Bacteriology Practice Questions

Q: A 60-year-old patient develops a central line infection with gram-positive cocci that are catalase-positive and coagulase-negative. What is the most likely pathogen?

Staphylococcus epidermidis. ***Staphylococcus epidermidis*** - This pathogen is a common cause of **catheter-related infections** and is characterized by being **catalase-positive** and **coagulase-negative**. - Its ability to form **biofilms** on medical devices makes it a significant cause of central line infections, especially in immunocompromised or hospitalized patients. *Staphylococcus aureus* - While *Staphylococcus aureus* is a gram-positive cocci and **catalase-positive**, it is uniquely characterized by being **coagulase-positive**, producing an enzyme that clots plasma. - It often causes more aggressive infections, including skin infections, pneumonia, and sepsis, which are usually not limited to indwelling devices. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is a gram-positive cocci, but it is **catalase-negative**, distinguishing it from *Staphylococcus* species. - It is a common cause of pneumonia, meningitis, and otitis media, and is less frequently associated with central line infections compared to staphylococci. *Enterococcus faecalis* - *Enterococcus faecalis* is a gram-positive cocci, but it is typically **catalase-negative** (though some strains can show weak catalase activity), and it is not coagulase-positive. - It commonly causes urinary tract infections, endocarditis, and hospital-acquired infections, but is not the most likely pathogen given the specific coagulase-negative characteristic in the context of a central line infection.

Q: What is the most common cause of urinary tract infections in children?

Escherichia coli. ***Escherichia coli*** - **Uropathogenic *E. coli*** (*UPEC*) is the most prevalent bacterium responsible for UTIs in children, accounting for approximately 80–90% of cases. - Its virulence factors, such as **P fimbriae**, enable it to adhere to uroepithelial cells and colonize the urinary tract. *Klebsiella* - While *Klebsiella* species can cause UTIs, they are a less common etiology than *E. coli*, particularly in uncomplicated cases. - More frequently associated with complicated UTIs, recurrent infections, or infections in hospitalized patients. *Proteus* - *Proteus mirabilis* is a notable cause of UTIs, particularly those associated with **struvite stones** due to its urease production. - However, its overall prevalence in pediatric UTIs is significantly lower compared to *E. coli*. *Staphylococcus aureus* - *S. aureus* is an uncommon cause of primary UTIs and is more typically associated with **hematogenous spread** to the kidneys, leading to renal abscesses, or with catheter-associated infections. - It almost never causes uncomplicated cystitis in children.

Q: A 30-year-old woman with a history of HIV presents with a persistent fever and night sweats. Blood cultures reveal non-lactose fermenting gram-negative rods. Which pathogen is most likely?

Salmonella typhi. ***Salmonella typhi*** - **Salmonella typhi** is a **non-lactose fermenting gram-negative rod** commonly associated with **persistent fever**, **night sweats**, and **bacteremia**, especially in immunocompromised individuals like those with HIV. - It causes **typhoid fever**, a systemic illness characterized by prolonged fever, **rose spots**, and hepatosplenomegaly. - **Note:** In HIV patients, non-typhoidal Salmonella species (e.g., *S. enteritidis*, *S. typhimurium*) are actually more common causes of recurrent bacteremia, but *S. typhi* remains a significant pathogen and is the only non-lactose fermenter among the given options. *Escherichia coli* - **Escherichia coli** is typically a **lactose-fermenting gram-negative rod**, which contradicts the blood culture finding of a non-lactose fermenter. - While *E. coli* can cause fever and bacteremia, it usually presents as a more acute infection, such as a **urinary tract infection (UTI)** or **sepsis**, rather than prolonged fever and night sweats in this context. *Klebsiella pneumoniae* - **Klebsiella pneumoniae** is a **lactose-fermenting gram-negative rod**, inconsistent with the blood culture result. - It commonly causes **pneumonia** and UTIs, and while it can lead to bacteremia, the fermentation characteristic excludes it in this scenario. *Enterobacter cloacae* - **Enterobacter cloacae** is a **lactose-fermenting gram-negative rod**, making it unlikely given the non-lactose fermenting finding. - It is often associated with nosocomial infections and can cause bacteremia, but the lactose fermentation characteristic is key in differentiation here.

Q: A 65-year-old man presents with fever and confusion. CSF analysis shows high protein and low glucose levels. Gram stain reveals small, pleomorphic, gram-negative coccobacilli. What is the most likely causative organism?

Haemophilus influenzae. ***Haemophilus influenzae*** - The combination of **fever**, **confusion**, **high CSF protein**, **low CSF glucose**, and **small, pleomorphic, gram-negative coccobacilli** on Gram stain strongly points to *Haemophilus influenzae* meningitis. - While vaccination has reduced its incidence, *H. influenzae* can still cause **meningitis** in unvaccinated individuals, particularly in older adults or those with underlying conditions. *Listeria monocytogenes* - This organism typically causes meningitis in **neonates**, **elderly individuals**, and **immunocompromised patients**. - On Gram stain, *Listeria* appears as **gram-positive rods**, which is inconsistent with the description of gram-negative coccobacilli. *Streptococcus pneumoniae* - *S. pneumoniae* is a common cause of bacterial meningitis, presenting with similar CSF findings (high protein, low glucose). - However, it appears as **gram-positive cocci in pairs or chains** on Gram stain, not gram-negative coccobacilli. *Neisseria meningitidis* - *N. meningitidis* also causes meningitis with high protein and low glucose in CSF, and it is a **gram-negative coccus** (often in pairs). - However, the description **"pleomorphic coccobacilli"** is more characteristic of *Haemophilus influenzae* than the typically more uniform cocci of *Neisseria*.

Q: A patient presents to the clinic with a fever and cough. A chest X-ray shows lobar pneumonia. Which organism is most likely responsible?

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - This is the **most common cause** of community-acquired bacterial pneumonia and is frequently associated with **lobar pneumonia** on chest X-ray. - Patients typically present with acute onset of **fever, cough**, and often **pleuritic chest pain**. *Haemophilus influenzae* - While it can cause pneumonia, especially in patients with **chronic lung disease** (e.g., COPD), it is a less common cause of classic **lobar pneumonia** than *S. pneumoniae*. - Often presents with features similar to other bacterial pneumonias but may also cause **epiglottitis** or **meningitis**. *Mycoplasma pneumoniae* - This typically causes **atypical pneumonia**, often referred to as "walking pneumonia," which tends to have a more **insidious onset** and milder symptoms. - Chest X-rays usually show **interstitial infiltrates**, not distinct lobar consolidation. *Legionella pneumophila* - This can cause severe pneumonia, often associated with a history of **exposure to contaminated water sources** (e.g., air conditioning systems). - Clinical features may include **hyponatremia** and **gastrointestinal symptoms** (diarrhea), which are not mentioned here, and while it can cause lobar consolidation, it is less common than *S. pneumoniae*.

Q: In a 60-year-old diabetic patient with necrotizing fasciitis, what is the most likely causative agent?

Streptococcus pyogenes. ***Streptococcus pyogenes*** - **Group A Streptococcus (GAS)**, particularly *S. pyogenes*, is the most common cause of **Type II necrotizing fasciitis**, a severe, rapidly progressing soft tissue infection. - Diabetic patients are at higher risk for severe infections due to **impaired immune response** and compromised microcirculation, making them susceptible to aggressive bacterial agents. *Staphylococcus aureus* - While *S. aureus*, especially **MRSA**, can cause skin and soft tissue infections, it is less common as the primary causative agent for typical **necrotizing fasciitis** compared to *S. pyogenes*. - *S. aureus* is more frequently associated with **abscesses, cellulitis, and wound infections**, rather than the rapidly spreading tissue destruction seen in classical necrotizing fasciitis. *Clostridium perfringens* - *C. perfringens* is the primary cause of **gas gangrene (clostridial myonecrosis)**, a distinct form of severe soft tissue infection characterized by gas production in tissues. - While also a rapidly progressive and life-threatening infection, it presents with specific signs like **crepitus** and is not typically the first suspected agent for general necrotizing fasciitis without these features. *Pseudomonas aeruginosa* - *P. aeruginosa* is often implicated in **healthcare-associated infections**, infections in immunocompromised patients, and **burn wounds**. - While it can cause serious soft tissue infections, it is not the most common causative agent of **community-acquired necrotizing fasciitis** in a diabetic patient without specific risk factors like extensive burn injuries or prolonged hospitalization.

Q: A newborn develops severe meningitis. Gram staining of the cerebrospinal fluid shows gram-positive cocci in chains. Which organism is most likely responsible?

Streptococcus agalactiae. ***Streptococcus agalactiae*** - **_Streptococcus agalactiae_** (Group B Streptococcus) is a common cause of **neonatal meningitis** and is characterized by **gram-positive cocci in chains**. - This organism is often acquired during **vaginal birth** from the mother's colonized birth canal. *Neisseria meningitidis* - **_Neisseria meningitidis_** causes **meningitis**, but it is characterized by **gram-negative diplococci**, not gram-positive cocci in chains. - While it can affect newborns, it is more commonly associated with **older infants, children, and young adults**. *Staphylococcus aureus* - **_Staphylococcus aureus_** is a **gram-positive coccus** that typically forms **clusters _(grape-like clusters)_**, not chains. - It is a common cause of skin infections, endocarditis, and osteomyelitis, but less commonly a primary cause of **neonatal meningitis** unless there's a predisposing factor like a shunt. *Listeria monocytogenes* - **_Listeria monocytogenes_** can cause **neonatal meningitis** and is acquired transplacentally or during birth. - However, it is a **gram-positive rod**, not a gram-positive coccus.

Q: A 30-year-old man presents with fever, night sweats, and weight loss. Acid-fast bacilli are found in his sputum. What is the primary mode of transmission of the causative agent?

Inhalation of respiratory droplets. ***Inhalation of respiratory droplets*** - The presence of **acid-fast bacilli in sputum**, along with symptoms like fever, night sweats, and weight loss, is highly suggestive of **pulmonary tuberculosis**. - **Tuberculosis** is primarily spread when an infected person coughs, sneezes, or talks, releasing airborne droplets containing *Mycobacterium tuberculosis* that can be inhaled by others. *Ingestion of contaminated food* - While some mycobacterial infections can be acquired through contaminated food (e.g., *Mycobacterium bovis* via unpasteurized dairy), the presence of **acid-fast bacilli in sputum** indicates a respiratory source. - This mode of transmission is typically associated with **gastrointestinal infections**, not primary pulmonary disease presenting in this manner. *Contact with infected blood* - Transmission via **blood contact** is characteristic of bloodborne pathogens like HIV, Hepatitis B, or Hepatitis C. - Tuberculosis is not transmitted through direct contact with infected blood, making this an unlikely mode for the described clinical scenario. *Vector-borne transmission* - **Vector-borne diseases** involve an animal or insect (e.g., mosquitoes, ticks) transmitting a pathogen to humans (e.g., malaria, Lyme disease). - This mode of transmission is not applicable to **tuberculosis**, which is spread directly from person to person via respiratory secretions.

Q: Which organism is most commonly associated with pyogenic osteomyelitis?

Staphylococcus aureus. ***Staphylococcus aureus*** - This organism is the **most frequent cause** of osteomyelitis in individuals of all ages. - It often gains entry through **trauma**, **surgery**, or **hematogenous spread**. *Pseudomonas aeruginosa* - This is a common cause of osteomyelitis in specific populations, such as **intravenous drug users** or individuals with **puncture wounds** through footwear. - It's also frequently associated with osteomyelitis in patients with **diabetes** or after **bone graft surgery**. *Salmonella* - While *Salmonella* can cause osteomyelitis, it is primarily associated with individuals suffering from **sickle cell disease** due to impaired reticuloendothelial function. - It is a **less common** cause overall compared to *Staphylococcus aureus*. *Eikenella corrodens* - This gram-negative bacillus is typically involved in osteomyelitis following **human bite wounds** or **head and neck infections** due to its presence in oral flora. - It is a **rare cause** of osteomyelitis in the general population.

Q: A 58-year-old male with chronic obstructive pulmonary disease presents with three weeks of productive cough and malaise. A chest X-ray shows a single lung cavity with an air-fluid level. Sputum culture grows filamentous bacteria with sulfur granules. What is the most likely pathogen?

Actinomyces israelii. ***Actinomyces israelii*** - The presence of **sulfur granules** in sputum, combined with a lung cavity and a history of COPD, is highly suggestive of **pulmonary actinomycosis**. - **Actinomyces israelii** is a normal inhabitant of the oral flora and can cause infection, particularly in individuals with compromised lung defenses or aspiration. *Mycobacterium tuberculosis* - While it can cause chronic productive cough, lung cavities, and is common in patients with impaired immunity, **Mycobacterium tuberculosis** does not produce **sulfur granules**. - It would typically be identified by **acid-fast staining** and specific culture methods, not by granules. *Nocardia asteroides* - **Nocardia** can cause pulmonary disease with cavities, especially in immunocompromised patients, but it typically presents with **partially acid-fast filaments** and does not form **sulfur granules**. - Infection often stems from inhalation of airborne organisms, and it can disseminate. *Staphylococcus aureus* - **Staphylococcus aureus** can cause lung abscesses and cavities (especially in patients with **COPD** or post-viral infections), but it typically does not produce **sulfur granules**. - Sputum cultures would reveal **Gram-positive cocci in clusters**, not filamentous bacteria.

Question 1

A 60-year-old patient develops a central line infection with gram-positive cocci that are catalase-positive and coagulase-negative. What is the most likely pathogen?

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Enterococcus faecalis

Question 2

What is the most common cause of urinary tract infections in children?

Accepted Answer

Escherichia coli

Answer

Klebsiella

Answer

Proteus

Answer

Staphylococcus aureus

Question 3

A 30-year-old woman with a history of HIV presents with a persistent fever and night sweats. Blood cultures reveal non-lactose fermenting gram-negative rods. Which pathogen is most likely?

Accepted Answer

Salmonella typhi

Answer

Klebsiella pneumoniae

Answer

Escherichia coli

Answer

Enterobacter cloacae

Question 4

A 65-year-old man presents with fever and confusion. CSF analysis shows high protein and low glucose levels. Gram stain reveals small, pleomorphic, gram-negative coccobacilli. What is the most likely causative organism?

Accepted Answer

Haemophilus influenzae

Answer

Listeria monocytogenes

Answer

Streptococcus pneumoniae

Answer

Neisseria meningitidis

Question 5

A patient presents to the clinic with a fever and cough. A chest X-ray shows lobar pneumonia. Which organism is most likely responsible?

Accepted Answer

Streptococcus pneumoniae

Answer

Haemophilus influenzae

Answer

Mycoplasma pneumoniae

Answer

Legionella pneumophila

Question 6

In a 60-year-old diabetic patient with necrotizing fasciitis, what is the most likely causative agent?

Accepted Answer

Streptococcus pyogenes

Answer

Staphylococcus aureus

Answer

Clostridium perfringens

Answer

Pseudomonas aeruginosa

Question 7

A newborn develops severe meningitis. Gram staining of the cerebrospinal fluid shows gram-positive cocci in chains. Which organism is most likely responsible?

Accepted Answer

Streptococcus agalactiae

Answer

Neisseria meningitidis

Answer

Staphylococcus aureus

Answer

Listeria monocytogenes

Question 8

A 30-year-old man presents with fever, night sweats, and weight loss. Acid-fast bacilli are found in his sputum. What is the primary mode of transmission of the causative agent?

Accepted Answer

Inhalation of respiratory droplets

Answer

Ingestion of contaminated food

Answer

Contact with infected blood

Answer

Vector-borne transmission

Question 9

Which organism is most commonly associated with pyogenic osteomyelitis?

Accepted Answer

Staphylococcus aureus

Answer

Pseudomonas aeruginosa

Answer

Salmonella

Answer

Eikenella corrodens

Question 10

A 58-year-old male with chronic obstructive pulmonary disease presents with three weeks of productive cough and malaise. A chest X-ray shows a single lung cavity with an air-fluid level. Sputum culture grows filamentous bacteria with sulfur granules. What is the most likely pathogen?

Accepted Answer

Actinomyces israelii

Answer

Mycobacterium tuberculosis

Answer

Nocardia asteroides

Answer

Staphylococcus aureus

Bacteriology — MCQs

Bacteriology — MCQs

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