule, which is their primary virulence factor. On culture media, this capsule gives them a characteristic "mucoid" appearance. **Analysis of Incorrect Options:** * **Helicobacter pylori:** Highly motile via multiple **unipolar (lophotrichous) flagella**, which allow it to burrow through the gastric mucus layer. * **Vibrio cholerae:** Demonstrates a characteristic **"darting motility"** due to a single polar flagellum. This is a high-yield diagnostic feature seen in hanging drop preparations. * **E. coli:** Most strains are motile via **peritrichous flagella** (distributed over the entire surface), though some specific strains (like enteroinvasive *E. coli*) may be non-motile. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Non-motile Bacteria:** "**S**ky **K**ites **A**re **B**eautiful" (**S**higella, **K**lebsiella, **A**nthrax, **B**rucella/Bordetella). * **Swarming Motility:** Characteristic of *Proteus mirabilis* and *Vibrio alginolyticus*. * **Tumbling Motility:** Characteristic of *Listeria monocytogenes* (at 25°C). * **Stately Motility:** Characteristic of *Bacillus anthracis* (though generally considered non-motile, this refers to its relatives/lookalikes). * **Corkscrew/Falling Leaf Motility:** Characteristic of *Spirochetes* and *Giardia* respectively.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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