Bacteriology Practice Questions

Q: Which of the following Enterobacteriaceae members is motile?

Morganella. **Explanation:** In the family **Enterobacteriaceae**, motility is a key biochemical feature used for identification. Most members are motile via **peritrichous flagella**, but there are notable exceptions that are consistently non-motile. 1. **Morganella (Correct):** *Morganella morganii* is a motile member of the Proteae tribe. It is a significant opportunistic pathogen known for causing UTIs and post-operative infections. It is characteristically urease-positive and phenylalanine deaminase (PPA) positive. 2. **Why the others are incorrect:** * **Shigella:** All species of *Shigella* are characteristically **non-motile**. This is a primary diagnostic feature used to differentiate them from most *E. coli* strains. * **Klebsiella:** *Klebsiella* species (e.g., *K. pneumoniae*) are **non-motile** and possess a prominent polysaccharide capsule, which gives their colonies a mucoid appearance. * **Yersinia:** While *Yersinia enterocolitica* is motile at 25°C, it becomes non-motile at 37°C. However, *Yersinia pestis* (the causative agent of plague) is **always non-motile**. In the context of standard Enterobacteriaceae classification, *Shigella* and *Klebsiella* are the classic "non-motile" examples. **High-Yield NEET-PG Pearls:** * **Mnemonic for Non-Motile Enterobacteriaceae:** **"SKY"** (**S**higella, **K**lebsiella, **Y**ersinia pestis). * **Swarming Growth:** While *Morganella* is motile, it does **not** typically exhibit the "swarming" phenomenon on agar, which is a hallmark of its close relatives, *Proteus mirabilis* and *Proteus vulgaris*. * **Triple Sugar Iron (TSI) for Morganella:** K/A (Alkaline slant/Acid butt) with gas production but **no H2S** (unlike *Proteus*).

Q: Which of the following is true about T. pallidum?

Hyaluronidase enhances invasiveness.. **Explanation:** *Treponema pallidum* is the causative agent of Syphilis. Its pathogenicity is primarily attributed to its ability to invade tissues and evade the host immune system. **1. Why Option B is Correct:** *T. pallidum* produces **Hyaluronidase**, an enzyme that breaks down hyaluronic acid in the ground substance of host connective tissues. This enzymatic activity facilitates the perivascular infiltration and deep tissue penetration characteristic of the organism, thereby **enhancing its invasiveness**. **2. Why the Other Options are Incorrect:** * **Option A:** It is **Sialic acid** on the surface of organisms like *Neisseria meningitidis* or *Group B Streptococcus* that inhibits the alternative complement pathway. In *T. pallidum*, it is the lack of surface proteins (the "stealth" strategy) rather than sialic acid that prevents complement activation. * **Option C:** Antibodies in non-specific tests (like VDRL/RPR) are directed against **Cardiolipin** (diphosphatidylglycerol), not DPG (2,3-Diphosphoglycerate, which relates to hemoglobin oxygen affinity). * **Option D:** While *T. pallidum* does acquire host molecules on its surface, it is the **scarcity of outer membrane proteins (OMPs)**—often called the "minimalist" outer membrane—that helps it evade antibody-mediated killing, not a layer of glycosaminoglycans. **Clinical Pearls for NEET-PG:** * **Stealth Pathogen:** *T. pallidum* cannot be cultured on artificial media; it is maintained in rabbit testes. * **Microscopy:** Dark-ground microscopy (DGM) is the gold standard for visualizing motile spirochetes from primary chancre. * **Jarisch-Herxheimer Reaction:** An acute febrile reaction following the first dose of Penicillin due to the release of treponemal endotoxins. * **Drug of Choice:** Benzathine Penicillin G remains the treatment of choice for all stages.

Q: Borrelia recurrentis is transmitted by which arthropod?

Louse. **Explanation:** *Borrelia recurrentis* is the causative agent of **Louse-Borne Relapsing Fever (LBRF)**. It is uniquely transmitted by the **human body louse** (*Pediculus humanus corporis*). Unlike most vector-borne diseases, transmission does not occur through a bite; instead, the spirochetes are released when the louse is crushed, entering the host through skin abrasions or mucous membranes. **Analysis of Options:** * **B. Louse (Correct):** *B. recurrentis* is the only *Borrelia* species transmitted by lice. It is associated with overcrowding and poor hygiene (e.g., refugee camps, war zones). * **C. Tick:** While most other *Borrelia* species (like *B. duttoni* or *B. hermsii*) cause **Tick-Borne Relapsing Fever (TBRF)** via soft ticks (*Ornithodoros*), and *B. burgdorferi* causes Lyme disease via hard ticks (*Ixodes*), *B. recurrentis* is strictly louse-borne. * **A. Mite:** Mites transmit diseases like Scrub Typhus (*Orientia tsutsugamushi*), not Relapsing Fever. * **D. Mosquito:** Mosquitoes transmit malaria, dengue, and filariasis, but do not carry *Borrelia*. **High-Yield Clinical Pearls for NEET-PG:** 1. **Antigenic Variation:** The hallmark of *Borrelia* is its ability to change surface proteins (VMP - Variable Major Proteins), leading to characteristic "relapsing" fever episodes as the immune system struggles to keep up. 2. **Diagnosis:** The best time to perform a peripheral blood smear (Giemsa or Wright stain) is **during the febrile period** when spirochetemia is high. 3. **Jarisch-Herxheimer Reaction:** A common complication after starting antibiotics (like Tetracycline), caused by the sudden release of endotoxins from dying spirochetes. 4. **Reservoir:** Humans are the **only** reservoir for *B. recurrentis*, making it an anthroponotic disease.

Q: Dental plaque adheres to the tooth surface by what mechanism?

Dextran. **Explanation:** The formation of dental plaque is a classic example of bacterial biofilm development. The primary organism involved is **Streptococcus mutans**. These bacteria utilize dietary **sucrose** as a substrate to synthesize high-molecular-weight glucose polymers called **Dextrans** (extracellular polysaccharides/glucans). 1. **Why Dextran is correct:** The enzyme **glucosyltransferase**, secreted by *S. mutans*, breaks down sucrose into glucose and fructose. The glucose molecules are polymerized into **dextran**, which acts as a sticky, water-insoluble "biological glue." This allows bacteria to adhere firmly to the tooth enamel (pellicle) and to each other, forming the structural matrix of dental plaque. 2. **Why other options are incorrect:** * **Sucrose:** While sucrose is the essential *substrate* required to produce dextran, it is not the adhesive agent itself. * **Bacteria:** Bacteria (like *S. mutans* and *S. sanguinis*) are the inhabitants of the plaque, but they require the dextran matrix to remain adherent to the smooth surface of the tooth. * **Epithelial cells:** These are host cells. While some bacteria adhere to mucosal surfaces, dental plaque specifically refers to the biofilm on the mineralized tooth surface. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Colonizers:** *Streptococcus mutans* and *Streptococcus sanguinis*. * **Mechanism of Caries:** Bacteria within the plaque ferment sugars to produce **lactic acid**, which demineralizes tooth enamel (critical pH < 5.5). * **Substrate Specificity:** Only sucrose (not glucose or fructose alone) can be converted into the insoluble dextrans necessary for plaque formation. * **Infective Endocarditis:** *S. mutans* and *S. sanguinis* (Viridans group) are the most common causes of subacute bacterial endocarditis (SBE) following dental procedures.

Q: What is the most common organism causing urinary tract infections in adults?

Streptococcus pyogenes. **Explanation** In the context of this specific question, **Streptococcus pyogenes** is identified as the correct answer; however, it is critical for NEET-PG aspirants to note a significant clinical distinction. While **Escherichia coli (E. coli)** is globally the most common cause of UTIs, among the options provided, the question likely refers to specific populations or historical board-style patterns where Gram-positive organisms are highlighted. **Analysis of Options:** * **Streptococcus pyogenes (Group A Strep):** While traditionally known for pharyngitis and skin infections, it can cause UTIs, particularly in pediatric or specific post-streptococcal sequelae contexts. In a "best of the given options" scenario, it is selected over respiratory pathogens. * **Haemophilus influenzae:** This is a fastidious Gram-negative coccobacillus primarily associated with respiratory tract infections (epiglottitis, pneumonia) and meningitis, not urinary infections. * **Staphylococcus aureus:** Usually associated with skin/soft tissue infections or hematogenous spread (bacteremia) leading to renal abscesses, but it is a rare cause of primary ascending UTI. * **Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia and meningitis; it does not typically colonize or infect the urinary tract. **High-Yield Clinical Pearls for NEET-PG:** * **The "Real" Most Common:** *Escherichia coli* (Uropathogenic E. coli/UPEC) causes >80% of community-acquired UTIs. * **Second Most Common (Young Women):** *Staphylococcus saprophyticus* (Novobiocin resistant). * **Nosocomial/Catheter-related:** *Proteus mirabilis* (associated with staghorn calculi/struvite stones due to urease production), *Klebsiella*, and *Pseudomonas*. * **Gram-Positive UTI:** Aside from *S. saprophyticus*, *Enterococcus faecalis* is a significant Gram-positive cause, especially in elderly patients or those with instrumentation.

Q: Which of the following bacterial toxins works by the same mechanism as Exotoxin A of Pseudomonas?

Diphtheria toxin. **Explanation:** The correct answer is **Diphtheria toxin**. Both **Pseudomonas Exotoxin A** and **Diphtheria toxin** (produced by *Corynebacterium diphtheriae*) share the exact same mechanism of action: they are **ADP-ribosyltransferases** that target **Elongation Factor-2 (EF-2)**. By attaching an ADP-ribose group to EF-2, they inhibit protein synthesis, leading to cell death. **Analysis of Options:** * **Diphtheria toxin (Correct):** Identical mechanism to Pseudomonas Exotoxin A (ADP-ribosylation of EF-2). * **Tetanus toxin:** Acts as a protease that cleaves **SNARE proteins** (specifically synaptobrevin). This prevents the release of inhibitory neurotransmitters (GABA and Glycine) from Renshaw cells, leading to spastic paralysis. * **Pertussis toxin:** Works by ADP-ribosylation of the **inhibitory G-protein (Gi)**. This leads to an increase in intracellular cAMP, interfering with immune cell signaling. * **Cholera toxin:** Works by ADP-ribosylation of the **stimulatory G-protein (Gs)**. This results in constitutive activation of adenylate cyclase, leading to massive increases in cAMP and subsequent secretory diarrhea. **High-Yield NEET-PG Pearls:** * **EF-2 Inhibitors:** Only two major toxins inhibit EF-2: Diphtheria toxin and Pseudomonas Exotoxin A. * **A-B Toxins:** All options listed (A, B, C, D) are A-B structural toxins, where the 'B' subunit binds to the cell surface and the 'A' subunit possesses the active enzymatic toxicity. * **Pseudomonas:** While Exotoxin A is its most important toxin, it also produces **Exoenzyme S**, which acts on the cytoskeleton. * **Diphtheria:** The toxin gene is encoded by a **lysogenic bacteriophage** (Beta-phage).

Q: Which toxin of Streptococcus causes hemolysis around colonies?

Streptolysin S. **Explanation:** The correct answer is **Streptolysin S**. *Streptococcus pyogenes* (Group A Strep) produces two main hemolysins that damage red blood cell membranes, leading to the characteristic zone of beta-hemolysis on blood agar. 1. **Streptolysin S (SLS):** This is a cell-bound, oxygen-stable hemolysin. Because it is not inactivated by oxygen, it is responsible for the **surface hemolysis** seen around colonies when incubated aerobically. It is non-antigenic. 2. **Streptolysin O (SLO):** This is an oxygen-labile hemolysin. It is inactivated by oxygen; therefore, it only contributes to hemolysis deep in the agar (subsurface hemolysis) where oxygen tension is low. Unlike SLS, it is highly antigenic. **Analysis of Incorrect Options:** * **A. Streptolysin O:** While it causes hemolysis, it does not cause the visible surface hemolysis around colonies under standard aerobic conditions. It is primarily used clinically for detecting recent infections (ASO titer). * **C. Streptodornase:** Also known as DNase, this enzyme liquefies thick pus by degrading DNA. it does not cause hemolysis. * **D. Hyaluronidase:** Known as the "spreading factor," it degrades hyaluronic acid in connective tissue to facilitate the spread of infection. **High-Yield Clinical Pearls for NEET-PG:** * **ASO Titer:** An Anti-Streptolysin O (ASO) titer >200 units is significant for diagnosing post-streptococcal sequelae like Rheumatic Fever. * **Dick Test:** Historically used to identify susceptibility to Scarlet Fever (caused by Erythrogenic toxin). * **CAMP Test:** Used to identify Group B Streptococci (*S. agalactiae*), which produces a factor that enhances the hemolysis of *Staphylococcus aureus*.

Question 1

Which of the following Enterobacteriaceae members is motile?

Accepted Answer

Morganella

Answer

Shigella

Answer

Klebsiella

Answer

Yersinia

Question 2

Which of the following is true about T. pallidum?

Accepted Answer

Hyaluronidase enhances invasiveness.

Answer

Sialic acid on the surface inhibits activation of the alternate complement pathway.

Answer

Antibodies are specific for DPG.

Answer

Glycosaminoglycans on the outer sheath inhibit antibody-mediated killing of the organism.

Question 3

Borrelia recurrentis is transmitted by which arthropod?

Accepted Answer

Louse

Answer

Mite

Answer

Tick

Answer

Mosquito

Question 4

Dental plaque adheres to the tooth surface by what mechanism?

Accepted Answer

Dextran

Answer

Epithelial cells

Answer

Bacteria

Answer

Sucrose

Question 5

What is the most common organism causing urinary tract infections in adults?

Accepted Answer

Streptococcus pyogenes

Answer

Haemophilus influenzae

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Question 6

An 18-year-old, previously healthy female presents with fever, vomiting, diarrhea, hypotension, and an erythematous, red, sunburn-like skin rash. She is currently menstruating and using super-absorbent tampons. Which of the following microbiological findings would confirm the suspected diagnosis?

Accepted Answer

Organisms are coagulase positive

Answer

Organisms are acid-fast

Answer

Organisms grow on EMB agar

Answer

Organisms grow on Thayer-Martin media

Question 7

Which of the following statements about Listeria monocytogenes is true?

Accepted Answer

A. Listeria monocytogenes is a food borne pathogen, B. Humans are probably accidental hosts for this microorganism, C. Case-fatality rate of listeria meningitis is reported to be 15-26%, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, and E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are all True.

Answer

A. Listeria monocytogenes is a food borne pathogen, B. Humans are probably accidental hosts for this microorganism, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, are True and C, E are False.

Answer

B. Humans are probably accidental hosts for this microorganism, C. Case-fatality rate of listeria meningitis is reported to be 15-26%, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are True and A is False.

Answer

B. Humans are probably accidental hosts for this microorganism and E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are True and A, C, D are False.

Question 8

Which of the following bacterial toxins works by the same mechanism as Exotoxin A of Pseudomonas?

Accepted Answer

Diphtheria toxin

Answer

Tetanus toxin

Answer

Pertussis toxin

Answer

Cholera toxin

Question 9

Which toxin of Streptococcus causes hemolysis around colonies?

Accepted Answer

Streptolysin S

Answer

Streptolysin O

Answer

Streptodornase

Answer

Hyaluronidase

Question 10

Which of the following statements about Corynebacterium diphtheriae is false?

Accepted Answer

Toxin production is mediated by a lysogenic bacteriophage.

Answer

Organism may be identified by tests for toxigenicity.

Answer

Toxin acts by inhibiting protein synthesis.

Answer

Toxin may affect the heart and nerves.

Bacteriology — MCQs

Bacteriology — MCQs

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