Bacteriology Practice Questions

Q: Which bacterium exhibits darting motility?

Campylobacter. **Explanation:** The correct answer is **Campylobacter**. Motility is a key diagnostic feature in bacteriology, often characterized by specific patterns under hanging drop preparation or dark-ground microscopy. **1. Why Campylobacter is correct:** *Campylobacter jejuni* (and *Vibrio cholerae*) exhibits a characteristic **"darting motility."** This rapid, purposeful movement is due to the presence of a single polar flagellum (monotrichous). Under the microscope, these bacteria appear to "dart" across the field of vision. Morphologically, they are described as "comma-shaped" or "seagull-wing" shaped, Gram-negative bacilli. **2. Analysis of Incorrect Options:** * **Listeria monocytogenes:** Exhibits **"tumbling motility"** at 25°C (room temperature) but is non-motile at 37°C. This is a classic NEET-PG favorite. * **Borrelia:** As a spirochete, it exhibits **corkscrew motility** (flexion-extension and rotation) facilitated by endoflagella (axial filaments) located in the periplasmic space. * **Mycoplasma:** These organisms lack a cell wall and are generally **non-motile**. However, some species (like *M. pneumoniae*) show a unique "gliding motility" on surfaces, but never darting motility. **3. High-Yield Clinical Pearls for NEET-PG:** * **Darting Motility:** *Vibrio cholerae* and *Campylobacter*. * **Tumbling Motility:** *Listeria monocytogenes*. * **Swarming Motility:** *Proteus mirabilis*, *Clostridium tetani*, and *Bacillus subtilis*. * **Stately Motility:** *Bacillus anthracis* (actually non-motile, but *B. cereus* shows stately movement). * **Falling Leaf Motility:** *Giardia lamblia* (Parasitology). * **Corkscrew/Lashing Motility:** *Treponema pallidum* and *Borrelia*.

Q: Which species of Shigella is aerogenic?

Shigella flexneri Type 6. ### Explanation **Correct Option: A (Shigella flexneri Type 6)** The genus *Shigella* is characteristically defined as **non-motile, non-capsulated, and non-aerogenic** (does not produce gas from glucose fermentation). However, there are rare exceptions to this rule that are frequently tested in competitive exams like NEET-PG. **Shigella flexneri Type 6** (specifically the Newcastle and Manchester biotypes) is the only member of the genus that can produce gas during carbohydrate fermentation, making it **aerogenic**. **Analysis of Incorrect Options:** * **B. Shigella dysenteriae Type 1:** Also known as *S. shigae*, it is the most virulent species and produces the Shiga toxin. It is strictly anaerogenic and is also catalase-negative (unlike other Shigella species). * **C. Shigella sonnei:** This is a Group D Shigella and is a late lactose fermenter. It is anaerogenic and typically causes milder "sonnei dysentery." * **D. Shigella boydii Type 4:** While *S. boydii* encompasses many serotypes, they are all characteristically anaerogenic. **High-Yield Clinical Pearls for NEET-PG:** * **The "Exceptions" Rule:** * Most *Shigella* are catalase positive; **Exception:** *S. dysenteriae* Type 1 (catalase negative). * Most *Shigella* are anaerogenic; **Exception:** *S. flexneri* Type 6 (aerogenic). * Most *Shigella* are non-lactose fermenters; **Exception:** *S. sonnei* (late lactose fermenter). * **Infective Dose:** *Shigella* has a very low infective dose (as few as 10–100 organisms), making it highly communicable via the fecal-oral route. * **Culture Media:** Deoxycholate Citrate Agar (DCA) and XLD agar are used, where *Shigella* appears as pale, colorless (NLF) colonies.

Q: What are the most commonly encountered bacterial pathogens in the lungs of patients with cystic fibrosis?

Staphylococcus and Pseudomonas. **Explanation:** Cystic Fibrosis (CF) is characterized by defective chloride transport (CFTR gene mutation), leading to thick, viscid endobronchial secretions. This impaired mucociliary clearance creates a niche for specific bacterial pathogens that colonize the lungs in a predictable age-related sequence. **Why Option D is Correct:** * **Staphylococcus aureus:** This is typically the **most common** pathogen isolated from the respiratory tract of infants and young children with CF. * **Pseudomonas aeruginosa:** This is the **most common** pathogen in adults and the leading cause of chronic pulmonary infection and progressive lung damage in CF patients. By early adulthood, up to 80% of CF patients are colonized with *Pseudomonas*. **Why Other Options are Incorrect:** * **Options A & B (Escherichia coli):** While *E. coli* can cause neonatal pneumonia, it is not a characteristic or primary pathogen associated with the chronic pulmonary pathology of CF. * **Option A (Alpha Streptococcus):** These are generally considered normal oral flora and are not primary drivers of CF lung disease. * **Option C (Proteus):** *Proteus* species are more commonly associated with urinary tract infections and are rarely implicated in CF respiratory exacerbations. **High-Yield NEET-PG Pearls:** 1. **Age Sequence:** *S. aureus* and *H. influenzae* dominate in childhood; *P. aeruginosa* dominates in adulthood. 2. **Burkholderia cepacia complex:** A highly feared pathogen in CF; it is associated with "Cepacia syndrome" (rapid clinical decline) and is often a contraindication for lung transplantation. 3. **Pseudomonas Phenotype:** In CF, *Pseudomonas* often converts to a **mucoid phenotype** (alginate production), which forms biofilms, making it highly resistant to antibiotics and host immune responses. 4. **Other Pathogens:** *Stenotrophomonas maltophilia* and *Aspergillus fumigatus* (ABPA) are also frequently encountered.

Q: Which of the following is NOT true about the Mantoux test?

The result is typically seen after 3 weeks.. The Mantoux test (Tuberculin Skin Test) is a screening tool for *Mycobacterium tuberculosis* infection. The correct answer is **B** because the statement is factually incorrect regarding the timing of the reaction. ### **Explanation of Options** * **Option B (Correct Answer):** The Mantoux test is a delayed-type hypersensitivity reaction. The result must be read **48 to 72 hours** (2–3 days) after the injection, not 3 weeks. If read after 72 hours, the result may be inaccurate due to the waning of the inflammatory response. * **Option A:** In India and many other regions, **PPD RT-23 (Purified Protein Derivative)** with **Tween 80** (a stabilizing detergent) is the standard reagent used. The standard dose is 2 Tuberculin Units (TU) in 0.1 ml. * **Option C:** In endemic areas like India, an induration (palpable raised hardening) of **10 mm or more** is generally considered positive. Therefore, a measurement of >9 mm is a standard threshold for a positive result in the general population. * **Option D:** The test is a classic clinical example of **Type IV (Delayed) Hypersensitivity**, mediated by T-lymphocytes (specifically Th1 cells) rather than antibodies. ### **High-Yield Clinical Pearls for NEET-PG** * **Injection Technique:** It is administered **intradermally** on the volar aspect of the forearm using a 26-gauge needle. * **Measurement:** Only the **induration** (the hard bump) is measured, not the erythema (redness). * **False Negatives:** Can occur in miliary TB, malnutrition, HIV/AIDS (low CD4 count), or recent viral infections (e.g., Measles). * **False Positives:** Can occur due to prior **BCG vaccination** or infection with Non-Tuberculous Mycobacteria (NTM). * **Booster Effect:** A second test may be positive if the first "primed" a dormant immune system; this is not a new infection.

Q: Lemierre's disease, an oropharyngeal infection, is most commonly caused by which of the following organisms?

F. necrophorum. **Explanation:** **Lemierre’s Disease** (also known as post-anginal septicemia) is a life-threatening condition characterized by a primary oropharyngeal infection (like tonsillitis or pharyngitis) that leads to **septic thrombophlebitis of the internal jugular vein (IJV)** and subsequent metastatic septic emboli, most commonly to the lungs. 1. **Why D is correct:** **_Fusobacterium necrophorum_** is the most common causative agent (found in >80% of cases). It is a Gram-negative, non-spore-forming, obligate anaerobic rod. It is highly virulent due to its ability to produce toxins (like leukocidin and hemolysin) and a potent endotoxin that promotes platelet aggregation and clot formation, leading to the characteristic thrombophlebitis. 2. **Why incorrect options are wrong:** * **A. *Streptococcus mutans*:** Primarily associated with dental caries and subacute bacterial endocarditis, not invasive oropharyngeal thrombophlebitis. * **B. *S. milleri* (Anginosus group):** While these can cause abscesses (brain, liver), they are less common primary drivers of Lemierre’s compared to *Fusobacterium*. * **C. *P. melanogenica* (Prevotella):** Though part of the oral flora and sometimes found in mixed anaerobic infections, it is not the classic primary pathogen for Lemierre’s syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Recent oropharyngeal infection + Internal Jugular Vein thrombosis + Metastatic septic emboli (Lungs are the #1 site). * **Microscopy:** *F. necrophorum* appears as pleomorphic, long, thin Gram-negative rods with rounded ends. * **Drug of Choice:** Metronidazole or Carbapenems (must cover anaerobes). * **Key Imaging:** Contrast-enhanced CT of the neck is the gold standard to visualize the IJV thrombus.

Q: Which of the following organisms can cause cervicitis?

Chlamydia. **Explanation:** **Cervicitis** is the inflammation of the uterine cervix, primarily categorized into ectocervicitis and endocervicitis. **Why Chlamydia is Correct:** *Chlamydia trachomatis* (Serotypes D-K) is the most common bacterial cause of **mucopurulent endocervicitis**. It is an obligate intracellular pathogen that specifically infects the columnar epithelium of the endocervical canal. Along with *Neisseria gonorrhoeae*, it is a major cause of Sexually Transmitted Infections (STIs) leading to Pelvic Inflammatory Disease (PID), infertility, and ectopic pregnancy. **Analysis of Incorrect Options:** * **A. Pseudomonas:** This is an opportunistic pathogen typically associated with nosocomial infections (UTIs or pneumonia), not primary cervicitis. * **B. Staphylococcus:** While *S. aureus* can be part of the vaginal flora or cause Toxic Shock Syndrome (TSS), it is not a recognized primary cause of infectious cervicitis. * **D. Trichomonas:** *Trichomonas vaginalis* primarily causes **vaginitis** and **ectocervicitis**. While it affects the cervix (producing the classic "Strawberry Cervix" or colpitis macularis), it involves the squamous epithelium of the exocervix rather than the endocervical canal. In the context of standard medical examinations, Chlamydia and Gonorrhea are the definitive answers for "cervicitis" (endocervicitis). **NEET-PG High-Yield Pearls:** 1. **Gold Standard Diagnosis:** Nucleic Acid Amplification Test (NAAT) is the investigation of choice for *Chlamydia*. 2. **Clinical Sign:** "Friable cervix" (bleeding on touch/swab) is a hallmark of infectious cervicitis. 3. **Treatment:** Azithromycin (1g stat) or Doxycycline (100mg BID for 7 days). Always treat the partner to prevent reinfection. 4. **Co-infection:** If *N. gonorrhoeae* is detected, always treat for *Chlamydia* empirically.

Q: A patient presented with trismus and opisthotonus. What is the probable causative organism?

Clostridium tetani. ### Explanation **Correct Answer: A. Clostridium tetani** The clinical presentation of **trismus** (lockjaw) and **opisthotonus** (generalized body arching due to muscle spasms) is pathognomonic for **Tetanus**, caused by *Clostridium tetani*. * **Mechanism:** *C. tetani* produces a potent exotoxin called **Tetanospasmin**. This toxin travels via retrograde axonal transport to the spinal cord, where it cleaves synaptobrevin (a SNARE protein). This inhibits the release of inhibitory neurotransmitters **GABA and Glycine** from Renshaw cells. The loss of inhibition leads to continuous motor neuron firing, resulting in spastic paralysis and muscle rigidity. **Why other options are incorrect:** * **B. Clostridium botulinum:** Causes Botulism, characterized by **symmetric descending flaccid paralysis** (not spastic). It acts at the neuromuscular junction by inhibiting Acetylcholine release. * **C. Clostridium difficile:** Primarily causes antibiotic-associated diarrhea and **pseudomembranous colitis** via Toxin A (enterotoxin) and Toxin B (cytotoxin). * **D. Clostridium perfringens:** Most commonly associated with **Gas Gangrene** (myonecrosis) and food poisoning. It produces Alpha-toxin (lecithinase). **NEET-PG High-Yield Pearls:** * **Risus Sardonicus:** An abnormal, sustained spasm of the facial muscles that appears to produce grinning. * **Morphology:** *C. tetani* is a Gram-positive, anaerobic bacillus with terminal spores, giving it a characteristic **"Drumstick" appearance**. * **Management:** Treatment involves wound debridement, Metronidazole (preferred over Penicillin as Penicillin is a GABA antagonist), and Tetanus Immunoglobulin (TIG). * **Neonatal Tetanus:** Often occurs due to unsterile umbilical cord cutting (Tetanus Neonatorum).

Q: Which of the following statements regarding Mycoplasma is false?

Penicillins inhibit Mycoplasma.. **Explanation:** **1. Why Option A is False (The Correct Answer):** The defining characteristic of *Mycoplasma* is the **complete absence of a cell wall**. Penicillins and other beta-lactam antibiotics (like Cephalosporins) act by inhibiting peptidoglycan synthesis in the bacterial cell wall. Since *Mycoplasma* lacks this target structure, it is **intrinsically resistant** to all cell-wall-acting agents. This is a high-yield concept for NEET-PG: they are treated with protein synthesis inhibitors like Macrolides (Azithromycin) or Tetracyclines. **2. Analysis of Other Options:** * **Option B:** Unlike Chlamydia or Rickettsia, *Mycoplasma* are **not obligate intracellular** parasites. They are the smallest free-living organisms and can be grown on enriched cell-free media (e.g., PPLO agar containing sterols). * **Option C:** *Mycoplasma* species (especially *M. pneumoniae*) possess specialized attachment organelles (like P1 adhesin) that allow them to adhere strongly to the respiratory epithelium and other mammalian cell membranes. * **Option D:** Due to their small size (0.15–0.3 µm) and **pleomorphic** nature (lack of a rigid cell wall allows them to squeeze through tight spaces), they easily pass through filters (450 nm) that typically retain other bacteria. **Clinical Pearls for NEET-PG:** * **Sterols:** *Mycoplasma* is the only bacterium that requires sterols (cholesterol) in its cell membrane for stability. * **Fried Egg Appearance:** Classic morphology of colonies on agar. * **Cold Agglutinins:** *M. pneumoniae* infection is associated with IgM antibodies that agglutinate RBCs at 4°C. * **Eaton’s Agent:** Another name for *M. pneumoniae*.

Question 1

Which bacterium exhibits darting motility?

Accepted Answer

Campylobacter

Answer

Listeria

Answer

Borrelia

Answer

Mycoplasma

Question 2

A 6.5-month-old infant presented with altered behavior, cyanosis, weak cry, drooling, diminished feeding, and constipation for 1 week. On examination, shallow respiration, generalized flaccidity, absent deep tendon reflexes, and drooping eyelids were noted. There was no history of fever, cough, or coryza. The prenatal and antenatal periods were uncomplicated, and developmental milestones were progressing normally. The child was recently started on pureed foods along with small amounts of honey. Laboratory and CSF studies were normal. SERS-based immunoassay revealed a positive test. The motility of the implicated organism is due to which type of flagella?

Accepted Answer

Lophotrichous

Answer

Peritrichial

Answer

Monotrichous

Answer

Amphitrichous

Question 3

Which species of Shigella is aerogenic?

Accepted Answer

Shigella flexneri Type 6

Answer

Shigella dysenteriae Type 1

Answer

Shigella sonnei

Answer

Shigella boydii Type 4

Question 4

What are the most commonly encountered bacterial pathogens in the lungs of patients with cystic fibrosis?

Accepted Answer

Staphylococcus and Pseudomonas

Answer

Escherichia coli and alpha Streptococcus

Answer

Escherichia coli and Pseudomonas

Answer

Staphylococcus and Proteus

Question 5

Which of the following is NOT true about the Mantoux test?

Accepted Answer

The result is typically seen after 3 weeks.

Answer

PPD RT-23 with Tween 80 is used as the strain.

Answer

An induration of > 9mm is considered positive.

Answer

It is an example of Type IV Hypersensitivity.

Question 6

Lemierre's disease, an oropharyngeal infection, is most commonly caused by which of the following organisms?

Accepted Answer

F. necrophorum

Answer

Streptococcus mutans

Answer

S. milleri

Answer

P. melanogenica

Question 7

A 5-year-old child presents with acute onset of high fever, abdominal cramps, and malaise. The child passes small amounts of bloody mucopurulent stools with increasing tenesmus. What is the most likely cause?

Accepted Answer

Shigella dysenteriae

Answer

Rotavirus

Answer

Vibrio cholerae

Answer

Entamoeba histolytica

Question 8

Which of the following organisms can cause cervicitis?

Accepted Answer

Chlamydia

Answer

Pseudomonas

Answer

Staphylococcus

Answer

Trichomonas

Question 9

A patient presented with trismus and opisthotonus. What is the probable causative organism?

Accepted Answer

Clostridium tetani

Answer

Clostridium botulinum

Answer

Clostridium difficile

Answer

Clostridium perfringens

Question 10

Which of the following statements regarding Mycoplasma is false?

Accepted Answer

Penicillins inhibit Mycoplasma.

Answer

Mycoplasma can reproduce in cell-free media.

Answer

Mycoplasma have an affinity for mammalian cell membranes.

Answer

Mycoplasma can pass through filters of 450 nm pore size.

Bacteriology — MCQs

Bacteriology — MCQs

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