Bacteriology Practice Questions

Q: Which organism is the most common cause of neonatal urinary tract infections?

E. coli. ***E. coli*** - **_Escherichia coli_** is the most frequent cause of **neonatal urinary tract infections (UTIs)**, accounting for a significant majority of cases (60-80%). - This is due to its prevalence in the **gastrointestinal tract** and ease of perineal contamination in infants, leading to ascending infections. *S. aureus* - **_Staphylococcus aureus_** is an important cause of skin and soft tissue infections, and in some cases, **sepsis**, but it is a relatively uncommon cause of primary neonatal UTIs. - While it can cause UTIs through **hematogenous spread**, especially in catheterized infants, it is not the most common causative agent. *Enterococcus* - **_Enterococcus_ species** are known causes of UTIs, particularly in hospitalized or immunocompromised patients, and can be isolated from neonatal UTIs. - However, they are **less common** than _E. coli_ in healthy neonates presenting with their first UTI. *Anaerobes* - **Anaerobic bacteria** are generally **not considered significant causes** of neonatal UTIs. - While they are part of the normal gut flora, their contribution to urinary tract infections is rare, as the urinary tract is typically aerobic.

Q: A 2-year-old child presents with fever and a maculopapular rash. Blood culture grows gram-positive cocci in chains that are catalase-negative. Which organism is likely responsible?

Streptococcus pyogenes. ***Streptococcus pyogenes*** - The presentation of fever and a **maculopapular rash** in a child, along with **gram-positive cocci in chains** that are **catalase-negative**, is highly indicative of *Streptococcus pyogenes* infection. - *S. pyogenes* is a common cause of **scarlet fever**, which presents with a characteristic maculopapular rash (scarlatiniform rash) and is caused by erythrogenic toxins produced by the bacteria. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is a **coagulase-negative staphylococcus** that is part of normal skin flora and is more commonly associated with medical device-related infections. - While it is a gram-positive coccus, it typically grows in **clusters** and is **catalase-positive**, differentiating it from the stated findings. *Neisseria meningitidis* - *Neisseria meningitidis* is a **gram-negative diplococcus** and is **catalase-positive**, making it inconsistent with the gram stain and catalase results. - It causes **meningitis** and **meningococcemia**, often with a petechial or purpuric rash, not typically a maculopapular rash in this context. *Enterococcus faecium* - *Enterococcus faecium* is a **gram-positive coccus that grows in chains** and is **catalase-negative**, which is consistent with the initial bacteriological findings. - However, while it can cause various infections, it is not typically associated with a maculopapular rash in a 2-year-old child as a primary presentation like *S. pyogenes* in scarlet fever.

Q: A child presents with a sudden onset of drooling, dysphagia, and respiratory distress. A lateral neck X-ray reveals a thumbprint sign. What is the most likely cause?

Haemophilus influenzae type B. ***Haemophilus influenzae type B*** - The classic presentation of sudden onset **drooling, dysphagia, and respiratory distress** in a child, coupled with the **thumbprint sign** on a lateral neck X-ray, is highly indicative of **epiglottitis**. - **_Haemophilus influenzae_ type B (Hib)** was historically the most common cause of bacterial epiglottitis; while its incidence has decreased significantly due to vaccination, it remains a primary pathogen in unvaccinated or partially vaccinated children. *Streptococcus pneumoniae* - While _Streptococcus pneumoniae_ is a common cause of respiratory infections (e.g., otitis media, pneumonia), it is **not typically associated with acute epiglottitis** or the thumbprint sign. - _S. pneumoniae_ can cause invasive disease, but its presentation differs from the rapid airway obstruction seen in epiglottitis. *Bordetella pertussis* - _Bordetella pertussis_ is the causative agent of **whooping cough (pertussis)**, characterized by severe coughing paroxysms, inspiratory whoop, and post-tussive vomiting. - It does not cause epiglottitis or present with the radiographic **thumbprint sign**. *Mycobacterium tuberculosis* - _Mycobacterium tuberculosis_ causes **tuberculosis**, a chronic infectious disease that can affect the lungs and other organs. - It does **not cause acute epiglottitis** with sudden onset drooling, dysphagia, and respiratory distress, nor is it associated with the thumbprint sign.

Q: Which disease is characterized by severe watery diarrhea and is associated with a toxin-producing bacterium?

Cholera. ***Cholera*** - Cholera is known for causing **severe, watery diarrhea** (often described as "rice water stools") due to the action of **cholera toxin** produced by *Vibrio cholerae* [1], [2]. - The toxin stimulates excessive fluid and electrolyte secretion in the small intestine, leading to rapid **dehydration** and electrolyte imbalance [2]. *Gastroenteritis* - This is a general term for **inflammation of the stomach and intestines**, which can be caused by various pathogens (viruses, bacteria, parasites) and toxins. - While it often presents with diarrhea, it doesn't specifically imply the **severe watery diarrhea** linked to a specific toxin as seen in cholera. *Dysentery* - Dysentery is characterized by **bloody diarrhea**, often accompanied by **fever and abdominal cramps**, indicating inflammation and damage to the intestinal lining [3]. - It is typically caused by bacteria like *Shigella* or *entamoeba histolytica*, distinct from the purely watery diarrhea of cholera [3]. *Typhoid fever* - Typhoid fever is a systemic illness caused by *Salmonella typhi*, characterized by **sustained fever, headache, malaise**, and can include **constipation or mild diarrhea**. - It does not primarily present with **severe watery diarrhea** induced by a specific toxin, unlike cholera.

Q: A 12-year-old girl presents with fever, shortness of breath, and cough. A chest X-ray reveals complete consolidation of the left lower lung lobe. What is the most probable organism?

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - This is the most common bacterial cause of **community-acquired pneumonia** in children and adults, and symptoms perfectly align with lobar consolidation. - **Lobar pneumonia**, as suggested by complete consolidation of a lung lobe on chest X-ray, is a classic presentation of *Streptococcus pneumoniae* infection. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause pneumonia, it often leads to **necrotizing pneumonia** or **empyema**, and is more common in hospitalized patients or those with predisposing factors like cystic fibrosis or influenza. - Its presentation is typically more severe and less frequently causes simple lobar consolidation in an otherwise healthy child. *Klebsiella pneumoniae* - *Klebsiella pneumoniae* typically causes **severe, necrotizing pneumonia** often seen in individuals with chronic alcohol abuse, diabetes, or immunocompromised states. - It characteristically produces a **"currant jelly" sputum** and is less common in healthy children with classic lobar pneumonia. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* pneumonia is primarily associated with **hospital-acquired infections**, **ventilator-associated pneumonia**, or in patients with underlying lung disease like **cystic fibrosis** or bronchiectasis. - It is highly unlikely to be the causative organism in an otherwise healthy 12-year-old presenting with typical community-acquired pneumonia.

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: Which pathogen is the most common cause of acute otitis media in children?

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - This pathogen is the most common bacterial cause of **acute otitis media (AOM)**, accounting for approximately 25-50% of cases in children. - Its prevalence underscores the importance of vaccination (pneumococcal vaccine) in preventing childhood ear infections. *Haemophilus influenzae* - While a significant cause of AOM, particularly in the post-pneumococcal vaccine era, *H. influenzae* is the **second most common** bacterial pathogen, making up about 20-40% of cases. - Non-typeable *H. influenzae* strains are often implicated. *Moraxella catarrhalis* - *M. catarrhalis* is another common bacterial cause of AOM but is less frequent than *S. pneumoniae* or *H. influenzae*, typically responsible for about 10-20% of cases. - It often causes milder infections compared to the other two primary pathogens. *Pseudomonas aeruginosa* - *P. aeruginosa* is an **uncommon cause** of acute otitis media in healthy children. - It is more typically associated with **otitis externa** (swimmer's ear) or chronic otitis media, especially in children with tympanostomy tubes.

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: 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 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*.

Question 1

Which organism is the most common cause of neonatal urinary tract infections?

Accepted Answer

E. coli

Answer

S. aureus

Answer

Enterococcus

Answer

Anaerobes

Question 2

A 2-year-old child presents with fever and a maculopapular rash. Blood culture grows gram-positive cocci in chains that are catalase-negative. Which organism is likely responsible?

Accepted Answer

Streptococcus pyogenes

Answer

Staphylococcus epidermidis

Answer

Neisseria meningitidis

Answer

Enterococcus faecium

Question 3

A child presents with a sudden onset of drooling, dysphagia, and respiratory distress. A lateral neck X-ray reveals a thumbprint sign. What is the most likely cause?

Accepted Answer

Haemophilus influenzae type B

Answer

Streptococcus pneumoniae

Answer

Bordetella pertussis

Answer

Mycobacterium tuberculosis

Question 4

Which disease is characterized by severe watery diarrhea and is associated with a toxin-producing bacterium?

Accepted Answer

Cholera

Answer

Gastroenteritis

Answer

Dysentery

Answer

Typhoid fever

Question 5

A 12-year-old girl presents with fever, shortness of breath, and cough. A chest X-ray reveals complete consolidation of the left lower lung lobe. What is the most probable organism?

Accepted Answer

Streptococcus pneumoniae

Answer

Staphylococcus aureus

Answer

Klebsiella pneumoniae

Answer

Pseudomonas aeruginosa

Question 6

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 7

Which pathogen is the most common cause of acute otitis media in children?

Accepted Answer

Streptococcus pneumoniae

Answer

Pseudomonas aeruginosa

Answer

Haemophilus influenzae

Answer

Moraxella catarrhalis

Question 8

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 9

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

Accepted Answer

Escherichia coli

Answer

Klebsiella

Answer

Proteus

Answer

Staphylococcus aureus

Question 10

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

Bacteriology — MCQs

Bacteriology — MCQs

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