Bacteriology Practice Questions

Q: Virulence determinant implicated in the pathogenesis of scarlet fever is -

Pyrogenic exotoxin (toxin produced by bacteria). ***Pyrogenic exotoxin (toxin produced by bacteria)*** - Scarlet fever is caused by strains of **_Streptococcus pyogenes_** (Group A Streptococcus, GAS) that produce **streptococcal pyrogenic exotoxins (SPEs)**, also known as erythrogenic toxins. - These toxins act as superantigens, leading to the characteristic **scarlet rash** and other systemic symptoms of scarlet fever. *Hyaluronidase (enzyme aiding tissue invasion)* - **Hyaluronidase** is an enzyme produced by _S. pyogenes_ that degrades hyaluronic acid, a component of connective tissue, facilitating bacterial spread. - While it contributes to the invasiveness of GAS infections, it is **not directly responsible for the characteristic symptoms of scarlet fever**. *M protein (immune evasion factor)* - The **M protein** is a major virulence factor of _S. pyogenes_ that helps the bacteria resist phagocytosis and adhere to host cells. - It plays a critical role in the pathogenesis of various GAS infections but is **not the direct cause of the scarlet rash or systemic toxicity of scarlet fever**. *Streptokinase (enzyme aiding bacterial spread)* - **Streptokinase** is an enzyme produced by _S. pyogenes_ that activates plasminogen, leading to the dissolution of fibrin clots and aiding bacterial spread through tissues. - While important for bacterial dissemination, it is **not the specific virulence factor responsible for the clinical features of scarlet fever**.

Q: What is the causative agent of Yaws?

Treponema pertenue. ***Treponema pertenue*** - This spirochete is the **causative agent** of **Yaws**, a chronic disfiguring infectious disease primarily affecting the skin, bones, and joints. - Yaws is typically transmitted through **direct skin-to-skin contact** with infectious lesions in tropical and subtropical regions. *Treponema carateum* - This bacterium is responsible for **Pinta**, another non-venereal treponemal infection. - Pinta primarily causes **skin lesions** characterized by dyschromic patches, but does not involve bone or joint destruction as is seen in Yaws. *Treponema pallidum* - This species has several subspecies, with *Treponema pallidum subspecies pallidum* being the cause of **venereal syphilis**. - While *T. pallidum* is a treponemal infection, its clinical presentation and mode of transmission are distinct from Yaws. *Treponema endemicum* - This subspecies of *Treponema pallidum* is the causative agent of **Bejel** (also known as endemic syphilis). - Like Yaws, Bejel is a non-venereal treponemal infection, but it primarily affects mucosal surfaces and bones, with different clinical characteristics than Yaws.

Q: Which organism is most commonly associated with the wearing of soft contact lenses?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - This is the **most common bacterial pathogen** causing contact lens-related microbial keratitis, accounting for 30-50% of cases. - Particularly associated with **soft contact lens wear**, it is known for rapid progression and severe corneal ulceration. - Risk factors include **overnight wear**, poor lens hygiene, and contaminated lens storage cases. - Can lead to **sight-threatening keratitis** within 24-48 hours if untreated. *Acanthamoeba* - While strongly associated with contact lens wear (especially poor hygiene and water exposure), it is **much less common** than bacterial causes. - Acanthamoeba keratitis is a severe protozoal infection but has a relatively low incidence (~1-2 per million contact lens wearers). - Characterized by severe pain disproportionate to clinical signs and ring-shaped stromal infiltrate. *Staphylococcus aureus* - A common cause of bacterial keratitis but **less specifically associated** with soft contact lens wear compared to Pseudomonas. - More commonly causes blepharitis and external eye infections. *Aspergillus* - Fungal keratitis is **rarely associated** with contact lens wear in developed countries. - More commonly linked to ocular trauma with vegetative matter in agricultural settings.

Q: A person developed severe vomiting after eating food from a Chinese restaurant, 3 hours prior. What is the most likely causative agent?

B. cereus. ***B. cereus*** - *Bacillus cereus* produces an **emetic toxin (cereulide)** that causes vomiting within **1-6 hours** after consumption, fitting the 3-hour onset perfectly. - This bacterium is commonly associated with **fried rice** in Chinese restaurants due to improperly cooked or reheated rice where spores survive cooking and germinate. - The preformed heat-stable toxin causes rapid-onset vomiting without significant diarrhea. *Staph. aureus* - While *Staphylococcus aureus* food poisoning also causes rapid-onset vomiting within **1-6 hours** due to preformed enterotoxin, making it a close differential. - However, *B. cereus* is more specifically associated with **starchy foods like rice**, making it the most likely culprit in the context of Chinese restaurant food. *C. difficile* - *Clostridioides difficile* infection primarily causes **diarrhea and pseudomembranous colitis**, typically after antibiotic use disrupting normal gut flora. - The onset of symptoms is typically **days to weeks** after exposure, not acute vomiting within hours of a meal. - Not associated with foodborne transmission. *Pseudomonas* - *Pseudomonas* species rarely cause foodborne illness and are not associated with rapid-onset vomiting. - They are primarily opportunistic pathogens causing infections in immunocompromised individuals or healthcare settings. - When implicated in food contamination, they affect raw produce, not typical Chinese restaurant cooked food.

Q: A 24-year-old woman is breastfeeding 3 weeks after giving birth to a normal term infant and notices fissures in the skin around her left nipple. Over the next 3 days, a 5-cm region near the nipple becomes erythematous and tender, with purulent exudate from a small abscess draining through a fissure. Which of the following organisms is most likely to be cultured from the exudate?

Staphylococcus aureus. ***Staphylococcus aureus*** - This presentation of **mastitis with abscess formation** is overwhelmingly caused by **_Staphylococcus aureus_**, especially in breastfeeding women. - The organism gains entry through **nipple fissures**, leading to erythema, tenderness, and purulent exudate characteristic of a bacterial infection. *Candida albicans* - While it can cause **nipple soreness** and **thrush** in infants, it typically presents as burning pain and shiny skin, not usually with a localized abscess and purulent exudate. - *Candida* infections are more common with **antibiotic use** or in women with compromised immune systems. *Lactobacillus acidophilus* - This is a common **probiotic organism** found in the gut and vagina, known for its beneficial role. - It does not cause purulent infections or mastitis in this context. *Listeria monocytogenes* - This bacterium is primarily known for causing **foodborne illness** and can lead to severe infections like meningitis or sepsis, particularly in pregnant women and neonates. - It is not a common cause of superficial abscesses or mastitis in breastfeeding women.

Q: Cholera is caused by:

V.cholerae O1 and O139. ***V.cholerae O1 and O139*** - Both serogroups, **O1** and **O139**, are known to cause epidemic and pandemic cholera. - They produce the **cholera toxin**, leading to the characteristic severe watery diarrhea. *V.cholerae O1* - While **V. cholerae O1** is historically the most common cause of cholera, it is not the *only* serogroup responsible for the disease globally. - Limiting the definition to O1 would exclude outbreaks caused by the O139 serogroup. *V. cholerae O139* - **V. cholerae O139** emerged as a significant cause of cholera in Asia in the early 1990s and is capable of causing large outbreaks. - However, stating only O139 would omit the more widespread and historically significant O1 serogroup responsible for most cholera cases. *NAG vibrios* - **NAG vibrios** (Non-agglutinable vibrios), also known as non-O1/non-O139 *Vibrio cholerae*, can cause sporadic cases of diarrheal disease. - They typically cause a milder illness and are not commonly associated with the severe epidemic or pandemic cholera caused by O1 and O139 serogroups.

Q: Which of the following is a motile member of the Enterobacteriaceae family?

Morganella. ***Morganella*** - *Morganella morganii* possesses **peritrichous flagella** and is motile at **25°C** (room temperature), which is the standard temperature for motility testing. - At 37°C (body temperature), motility may be reduced or absent, but in standard microbiology classification, Morganella is considered a **motile organism**. - This distinguishes it from consistently non-motile genera like Shigella and Klebsiella. *Shigella* - All species within the genus *Shigella* are inherently **non-motile** at all temperatures. - Complete absence of flagella is a fundamental differentiating feature from other Enterobacteriaceae. - This lack of motility is one of the key characteristics used in laboratory identification. *Klebsiella* - Members of the *Klebsiella* genus, notably *Klebsiella pneumoniae*, are **non-motile** and possess a prominent polysaccharide capsule. - Their immobility and large capsule are important identifying characteristics in laboratory settings. - The absence of flagella is consistent across all species. *Yersinia* - *Yersinia enterocolitica* exhibits **temperature-dependent motility** (motile at 25°C, non-motile at 37°C). - However, *Yersinia pestis*, the most clinically significant species causing plague, is **non-motile at all temperatures**. - In standard teaching, Yersinia is often represented by Y. pestis and characterized as non-motile, making Morganella the better answer for a consistently recognized motile member.

Q: Which of the following conditions is most commonly classified as a sexually transmitted disease?

Chlamydia. ***Chlamydia*** - **Chlamydia trachomatis** is a bacterial infection that is exclusively transmitted through **sexual contact**, making it a classic sexually transmitted disease. - It often presents with **asymptomatic infections** but can lead to serious complications if untreated, such as pelvic inflammatory disease (PID) and infertility. *Trichomoniasis* - Caused by the **protozoan Trichomonas vaginalis**, it is also a sexually transmitted infection. - While sexually transmitted, **Chlamydia** is generally more prevalent and often considered the *most commonly* classified or reported bacterial STD. *Giardiasis* - Caused by the parasite **Giardia lamblia**, it is typically acquired through the ingestion of contaminated water or food. - It is primarily an **enteric infection** and not classified as a sexually transmitted disease, although anal-oral contact can be a route of transmission in some cases. *Amoebiasis* - Caused by the parasite **Entamoeba histolytica**, it is spread through the fecal-oral route, usually via contaminated food or water. - **Amoebiasis** is an infection of the intestines and liver, and it is not considered a sexually transmitted disease.

Q: Which of the following bacteria is microaerophilic?

Campylobacter. ***Campylobacter*** - **Campylobacter jejuni** is a classic example of a microaerophilic bacterium, thriving in environments with **reduced oxygen (5-10% O2)** and **increased CO2 (5-10%)**. - This specific atmospheric requirement is crucial for its **growth** and **virulence**, often leading to gastroenteritis. *Vibrio cholerae* - **Vibrio cholerae** is a **facultative anaerobe**, meaning it can grow in both the presence and absence of oxygen. - It does not require low oxygen environments; optimal growth occurs **aerobically**. *Pseudomonas* - **Pseudomonas aeruginosa** is an **obligate aerobe**; it requires oxygen for respiration and growth. - It uses oxygen as the **final electron acceptor** in its electron transport chain. *Salmonella* - **Salmonella enterica** is a **facultative anaerobe**, capable of switching between aerobic and anaerobic respiration. - It can grow in the presence of oxygen, as well as in **anaerobic conditions** by fermenting sugars.

Question 1

Which of the following statements about Campylobacter jejuni is correct?

Accepted Answer

Most often occurs several days after consumption of undercooked chicken.

Answer

Gram-positive coccus

Answer

Symptoms may initially mimic appendicitis

Answer

Macrolides should be used in all cases

Question 2

Virulence determinant implicated in the pathogenesis of scarlet fever is -

Accepted Answer

Pyrogenic exotoxin (toxin produced by bacteria)

Answer

M protein (immune evasion factor)

Answer

Hyaluronidase (enzyme aiding tissue invasion)

Answer

Streptokinase (enzyme aiding bacterial spread)

Question 3

What is the causative agent of Yaws?

Accepted Answer

Treponema pertenue

Answer

Treponema endemicum

Answer

Treponema pallidum

Answer

Treponema carateum

Question 4

Which organism is most commonly associated with the wearing of soft contact lenses?

Accepted Answer

Pseudomonas aeruginosa

Answer

Acanthamoeba

Answer

Aspergillus

Answer

Staphylococcus aureus

Question 5

A person developed severe vomiting after eating food from a Chinese restaurant, 3 hours prior. What is the most likely causative agent?

Accepted Answer

B. cereus

Answer

C. difficile

Answer

Staph. aureus

Answer

Pseudomonas

Question 6

A 24-year-old woman is breastfeeding 3 weeks after giving birth to a normal term infant and notices fissures in the skin around her left nipple. Over the next 3 days, a 5-cm region near the nipple becomes erythematous and tender, with purulent exudate from a small abscess draining through a fissure. Which of the following organisms is most likely to be cultured from the exudate?

Accepted Answer

Staphylococcus aureus

Answer

Candida albicans

Answer

Lactobacillus acidophilus

Answer

Listeria monocytogenes

Question 7

Cholera is caused by:

Accepted Answer

V.cholerae O1 and O139

Answer

V.cholerae O1

Answer

NAG vibrios

Answer

V.cholerae O139

Question 8

Which of the following is a motile member of the Enterobacteriaceae family?

Accepted Answer

Morganella

Answer

Shigella

Answer

Yersinia

Answer

Klebsiella

Question 9

Which of the following conditions is most commonly classified as a sexually transmitted disease?

Accepted Answer

Chlamydia

Answer

Trichomoniasis

Answer

Giardiasis

Answer

Amoebiasis

Question 10

Which of the following bacteria is microaerophilic?

Accepted Answer

Campylobacter

Answer

Pseudomonas

Answer

Salmonella

Answer

Vibrio cholerae

Bacteriology — MCQs

Bacteriology — MCQs

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