Bacteriology Practice Questions

Q: How does Treponema pallidum evade host immune response during early infection?

By having few surface proteins. ***By having few surface proteins*** - *Treponema pallidum* is characterized by its **reduced number of outer membrane proteins (OMPs)** visible to the host immune system. - This **"stealth" mechanism** limits the host's ability to recognize and target the bacterium during early infection. - The paucity of surface antigens (only 10-20 OMPs compared to hundreds in typical Gram-negative bacteria) makes *T. pallidum* nearly invisible to immune surveillance. *By producing endotoxins* - **Endotoxins** (lipopolysaccharide or LPS) are components of the outer membrane of Gram-negative bacteria. - While *T. pallidum* is technically Gram-negative, **endotoxin production is not its primary immune evasion mechanism** during early infection. - The organism's survival depends on immune evasion through reduced antigenicity, not endotoxin-mediated effects. *Through antigenic variation* - While **antigenic variation** is a common immune evasion strategy for many pathogens (e.g., *Neisseria gonorrhoeae*, *Borrelia recurrentis*), it is **not a primary mechanism** employed by *T. pallidum*. - *T. pallidum* lacks the extensive genetic machinery for significant antigenic switching of its surface proteins. - The organism relies on having few targets rather than changing targets. *By forming biofilms* - **Biofilm formation** allows some bacteria to evade host immunity and antibiotics by creating a protective matrix. - This is **not a well-documented or primary immune evasion strategy** for *T. pallidum* in the context of syphilis infection. - *T. pallidum* typically exists as planktonic cells or within host tissues rather than forming robust biofilms in vivo.

Q: A 42-year-old woman comes to her primary care physician with 2 days of fever and malaise. She also says that she has a painful red lesion on her left hand that she noticed after shucking oysters at a recent family reunion. Physical exam reveals a well-demarcated swollen, tender, warm, red lesion on her left hand. Pressing the lesion causes a small amount of purulent drainage. The material is cultured and the causative organism is identified. Which of the following characteristics describes the organism that is most associated with this patient's mechanism of infection?

Gram-negative facultative anaerobe. ***Gram-negative facultative anaerobe*** - This clinical presentation, including oyster exposure and a painful, red hand lesion with purulent drainage, is highly suggestive of infection with **Vibrio vulnificus**. - **Vibrio vulnificus** is a classic **Gram-negative facultative anaerobe** that thrives in warm, brackish water and is known to cause severe wound infections and sepsis, particularly in individuals who consume raw seafood or have open wounds exposed to contaminated water. *Gram-negative aerobe* - While some Gram-negative bacteria are aerobes, a purely aerobic organism would not fit the typical growth characteristics of **Vibrio vulnificus**, which is **facultative anaerobic**. - No common obligate Gram-negative aerobes are primarily associated with the rapid, severe wound infection seen after oyster exposure. *Gram-positive clusters* - **Staphylococcus** species are Gram-positive bacteria that grow in clusters and can cause skin infections, but they are generally not associated with oyster exposure or the rapid progression described. - The mechanism of infection via oyster shucking points away from typical staphylococcal wound infections. *Gram-positive chains* - **Streptococcus** species are Gram-positive bacteria that grow in chains and can cause various infections, including cellulitis, but they are not typically linked to seafood-related wound infections like **Vibrio vulnificus**. - The clinical context strongly favors a different causative agent. *Gram-negative anaerobe* - While there are Gram-negative anaerobes, such as **Bacteroides**, they are primarily found in the gut flora and are less commonly implicated in acute wound infections acquired from environmental exposure like oysters. - **Vibrio vulnificus** is a **facultative anaerobe**, meaning it can grow with or without oxygen, which is distinct from obligate anaerobes.

Q: A 34-year-old man comes to the physician because of fatigue and shortness of breath with moderate exertion for the past 2 months. Over the past 10 days, he has had low-grade fevers and night sweats. He has no history of serious illness except for a bicuspid aortic valve diagnosed 5 years ago. He has smoked one pack of cigarettes daily for 10 years and drinks 3–5 beers on social occasions. He does not use illicit drugs. The patient takes no medications. He appears weak. His temperature is 37.7°C (99.9°F), pulse is 70/min, and blood pressure is 128/64 mm Hg. The lungs are clear to auscultation. A grade 2/6 systolic murmur is heard best at the right sternal border and second intercostal space. There are several hemorrhages underneath his fingernails on both hands and multiple tender, red nodules on his fingers. Which of the following is the most likely causal organism?

Streptococcus sanguinis. ***Streptococcus sanguinis*** - The patient's history of **bicuspid aortic valve** represents a predisposing cardiac lesion for **infective endocarditis**. - **S. sanguinis** is part of the **viridans group streptococci**, common inhabitants of the oral flora, and is a frequent cause of subacute bacterial endocarditis, especially in individuals with damaged heart valves. - The clinical presentation of **splinter hemorrhages** and **Osler nodes** (tender nodules on fingers) along with low-grade fever and constitutional symptoms is classic for subacute bacterial endocarditis. *Candida albicans* - While *Candida* can cause endocarditis, it is typically seen in specific risk groups such as **intravenous drug users**, immunocompromised patients, or after prosthetic valve surgery, none of which apply here. - Fungal endocarditis often presents with larger **vegetations** and a more subacute course, but bacterial causes (especially viridans streptococci) are far more common in this clinical setting. *Streptococcus pneumoniae* - *S. pneumoniae* (pneumococcus) is a known cause of **pneumonia**, **meningitis**, and **otitis media**, but it is an uncommon cause of endocarditis. - Pneumococcal endocarditis, when it occurs, typically presents with a more fulminant course and may be associated with other sites of pneumococcal infection. *Staphylococcus epidermidis* - *S. epidermidis* is a common cause of **prosthetic valve endocarditis** and infections related to foreign bodies or catheters. - Given the patient's **native valve** issue and absence of prosthetic material or recent invasive procedures, it is less likely than **viridans streptococci**.

Q: A bacterial culture shows 'tumbling motility' pattern. Which microscopic finding would confirm Listeria monocytogenes?

Gram-positive coccobacilli. ***Gram-positive coccobacilli*** - *Listeria monocytogenes* classically presents as **Gram-positive coccobacilli** (short rods) and exhibits a characteristic **"tumbling motility"** at **room temperature (20-25°C)** due to peritrichous flagella. - This combination of tumbling motility and Gram-positive coccobacilli morphology is highly characteristic and aids in confirming *Listeria* identification. - The organism shows this distinctive end-over-end tumbling movement in wet mount or hanging drop preparations. *Gram-variable pleomorphic rods* - **Gram-variable** and **pleomorphic rods** describe organisms with inconsistent Gram stain reactions and variable shapes, which does not fit the typical morphology of *Listeria monocytogenes*. - *Listeria* consistently stains Gram-positive and appears as short rods or coccobacilli, not pleomorphic. *Gram-negative diplococci* - **Gram-negative diplococci** are characteristic of *Neisseria* species (*N. meningitidis*, *N. gonorrhoeae*), which have distinctly different morphology and do not exhibit tumbling motility. - This finding would completely rule out *Listeria monocytogenes*. *Acid-fast bacilli* - **Acid-fast bacilli** (AFB) are identified using the Ziehl-Neelsen stain and are characteristic of mycobacteria (e.g., *Mycobacterium tuberculosis*), which have a waxy cell wall rich in mycolic acids. - Mycobacteria are non-motile and morphologically distinct from *Listeria monocytogenes*.

Q: A bacterial culture shows small colonies with narrow zones of β-hemolysis on blood agar and exhibits tumbling motility at 25°C. Which biochemical test would confirm Listeria monocytogenes?

CAMP test positivity. ***CAMP test positivity*** - A positive **CAMP test** result, characterized by enhanced hemolysis when *Listeria monocytogenes* is cultured near *Staphylococcus aureus* on blood agar, is a **key confirmatory test** for identifying this organism. - The test demonstrates the synergistic hemolytic activity between the CAMP factor of *Listeria* and the β-hemolysin of *S. aureus*, producing an arrowhead-shaped zone of enhanced hemolysis. - This test, combined with the characteristic **tumbling motility** at room temperature and **narrow β-hemolysis**, provides strong confirmation of *Listeria monocytogenes*. *Catalase positivity* - While *Listeria monocytogenes* is **catalase-positive**, this characteristic is **not specific** as many bacteria including *Staphylococcus* species, *Bacillus* species, and other gram-positive organisms are also catalase-positive. - Catalase testing alone cannot differentiate *Listeria* from other catalase-positive bacteria. *Oxidase positivity* - *Listeria monocytogenes* is **oxidase-negative**, meaning it does not produce cytochrome c oxidase. - This is a useful negative characteristic but **oxidase negativity alone is not confirmatory** as many bacteria are oxidase-negative. *Urease positivity* - *Listeria monocytogenes* is **urease-negative**, meaning it does not produce the enzyme urease which hydrolyzes urea into ammonia and carbon dioxide. - Urease-positive bacteria include *Proteus* species, *Helicobacter pylori*, and *Yersinia enterocolitica*, making this test useful for ruling out these organisms but not for confirming *Listeria*.

Q: A fluid culture shows 'spoked wheel' colony appearance. Which microscopic finding would confirm Actinomyces species?

Branching filaments. ***Branching filaments*** - **Actinomyces** species are characterized by their **gram-positive, branching filamentous** morphology on microscopy. - The "spoked wheel" (or "molar tooth") colony appearance on culture combined with **branching filaments on Gram stain** is diagnostic for Actinomyces. - These are **anaerobic or facultatively anaerobic** bacteria that can form intricate structures, often visible as "sulfur granules" in clinical specimens, which are microcolonies of the organism. *Pleomorphic rods* - While Actinomyces can appear somewhat pleomorphic, this description alone is **not specific enough** to distinguish it from many other bacterial species. - The key distinguishing feature for Actinomyces is the **branching filamentous** nature, not just pleomorphic rods. *Diplococci* - **Diplococci** refer to cocci that occur in pairs (e.g., *Neisseria* species, *Streptococcus pneumoniae*). - This morphology is distinctly different from the **rod-shaped, branching filamentous structure** of Actinomyces. *Curved bacilli* - **Curved bacilli** are characteristic of bacteria like *Vibrio cholerae* or *Campylobacter jejuni*. - This morphology does not fit the typical appearance of Actinomyces, which are known for their **branching filaments**, not curved shapes.

Q: A bacterial culture shows 'medusa head' colonies. Which biochemical finding is characteristic of Bacillus cereus?

Catalase positive. ***Catalase positive*** - *Bacillus cereus* is a **catalase-positive**, gram-positive, spore-forming rod. Catalase production is a characteristic shared by many *Bacillus* species. [2] - The **"medusa head" colonies** are a distinctive morphological feature of *Bacillus anthracis* (a close relative) and also seen with *B. cereus*, indicating a large, spreading, rough colony appearance. [1] *Optochin sensitive* - **Optochin sensitivity** is a key characteristic used primarily to differentiate *Streptococcus pneumoniae* from other alpha-hemolytic streptococci. - *Bacillus cereus* is typically **resistant to optochin**. *CAMP test positive* - The **CAMP test** is used to identify **Group B Streptococcus** (*Streptococcus agalactiae*) by detecting an enhanced zone of hemolysis when grown next to *Staphylococcus aureus*. [2] - This test is **not applicable or positive for *Bacillus cereus***. *Oxidase positive* - Being **oxidase positive** indicates the presence of cytochrome c oxidase, an enzyme found in some bacteria. - *Bacillus cereus* is typically **oxidase negative**.

Q: A bacterial culture shows flat, irregular colonies with metallic sheen. Which additional finding would confirm Pseudomonas aeruginosa?

Blue-green pigment with grape odor. ***Blue-green pigment with grape odor*** - *Pseudomonas aeruginosa* characteristically produces **pyocyanin**, a blue-green pigment, and often has a distinct **grape-like odor** due to aminoacetophenone. - The combination of **flat, irregular colonies** with a metallic sheen and this specific pigment and odor is highly indicative of *P. aeruginosa*. *Black pigment* - **Black pigmentation** is typically associated with other bacteria, such as some *Proteus* species or certain anaerobic bacteria like *Prevotella intermedia*. - It is not a characteristic feature used to identify *Pseudomonas aeruginosa*. *Beta-hemolysis* - While *Pseudomonas aeruginosa* can be **beta-hemolytic** on blood agar, this finding is not specific enough to confirm its identity as many other bacteria also exhibit beta-hemolysis. - Other features like pigment production and odor are more unique and definitive for *P. aeruginosa*. *Yellow pigment* - **Yellow pigmentation** is characteristic of various bacteria, including some *Staphylococcus* species (e.g., *Staphylococcus aureus* often produces a golden-yellow pigment) and *Micrococcus* species. - This is not a typical characteristic of *Pseudomonas aeruginosa* and would not help confirm its identification.

Q: In a Gram stain preparation of CSF, which organism appears as pairs of kidney-shaped diplococci with adjacent flattened sides?

Neisseria meningitidis. ***Neisseria meningitidis*** - This description is classic for **Gram-negative diplococci** of *Neisseria meningitidis*, often appearing in pairs with adjacent flattened sides. - Its presence in CSF is a strong indicator of **bacterial meningitis**, especially given the characteristic morphology. *Streptococcus pneumoniae* - This bacterium also appears as **Gram-positive diplococci**, but they are typically **lancet-shaped** (pointed ends), not kidney-shaped with flattened sides. - While a common cause of meningitis, its distinctive morphology differentiates it from *Neisseria*. *Moraxella catarrhalis* - While *Moraxella catarrhalis* is a **Gram-negative diplococcus**, it is typically found in the respiratory tract and rarely causes **meningitis**; when it does, it usually presents as spherical diplococci. - Its morphology is generally not described as kidney-shaped with flattened sides in the context of CSF. *Staphylococcus aureus* - *Staphylococcus aureus* is a **Gram-positive coccus** that forms **clusters resembling grapes** rather than diplococci. - Its morphology is distinctly different from the kidney-shaped diplococci described.

Question 1

How does Treponema pallidum evade host immune response during early infection?

Accepted Answer

By having few surface proteins

Answer

By producing endotoxins

Answer

Through antigenic variation

Answer

By forming biofilms

Question 2

A 34-year-old female visits her primary care physician because recently she has started to have painful, numb, and discolored toes. She is otherwise healthy and has no family history of similar conditions that she can recall. Occasionally during these episodes, her fingers and nose will also have similar symptoms. On examination, the patient's appearance is completely normal with warm and well perfused extremities. No evidence of discoloration is found. On closer questioning, she reveals that several months ago during the summer, she succumbed to a viral illness that caused her to feel fatigued and have a long bout of cold symptoms with sore throat and swollen lymph nodes. The bacterial species that is also associated with this patient's most likely condition has which of the following characteristics?

Accepted Answer

No cell wall

Answer

Gram-positive

Answer

Acid-fast

Answer

Spirochete

Answer

Gram-negative

Question 3

A 42-year-old woman comes to her primary care physician with 2 days of fever and malaise. She also says that she has a painful red lesion on her left hand that she noticed after shucking oysters at a recent family reunion. Physical exam reveals a well-demarcated swollen, tender, warm, red lesion on her left hand. Pressing the lesion causes a small amount of purulent drainage. The material is cultured and the causative organism is identified. Which of the following characteristics describes the organism that is most associated with this patient's mechanism of infection?

Accepted Answer

Gram-negative facultative anaerobe

Answer

Gram-negative aerobe

Answer

Gram-positive clusters

Answer

Gram-positive chains

Answer

Gram-negative anaerobe

Question 4

A 34-year-old man comes to the physician because of fatigue and shortness of breath with moderate exertion for the past 2 months. Over the past 10 days, he has had low-grade fevers and night sweats. He has no history of serious illness except for a bicuspid aortic valve diagnosed 5 years ago. He has smoked one pack of cigarettes daily for 10 years and drinks 3–5 beers on social occasions. He does not use illicit drugs. The patient takes no medications. He appears weak. His temperature is 37.7°C (99.9°F), pulse is 70/min, and blood pressure is 128/64 mm Hg. The lungs are clear to auscultation. A grade 2/6 systolic murmur is heard best at the right sternal border and second intercostal space. There are several hemorrhages underneath his fingernails on both hands and multiple tender, red nodules on his fingers. Which of the following is the most likely causal organism?

Accepted Answer

Streptococcus sanguinis

Answer

Candida albicans

Answer

Streptococcus pneumoniae

Answer

Staphylococcus epidermidis

Question 5

A bacterial culture shows 'tumbling motility' pattern. Which microscopic finding would confirm Listeria monocytogenes?

Accepted Answer

Gram-positive coccobacilli

Answer

Gram-variable pleomorphic rods

Answer

Gram-negative diplococci

Answer

Acid-fast bacilli

Question 6

A bacterial culture shows small colonies with narrow zones of β-hemolysis on blood agar and exhibits tumbling motility at 25°C. Which biochemical test would confirm Listeria monocytogenes?

Accepted Answer

CAMP test positivity

Answer

Catalase positivity

Answer

Oxidase positivity

Answer

Urease positivity

Question 7

A fluid culture shows 'spoked wheel' colony appearance. Which microscopic finding would confirm Actinomyces species?

Accepted Answer

Branching filaments

Answer

Pleomorphic rods

Answer

Diplococci

Answer

Curved bacilli

Question 8

A bacterial culture shows 'medusa head' colonies. Which biochemical finding is characteristic of Bacillus cereus?

Accepted Answer

Catalase positive

Answer

Optochin sensitive

Answer

CAMP test positive

Answer

Oxidase positive

Question 9

A bacterial culture shows flat, irregular colonies with metallic sheen. Which additional finding would confirm Pseudomonas aeruginosa?

Accepted Answer

Blue-green pigment with grape odor

Answer

Black pigment

Answer

Beta-hemolysis

Answer

Yellow pigment

Question 10

In a Gram stain preparation of CSF, which organism appears as pairs of kidney-shaped diplococci with adjacent flattened sides?

Accepted Answer

Neisseria meningitidis

Answer

Streptococcus pneumoniae

Answer

Moraxella catarrhalis

Answer

Staphylococcus aureus

Bacteriology — MCQs

Bacteriology — MCQs

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