Bacteriology Practice Questions

Q: Which of the following is the most diagnostically significant characteristic of Campylobacter jejuni?

Grows at 42°C. ***Grows at 42°C*** - The ability to grow optimally at **42°C (thermophilic growth)** is a key discriminating characteristic for *Campylobacter jejuni*, differentiating it from other common enteric pathogens. - This property is exploited in selective media (e.g., Skirrow agar) incubated at this higher temperature to aid in its isolation from fecal samples. *Obligate aerobe* - *Campylobacter jejuni* is actually **microaerophilic**, meaning it requires reduced oxygen levels (typically 5% O2) for optimal growth, not completely aerobic conditions. - Obligate aerobes require atmospheric oxygen concentrations for survival, which is contrary to the growth requirements of *C. jejuni*. *Oxidase negative* - *Campylobacter jejuni* is **oxidase positive**, which means it produces the enzyme cytochrome c oxidase. - An oxidase negative result would rule out *Campylobacter jejuni* and is characteristic of other organisms like *Escherichia coli*. *Non-motile* - *Campylobacter jejuni* is **highly motile** due to its characteristic single polar flagellum, which contributes to its invasiveness in the intestinal mucosa. - Non-motility would be inconsistent with the typical microscopic appearance and pathogenic mechanisms of *C. jejuni*.

Q: Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?

Acute cystitis. ***Acute cystitis*** - **Staphylococcus saprophyticus** is a common cause of **urinary tract infections (UTIs)**, particularly acute cystitis, in young sexually active women. - This bacterium has a high affinity for **uroepithelial cells**, facilitating its colonization and subsequent infection of the bladder. *Tension pneumothorax* - A **tension pneumothorax** is a medical emergency characterized by air accumulation in the pleural space, leading to lung collapse and mediastinal shift. - It is typically caused by trauma or iatrogenic factors, not directly by bacterial infection from either *Staphylococcus saprophyticus* or *Staphylococcus aureus*. *Burns* - Burn wounds are highly susceptible to bacterial colonization and infection, with **Staphylococcus aureus** being a primary pathogen in this context. - *Staphylococcus saprophyticus* is rarely associated with burn wound infections. *Osteomyelitis* - **Osteomyelitis**, an infection of the bone, is most frequently caused by **Staphylococcus aureus** via hematogenous spread or direct inoculation. - *Staphylococcus saprophyticus* is not a common pathogen in osteomyelitis.

Q: A healthy 21-year-old woman develops a profuse, watery diarrhea one day after a meal of raw oysters. On physical examination, her temperature is 37.5°C. A stool sample is negative for occult blood. There is no abdominal distension or tenderness, and bowel sounds are present. Diarrhea subsides over the next three days. Which of the following organisms is most likely to produce these findings?

Vibrio parahaemolyticus. ***Vibrio parahaemolyticus*** - **Raw seafood consumption** (raw oysters) is a classical risk factor for *Vibrio parahaemolyticus* infection, which commonly causes **watery diarrhea** and abdominal cramps. - The symptoms, including the self-limiting nature and absence of fever or severe abdominal findings, are consistent with a typical onset of 24 hours after exposure to *Vibrio parahaemolyticus*. *Cryptosporidium parvum* - This parasite typically causes **prolonged watery diarrhea**, especially in immunocompromised individuals, and is not usually associated with rapid onset following consumption of raw oysters. - While it causes watery diarrhea, its incubation period can range from 2 to 10 days, and the illness often lasts for weeks, not just three days. *Entamoeba histolytica* - *Entamoeba histolytica* causes **amoebic dysentery**, characterized by **bloody diarrhea**, fever, and abdominal pain, which is not seen in this patient (stool negative for occult blood). - Transmission is typically via contaminated food or water, but raw oysters are not a common source, and the symptoms described do not match amoebiasis. *Staphylococcus aureus* - *Staphylococcus aureus* causes **food poisoning** due to preformed toxins, leading to very rapid onset (1-6 hours) of severe nausea, vomiting, and abdominal cramps, with diarrhea sometimes present. - The 24-hour incubation period and predominantly diarrheal presentation make *Vibrio parahaemolyticus* a more likely culprit than *Staphylococcus aureus*, where vomiting is usually prominent.

Q: Which of the following statements about Enterohemorrhagic E. coli (EHEC) is false?

Sereny test is positive. ***Sereny test is positive*** - The **Sereny test** assesses the invasiveness of bacteria by observing their ability to invade and replicate in epithelial cells, typically in guinea pig conjunctiva. - *Enterohemorrhagic E. coli* (**EHEC**) is **non-invasive**; its pathogenicity stems from its production of **Shiga-like toxins**, not from cellular invasion, making a positive Sereny test statement false. *Commonly causes HUS* - **EHEC**, particularly serotype **O157:H7**, is a well-known cause of **Hemolytic Uremic Syndrome (HUS)**, especially in children and the elderly. - The **Shiga-like toxins** produced by EHEC are absorbed into the bloodstream, damaging endothelial cells and leading to the characteristic triad of **hemolytic anemia**, **thrombocytopenia**, and **acute kidney injury** seen in HUS. *Bloody diarrhea and HUS linked to O157:H7* - **EHEC O157:H7** is the most common serotype associated with outbreaks of **bloody diarrhea** and the subsequent development of **HUS**. - The toxins cause widespread damage to the intestinal lining, leading to **hemorrhagic colitis** (bloody diarrhea), and can then disseminate to distant organs like the kidneys, causing HUS. *Produces Shiga-like toxin* - The primary virulence factor of **EHEC** is the production of **Shiga-like toxins** (also known as **verotoxins**), specifically Stx1 and Stx2. - These toxins inhibit protein synthesis in eukaryotic cells, leading to cell death and tissue damage, which contribute to the severe gastrointestinal symptoms and systemic complications like HUS.

Q: In a clinical scenario where a patient presents with flaccid paralysis, which of the following toxins is most likely responsible?

Botulism. ***Correct Option: Botulism*** - **Botulinum toxin** blocks the release of **acetylcholine** at the neuromuscular junction, leading to **flaccid paralysis**. - This paralysis typically progresses symmetrically and can affect ocular, bulbar, and peripheral muscles. *Incorrect Option: Tetanus* - **Tetanus toxin** inhibits the release of **inhibitory neurotransmitters** (**GABA** and **glycine**) in the central nervous system. - This leads to uncontrolled muscle contractions, presenting as **spastic paralysis** and **lockjaw**. *Incorrect Option: Diphtheria* - **Diphtheria toxin** causes local inflammation and can lead to systemic effects, including **myocarditis** and **neuropathy**. - The neuropathy can cause weakness, but it is typically a **delayed polyneuropathy** rather than acute flaccid paralysis. *Incorrect Option: Cholera* - **Cholera toxin** activates adenylyl cyclase in intestinal cells, leading to excessive fluid and electrolyte secretion. - The primary symptom is **severe watery diarrhea** and dehydration, not paralysis.

Q: Blood culture is positive in which infection caused by Staphylococcus aureus?

Infective endocarditis. ***Infective endocarditis*** - **Infective endocarditis** is characterized by the presence of bacteria in the bloodstream, leading to a **positive blood culture** [2]. *Staphylococcus aureus* is a common cause, particularly in intravenous drug users [1], [2]. - The infection involves the **endothelial lining of the heart**, often affecting heart valves, causing vegetations that can shed bacteria into the circulation [2], [3]. *Toxic Shock Syndrome (TSS)* - TSS is caused by toxins (e.g., **TSST-1**) produced by *Staphylococcus aureus*, not by the direct presence of bacteria in the bloodstream in high numbers that would consistently yield a positive blood culture. - While *S. aureus* is present, the systemic effects are primarily **toxin-mediated**, and blood cultures are often negative. *Impetigo* - Impetigo is a **superficial skin infection** caused by *Staphylococcus aureus* or *Streptococcus pyogenes*. - It does not involve systemic bacteremia, so **blood cultures are typically negative**. *Staphylococcal Scalded Skin Syndrome (SSSS)* - SSSS is a **toxin-mediated disease** caused by exfoliatin toxins produced by *Staphylococcus aureus*. - The bacteria usually remain localized at the site of infection (e.g., nose, throat, or skin), and **blood cultures are generally negative**.

Q: Cholera is caused by?

Vibrio cholerae O1. ***Vibrio cholerae O1*** - **Cholera** is primarily caused by **toxigenic strains** of *Vibrio cholerae*, with serogroup **O1** being the most common and historically significant cause of epidemics and pandemics. - This bacterium produces **cholera toxin**, which leads to profuse watery diarrhea by acting on the intestinal epithelial cells. *Enterococcus* - **Enterococcus** species are common inhabitants of the human gastrointestinal tract and are often associated with **nosocomial infections**, such as urinary tract infections, endocarditis, and bacteremia. - They do not typically cause the severe, watery diarrhea characteristic of cholera. *Vibrio parahemolyticus* - *Vibrio parahemolyticus* is a common cause of **foodborne illness**, particularly associated with consuming **raw or undercooked seafood**. - It causes a form of gastroenteritis characterized by abdominal pain, watery diarrhea, nausea, vomiting, and fever, but it is distinct from cholera. *E.coli* - While various strains of **Escherichia coli (E. coli)** can cause diarrheal diseases (e.g., enterotoxigenic *E. coli* causing traveler's diarrhea, enterohemorrhagic *E. coli* causing bloody diarrhea), **E. coli** is not the causative agent of cholera. - The pathogenesis and specific toxins produced differ significantly from those of *Vibrio cholerae*.

Q: Which of the following is more frequently associated with Klebsiella pneumoniae than with Pseudomonas aeruginosa?

Upper lobe cavitation. Upper lobe cavitation - **Upper lobe cavitation** is a characteristic feature of pneumonia caused by **Klebsiella pneumoniae**, particularly in chronic alcoholics or individuals with poor host defenses [1]. - While *Pseudomonas aeruginosa* can cause severe pneumonia, **cavitary lesions** are less frequently reported and typically occur in immunocompromised patients. *Artificial ventilation* - **Artificial ventilation** is a significant risk factor for nosocomial pneumonia caused by both *Klebsiella pneumoniae* and *Pseudomonas aeruginosa*. - However, the question asks for a feature *more frequently associated* with *Klebsiella pneumoniae*, and ventilation is a common risk for both. *Cystic fibrosis* - **Cystic fibrosis** is overwhelmingly associated with chronic *Pseudomonas aeruginosa* infections, which establish persistent biofilms in the respiratory tract. - While *Klebsiella pneumoniae* can colonize CF patients, it is not the primary or most frequent pathogen linked to this condition. *Diabetes mellitus* - **Diabetes mellitus** increases susceptibility to various infections, including those caused by *Klebsiella pneumoniae* and *Pseudomonas aeruginosa*. - However, it is an underlying condition and not a specific clinical characteristic of the pneumonia itself that differentiates the two organisms.

Q: Atrophic dry nasal mucosa, extensive encrustations, and a woody hard external nose are most suggestive of:

Klebsiella pneumoniae. ***Klebsiella pneumoniae*** - This constellation of symptoms—**atrophic dry nasal mucosa**, **extensive encrustations**, and a **woody hard external nose**—is characteristic of **primary atrophic rhinitis (ozena)** caused by *Klebsiella pneumoniae* subspecies *ozaenae*. - **Ozena** presents with the classic triad of **progressive nasal mucosal atrophy**, **foul-smelling greenish crusts**, and **anosmia** due to destruction of olfactory epithelium. - The **woody hard external nose** suggests either advanced ozena with fibrosis or **rhinoscleroma** (caused by *Klebsiella rhinoscleromatis*), both of which are Klebsiella-related chronic granulomatous conditions. - *K. pneumoniae* subspecies *ozaenae* is the **classic etiological agent** for this severe destructive form of atrophic rhinitis. *Staphylococcus aureus* - *S. aureus* causes **rhinitis sicca anterior** (anterior nasal vestibulitis), characterized by crusting and inflammation **limited to the anterior nasal vestibule**. - Unlike ozena, S. aureus infection does **not cause progressive atrophy** of the entire nasal mucosa or the extensive encrustations throughout the nasal cavity described here. - The **woody hard external nose** is not a feature of staphylococcal nasal infections, which remain superficial. *Peptostreptococcus* - **Peptostreptococcus** species are anaerobic bacteria typically involved in **polymicrobial infections** such as chronic sinusitis, dental abscesses, or deep neck space infections. - They are **not primary pathogens** in chronic atrophic rhinitis and do not cause the specific progressive nasal atrophy and external nasal changes described. *Bacteroides* - **Bacteroides** species are obligate anaerobes that are part of the normal gut flora and commonly cause **intra-abdominal infections** and abscesses. - They are **not associated** with chronic rhinitis, nasal mucosal atrophy, or the external nasal deformities characteristic of ozena or rhinoscleroma.

Q: Which of the following is the MOST CHARACTERISTIC feature of Campylobacter?

Comma-shaped. ***Comma-shaped*** - **Campylobacter** species are classically described as **comma-shaped** or S-shaped (seagull-shaped) **gram-negative rods** when viewed under a microscope. - This distinctive curved morphology is the **most characteristic feature** used in their identification. - The characteristic "darting motility" combined with this morphology makes identification straightforward. *Encapsulated* - **Campylobacter** species are generally **not encapsulated**, differentiating them from other pathogenic bacteria like *Streptococcus pneumoniae*. - Encapsulation is a **virulence factor** that helps bacteria evade phagocytosis but is not typical for **Campylobacter**. *Non-motile* - **Campylobacter** species are **highly motile**, possessing a **single polar flagellum** at one or both ends. - Their characteristic **darting motility** contributes to pathogenicity and ability to penetrate the intestinal mucus layer. - This motility is a crucial feature allowing movement through viscous environments. *Spore-forming* - **Campylobacter** species are **non-spore-forming** bacteria. - Spore formation is characteristic of certain gram-positive bacteria like *Bacillus* and *Clostridium* species. - This helps differentiate Campylobacter from spore-forming enteric pathogens.

Question 1

Which of the following is the most diagnostically significant characteristic of Campylobacter jejuni?

Accepted Answer

Grows at 42°C

Answer

Obligate aerobe

Answer

Oxidase negative

Answer

Non-motile

Question 2

Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?

Accepted Answer

Acute cystitis

Answer

Burns

Answer

Tension pneumothorax

Answer

Osteomyelitis

Question 3

A healthy 21-year-old woman develops a profuse, watery diarrhea one day after a meal of raw oysters. On physical examination, her temperature is 37.5°C. A stool sample is negative for occult blood. There is no abdominal distension or tenderness, and bowel sounds are present. Diarrhea subsides over the next three days. Which of the following organisms is most likely to produce these findings?

Accepted Answer

Vibrio parahaemolyticus

Answer

Cryptosporidium parvum

Answer

Entamoeba histolytica

Answer

Staphylococcus aureus

Question 4

Which of the following statements about Enterohemorrhagic E. coli (EHEC) is false?

Accepted Answer

Sereny test is positive

Answer

Commonly causes HUS

Answer

Bloody diarrhea and HUS linked to O157:H7

Answer

Produces Shiga-like toxin

Question 5

In a clinical scenario where a patient presents with flaccid paralysis, which of the following toxins is most likely responsible?

Accepted Answer

Botulism

Answer

Tetanus

Answer

Diphtheria

Answer

Cholera

Question 6

Blood culture is positive in which infection caused by Staphylococcus aureus?

Accepted Answer

Infective endocarditis

Answer

Impetigo

Answer

Toxic Shock Syndrome (TSS)

Answer

Staphylococcal Scalded Skin Syndrome (SSSS)

Question 7

Cholera is caused by?

Accepted Answer

Vibrio cholerae O1

Answer

Enterococcus

Answer

E.coli

Answer

Vibrio parahemolyticus

Question 8

Which of the following is more frequently associated with Klebsiella pneumoniae than with Pseudomonas aeruginosa?

Accepted Answer

Upper lobe cavitation

Answer

Cystic fibrosis

Answer

Diabetes mellitus

Answer

Artificial ventilation

Question 9

Atrophic dry nasal mucosa, extensive encrustations, and a woody hard external nose are most suggestive of:

Accepted Answer

Klebsiella pneumoniae

Answer

Bacteroides

Answer

Staphylococcus aureus

Answer

Peptostreptococcus

Question 10

Which of the following is the MOST CHARACTERISTIC feature of Campylobacter?

Accepted Answer

Comma-shaped

Answer

Non-motile

Answer

Encapsulated

Answer

Spore-forming

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?