Bacteriology Practice Questions

Q: A patient was diagnosed with Escherichia coli O157:H7 infection. In this designation, what does the "H" stand for?

Flagella. **Flagella** - The "H" in bacterial designations like *Escherichia coli* O157:H7 refers to the **H antigen**, which is associated with bacterial **flagella**. - Flagellar antigens are important for serotyping bacteria, identifying different strains based on their unique flagellar protein compositions. *Lipopolysaccharide (LPS)* - The "O" in bacterial designations like *E. coli* O157 refers to the **O antigen**, which is part of the **lipopolysaccharide (LPS)** layer on the outer membrane of Gram-negative bacteria. - LPS is a major component of the bacterial outer membrane and acts as an **endotoxin**. *Capsule* - The "K" in some bacterial serotypes (e.g., *Klebsiella pneumoniae* K1) refers to the **K antigen**, which is associated with the **capsule**. - The capsule is a protective layer surrounding some bacteria that helps them evade phagocytosis. *Fimbriae* - **Fimbriae** (also known as pili) are short, hair-like appendages on the surface of some bacteria that aid in **adhesion** to host cells. - While fimbriae have their own antigenic properties, they are not typically designated by the "H" prefix in standard serotyping, which specifically refers to flagellar antigens.

Q: A patient was admitted with bloody diarrhea after consumption of oysters. The organism exhibits the Kanagawa phenomenon. What is the correct organism?

Vibrio parahaemolyticus. ***Vibrio parahaemolyticus*** - This organism is a common cause of **gastroenteritis** linked to consumption of **raw or undercooked seafood**, especially oysters, and is known for causing **bloody diarrhea**. - The **Kanagawa phenomenon** refers to the production of **thermostable direct hemolysin (TDH)** by *V. parahaemolyticus*, which is a key virulence factor causing hemolysis and is detectable by a specific agar assay. *Staphylococcus aureus* - While *S. aureus* can cause **food poisoning** through its toxins, it typically presents with **nausea, vomiting**, and **abdominal cramps**, not bloody diarrhea. - Food poisoning from *S. aureus* is usually rapid-onset (within hours) and is associated with **improperly handled foods**, particularly those requiring no cooking after preparation (e.g., salads, custards). *Campylobacter jejuni* - This bacterium is a common cause of **bacterial gastroenteritis** and can lead to **bloody diarrhea**, but it's typically acquired from **poultry** or contaminated water, not specifically oysters. - **Guillain-Barré syndrome** is a serious complication that can follow *C. jejuni* infection, which is not characteristic of *Vibrio* infections. *Salmonella typhi* - *Salmonella typhi* causes **typhoid fever**, a systemic illness characterized by **sustained fever, headache, malaise**, and can lead to a "rose spot" rash. - While it can cause gastrointestinal symptoms, **bloody diarrhea is not the predominant symptom**, and raw oyster consumption is not its typical mode of transmission; it's usually spread through contaminated food or water by asymptomatic carriers.

Q: An adult patient with a military background is admitted with a rash, fever, altered sensorium, and a deficiency of the membrane attack complex. What is the most likely etiological agent?

Neisseria meningitidis. ***Neisseria meningitidis*** - A deficiency in the **membrane attack complex (MAC)**, particularly **C5-C9 components**, predisposes individuals to recurrent infections with encapsulated bacteria, especially *N. meningitidis*. - *N. meningitidis* is a common cause of **meningitis**, presenting with **fever, altered sensorium**, and often a **petechial rash** due to disseminated intravascular coagulation (DIC), which align with the patient's symptoms. *Klebsiella pneumoniae* - While *K. pneumoniae* can cause severe infections, including pneumonia and meningitis, it is not specifically associated with **MAC deficiency**. - Its infections more commonly manifest as **severe pneumonia** or **urinary tract infections** in immunocompromised patients. *Haemophilus influenzae* - *H. influenzae* can cause meningitis and other invasive infections, especially in children, but it is not typically linked to **MAC deficiency**. - The classic presentation involving **rash** and severe systemic symptoms as described is more characteristic of **meningococcal disease**. *CMV* - **Cytomegalovirus (CMV)** is a herpesvirus that causes a wide range of diseases, particularly in immunocompromised individuals. - However, CMV infections are primarily associated with **cellular immunity defects** rather than a deficiency in the **membrane attack complex** of the complement system.

Q: A male patient presents with white discharge from the urethra, as shown in the image. What is the most probable causative organism?

Neisseria gonorrhoeae. ***Neisseria gonorrhoeae*** - Among the given options, *Neisseria gonorrhoeae* is the **most probable causative organism** for **urethral discharge** in males. - Gonorrhea is a common sexually transmitted infection (STI) presenting with **purulent urethritis**, typically with thick, yellow-green discharge, though appearance can vary. - **Clinical note:** While classic gonococcal discharge is purulent and yellow-green, the clinical presentation can vary. None of the other organisms listed cause urethritis with discharge. *Haemophilus ducreyi* - This bacterium causes **chancroid**, a sexually transmitted infection characterized by **painful genital ulcers (chancres)** and **inguinal lymphadenopathy**, not urethral discharge. - Presents with ulcerative lesions, not discharge. *Klebsiella granulomatis* - This organism is responsible for **donovanosis** (granuloma inguinale), which manifests as **painless, progressive ulcerative lesions** on the genitals. - It does not cause urethral discharge; presents with beefy red granulomatous lesions. *Treponema pallidum* - This spirochete causes **syphilis**, which presents with **painless chancres** in the primary stage, **maculopapular rash** in the secondary stage, and gummas or neurological symptoms in later stages. - Urethral discharge is not a typical symptom of syphilis; primary lesions are ulcerative.

Q: A group of people ate patty late at night and experienced bouts of vomiting early in the morning. What is the most likely cause?

Staphylococcus aureus. ***Staphylococcus aureus*** - This scenario describes classic **Staphylococcal food poisoning** with rapid onset of vomiting (4-8 hours after ingestion). - *S. aureus* produces preformed **heat-stable enterotoxins** in contaminated foods left at room temperature, particularly **meat products, pastries, and cream-filled items**. - Clinical hallmark: **Prominent vomiting** with minimal diarrhea and rapid resolution (24 hours). - The timing (late night eating → early morning vomiting) and food item (patty) make this the **definitive diagnosis**. *Escherichia coli* - Most pathogenic *E. coli* strains (ETEC, EHEC, EPEC) have **longer incubation periods** (1-3 days). - Typically presents with **diarrhea as the predominant symptom**, not isolated vomiting. - The rapid onset of symptoms excludes direct bacterial infection and indicates preformed toxin ingestion. *Lactobacillus* - These are **probiotic bacteria** and part of normal human flora (gut, vagina, mouth). - Not pathogenic and not associated with food poisoning. - Used therapeutically to prevent antibiotic-associated diarrhea. *Bacillus cereus* - While *B. cereus* has an emetic form with rapid onset (1-5 hours), it is **classically associated with reheated fried rice** and starchy foods, not meat products. - The emetic toxin (cereulide) is produced in improperly stored rice, not patties. - The food item (patty) clearly points to *S. aureus* rather than *B. cereus*.

Q: A patient presented with 70% burns, and a sample was collected from the burn site. The image shows Gram-negative rods, and the suspected organism is an obligate aerobe. What is the most likely causative microbe?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - The image shows **Gram-negative rods**, and the patient has extensive **burns**, making *Pseudomonas aeruginosa* a highly likely causative agent due to its common association with burn wound infections. - *Pseudomonas aeruginosa* is an **obligate aerobe** and thrives in moist environments, making it a frequent colonizer of burn wounds, which are large, often moist surfaces. *Neisseria meningitidis (Meningococcus)* - *Neisseria meningitidis* is a **Gram-negative coccus**, typically appearing as diplococci, not rods, on Gram stain. - While it can cause severe infections, it is primarily associated with **meningitis** and **sepsis**, not typically burn wound infections. *Streptococcus pneumoniae (Pneumococcus)* - *Streptococcus pneumoniae* is a **Gram-positive coccus**, appearing as lancet-shaped diplococci or short chains, which contradicts the Gram-negative rod morphology seen in the image. - It is a common cause of **pneumonia** and **otitis media**, not primarily associated with burn wound infections. *Streptococcus pyogenes* - *Streptococcus pyogenes* is a **Gram-positive coccus** that grows in chains, which is inconsistent with the Gram-negative rod morphology. - Although it can cause skin infections like cellulitis and impetigo, it is not a typical cause of **burn wound infections** in the way *Pseudomonas aeruginosa* is.

Q: A patient presents with a history of pastry intake causing food poisoning. What is the most likely causative agent?

Staphylococcus aureus. ***Staphylococcus aureus*** - *S. aureus* is a common cause of food poisoning linked to **creamy foods** like pastries, salads, and custards, as it produces **heat-stable enterotoxins** when allowed to proliferate. - The symptoms, typically rapid onset **nausea, vomiting**, and abdominal cramps, occur because of the **preformed toxins** in the food, not necessarily active infection. *Verotoxin-producing E. coli* - This strain, often **E. coli O157:H7**, is primarily associated with **undercooked beef** or contaminated produce, and typically causes **bloody diarrhea** and can lead to hemolytic uremic syndrome (HUS). - Its mechanism involves **verotoxins** directly damaging intestinal cells and blood vessels, which is different from the rapid, emetic-focused symptoms of *S. aureus* food poisoning. *Bacillus cereus* - *B. cereus* causes two main types of food poisoning: **emetic (vomiting)**, typically from **reheated rice**, and **diarrheal**, from meat products or vegetables. - While the emetic form can cause vomiting, it is most strongly associated with **rice dishes** and usually has a shorter incubation period than the diarrheal form, making *Staphylococcus aureus* a more classic cause for pastry-related outbreaks. *Enteroinvasive E. coli (EIEC)* - EIEC causes a disease similar to **shigellosis**, involving direct invasion of intestinal epithelial cells, leading to **bloody diarrhea** and fever. - It is typically spread through contaminated food and water and not specifically linked to pastry intake or characterized by the rapid onset emetic symptoms seen with preformed toxins.

Q: Which of the following is a gram-positive organism that shows the following appearance on Ziehl-Neelsen staining?

Nocardia. ***Nocardia*** * The image displays delicate, branching, **filamentous rods** that are stained **red/pink** against a blue background, which is characteristic of partially acid-fast organisms like *Nocardia* on a Ziehl-Neelsen stain. * *Nocardia* species are **gram-positive**, aerobic bacteria that can cause opportunistic infections, particularly in immunocompromised individuals. They are distinguished by their **partial acid-fastness** due to their mycolic acid content, similar to mycobacteria but to a lesser degree. * The characteristic **branching filamentous morphology** combined with partial acid-fastness on Ziehl-Neelsen staining is pathognomonic for *Nocardia*. *Incorrect: Mycobacterium tuberculosis* * While *M. tuberculosis* is **strongly acid-fast** on Ziehl-Neelsen staining (appearing red), it is **not truly gram-positive**—it is gram-variable or weakly gram-positive. * *Mycobacterium* appears as **straight or slightly curved rods**, NOT branching filaments like those shown in the image. *Incorrect: Actinomyces* * *Actinomyces* is a **gram-positive**, filamentous, branching organism that can morphologically resemble *Nocardia*. * However, *Actinomyces* is **NOT acid-fast** and would appear **blue** (not red/pink) on Ziehl-Neelsen staining as it takes up the counterstain. * *Actinomyces* is also anaerobic, whereas *Nocardia* is aerobic. *Incorrect: Rhodococcus* * *Rhodococcus* is a gram-positive organism that can show **partial acid-fastness**, similar to *Nocardia*. * However, *Rhodococcus* typically appears as **coccoid to short rods**, occasionally forming short chains, but does NOT show the extensive **branching filamentous** pattern characteristic of *Nocardia* seen in the image.

Q: Which of the following microorganisms will be resistant to meropenem and aminoglycosides but sensitive to piperacillin tazobactam and cotrimoxazole?

Stenotrophomonas. ***Stenotrophomonas maltophilia*** - *Stenotrophomonas maltophilia* exhibits **intrinsic resistance to carbapenems (like meropenem)** due to the presence of L1 and L2 metallo-beta-lactamases and chromosomally encoded beta-lactamases. - It is **resistant to aminoglycosides** via aminoglycoside-modifying enzymes and efflux pump mechanisms. - **Trimethoprim-sulfamethoxazole (cotrimoxazole) is the drug of choice** with consistent susceptibility, making it the first-line treatment. - **Susceptibility to piperacillin-tazobactam is variable** - while some isolates may show in vitro susceptibility, clinical efficacy is inconsistent and it is not considered a reliable first-line agent. Among the options given, this organism best fits the described pattern. *Pseudomonas aeruginosa* - **Generally susceptible to carbapenems (meropenem) and aminoglycosides**, which are important therapeutic options. - Does not match the resistance pattern described in the question. *Acinetobacter baumannii* - Shows **multidrug resistance including carbapenems and aminoglycosides** in most clinical isolates. - However, typically also **resistant to piperacillin-tazobactam and cotrimoxazole**, making it inconsistent with the described susceptibility pattern. *Burkholderia cepacia complex* - Exhibits **intrinsic resistance to multiple antibiotics** including aminoglycosides and often carbapenems. - **Variable and often resistant to piperacillin-tazobactam**, and susceptibility to cotrimoxazole is inconsistent. - Does not reliably match the described antibiotic profile.

Q: A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

Stenotrophomonas. ***Stenotrophomonas*** - *Stenotrophomonas maltophilia* is intrinsically **resistant to azithromycin** and other macrolides due to its efflux pumps. - It is known to be sensitive to **ticarcillin-clavulanic acid** (a beta-lactam/beta-lactamase inhibitor combination) and **cotrimoxazole** (trimethoprim-sulfamethoxazole), which are common treatment options. *Pseudomonas* - *Pseudomonas aeruginosa* is generally **resistant to macrolides like azithromycin** but can exhibit varying sensitivity to antibiotics. - However, it often shows resistance to cotrimoxazole and ticarcillin-clavulanic acid is not a first-line agent, and it would typically be sensitive to other antipseudomonal beta-lactams (e.g., piperacillin-tazobactam, carbapenems). *Staphylococcus* - Many *Staphylococcus* species, particularly **MRSA**, are resistant to azithromycin. - However, *Staphylococcus* species are typically sensitive to other antibiotics like **vancomycin**, and are generally not sensitive to ticarcillin-clavulanic acid, and their susceptibility to cotrimoxazole varies depending on the species and resistance mechanisms. *Mycoplasma* - *Mycoplasma* species are **intrinsically resistant to beta-lactam antibiotics like ticarcillin** because they lack a cell wall, which is the target of these drugs. - They are typically sensitive to macrolides like azithromycin and also to cotrimoxazole.

Question 1

A patient was diagnosed with Escherichia coli O157:H7 infection. In this designation, what does the "H" stand for?

Accepted Answer

Flagella

Answer

Lipopolysaccharide (LPS)

Answer

Capsule

Answer

Fimbriae

Question 2

A patient was admitted with bloody diarrhea after consumption of oysters. The organism exhibits the Kanagawa phenomenon. What is the correct organism?

Accepted Answer

Vibrio parahaemolyticus

Answer

Staphylococcus aureus

Answer

Campylobacter jejuni

Answer

Salmonella typhi

Question 3

An adult patient with a military background is admitted with a rash, fever, altered sensorium, and a deficiency of the membrane attack complex. What is the most likely etiological agent?

Accepted Answer

Neisseria meningitidis

Answer

Klebsiella pneumoniae

Answer

Haemophilus influenzae

Answer

CMV

Question 4

A male patient presents with white discharge from the urethra, as shown in the image. What is the most probable causative organism?

Accepted Answer

Neisseria gonorrhoeae

Answer

Haemophilus ducreyi

Answer

Klebsiella granulomatis

Answer

Treponema pallidum

Question 5

A group of people ate patty late at night and experienced bouts of vomiting early in the morning. What is the most likely cause?

Accepted Answer

Staphylococcus aureus

Answer

Escherichia coli

Answer

Lactobacillus

Answer

Bacillus cereus

Question 6

A patient presented with 70% burns, and a sample was collected from the burn site. The image shows Gram-negative rods, and the suspected organism is an obligate aerobe. What is the most likely causative microbe?

Accepted Answer

Pseudomonas aeruginosa

Answer

Neisseria meningitidis (Meningococcus)

Answer

Streptococcus pneumoniae (Pneumococcus)

Answer

Streptococcus pyogenes

Question 7

A patient presents with a history of pastry intake causing food poisoning. What is the most likely causative agent?

Accepted Answer

Staphylococcus aureus

Answer

Verotoxin-producing E. coli

Answer

Bacillus cereus

Answer

Enteroinvasive E. coli (EIEC)

Question 8

Which of the following is a gram-positive organism that shows the following appearance on Ziehl-Neelsen staining?

Accepted Answer

Nocardia

Answer

Mycobacterium tuberculosis

Answer

Actinomyces

Answer

Rhodococcus

Question 9

Which of the following microorganisms will be resistant to meropenem and aminoglycosides but sensitive to piperacillin tazobactam and cotrimoxazole?

Accepted Answer

Stenotrophomonas

Answer

Pseudomonas

Answer

Acinetobacter

Answer

Burkholderia cepacia

Question 10

A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

Accepted Answer

Stenotrophomonas

Answer

Pseudomonas

Answer

Staphylococcus

Answer

Mycoplasma

Bacteriology — MCQs

Bacteriology — MCQs

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