Bacteriology Practice Questions

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.

Q: Whipple's disease is caused by:

Bacteria. **Bacteria** - Whipple's disease is caused by the bacterium **_Tropheryma whipplei_**, a rod-shaped, gram-positive actinobacterium. - This bacterium causes a chronic, systemic infection affecting various organ systems, most commonly the gastrointestinal tract. *Virus* - Viruses are intracellular parasites that reproduce inside living cells, and they are not the causative agent of Whipple's disease. - While viral infections can cause gastrointestinal symptoms, they do not lead to the distinct histopathological findings of Whipple's disease, such as **PAS-positive macrophages**. *Helminths* - Helminths are parasitic worms (e.g., tapeworms, roundworms) that can cause intestinal and systemic infections. - These organisms are multicellular eukaryotes and are distinct from the bacterial etiology of Whipple's disease. *Protozoa* - Protozoa are single-celled eukaryotic organisms that can cause a range of infections, often gastrointestinal (e.g., Giardia, Entamoeba). - While parasitic infections can cause malabsorption, the specific **PAS-positive macrophage infiltrates** seen in Whipple's disease are characteristic of bacterial infection, not protozoan.

Q: What is the causative agent for malignant otitis externa?

Pseudomonas. ***Pseudomonas*** - **Malignant otitis externa** is a severe, rapidly progressive infection of the external auditory canal and skull base, primarily caused by **Pseudomonas aeruginosa**. - This opportunistic bacterium thrives in moist environments and commonly affects immunocompromised individuals, such as diabetics or the elderly. *Streptococcus* - While various *Streptococcus* species can cause common ear infections (e.g., **otitis media**), they are not the primary causative agent for malignant otitis externa. - Infections by *Streptococcus* typically present differently and affect other parts of the ear or body more commonly. *Staphylococcus aureus* - **Staphylococcus aureus** is a common cause of skin infections, including localized forms of otitis externa (e.g., furuncles), but it is rarely implicated in the aggressive, invasive form known as malignant otitis externa. - When *Staphylococcus aureus* is involved, it often presents differently and is less likely to cause skull base osteomyelitis. *Influenza* - **Influenza** is a viral infection primarily affecting the **respiratory system** and is not a bacterial pathogen responsible for otitis externa, malignant or otherwise. - Viral infections like influenza can predispose individuals to secondary bacterial infections, but they are not directly causative of this condition.

Q: Bacterial vaginosis is caused by all except

Proteus. ***Proteus*** - **Proteus** species are commonly associated with **urinary tract infections (UTIs)** and wound infections, not typically with bacterial vaginosis. - The presence of **Proteus** in vaginal flora is generally considered an opportunistic infection rather than a causative agent of BV. *Coccobacillus* - **Coccobacillary bacteria**, such as **Gardnerella vaginalis**, are primary organisms contributing to the polymicrobial shift that defines bacterial vaginosis. - The disruption of normal lactobacilli dominance allows for the overgrowth of these specific anaerobic and facultative organisms. *Mobiluncus* - **Mobiluncus species** are curved, rod-shaped anaerobic bacteria frequently found in high numbers in women with bacterial vaginosis. - They are considered key anaerobic bacteria involved in the microbial imbalance characteristic of BV. *Gardnerella* - **Gardnerella vaginalis** is the most commonly isolated bacterium in bacterial vaginosis and is considered the primary initiating organism. - While BV is polymicrobial, **Gardnerella's** overgrowth is central to the diagnostic criteria and pathogenesis.

Q: Most common cause of bacterial meningitis in post-neonatal period:

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - *S. pneumoniae* is the most common cause of **bacterial meningitis** in the post-neonatal period (1 month to 1 year of age), particularly in regions with high vaccination rates against Hib. - Its polysaccharide capsule and ability to evade the immune system contribute to its virulence in causing **central nervous system infections**. - Accounts for approximately 40-50% of bacterial meningitis cases in this age group. *Mycobacterium tuberculosis* - While it can cause **tuberculous meningitis**, this is a less common form of meningitis, typically with a more **insidious onset** and often associated with immunosuppression or endemic areas. - Represents a chronic form of meningitis rather than acute bacterial meningitis. *Staphylococcus aureus* - *S. aureus* meningitis typically occurs in specific contexts such as **post-neurosurgery**, following head trauma, or in patients with indwelling catheters or bacteremia. - It is not the most frequent pathogen in community-acquired meningitis in infants. *Klebsiella* - **Klebsiella pneumoniae** can cause meningitis, especially in **neonates** (first 28 days of life), immunocompromised individuals, or patients with healthcare-associated infections. - However, it is not the most common cause of meningitis in the post-neonatal period.

Q: Bacillus anthracis is:

Gram positive rods with square ends. ***Gram positive rods with square ends*** - *Bacillus anthracis* is a **gram-positive bacterium**, meaning it retains the crystal violet stain due to its thick peptidoglycan layer. - Microscopically, it appears as **large, rod-shaped bacteria** (bacilli) with characteristic **square-cut ends**, often arranged in chains. *Gram negative cocci in cluster* - This describes organisms like **Neisseria gonorrhoeae** or **meningitidis**, which are spherical (cocci) and appear pink/red after Gram staining. - *Bacillus anthracis* is a **rod-shaped bacterium** that stains Gram-positive. *Gram positive cocci in cluster* - This morphology is characteristic of **Staphylococcus species**, which are spherical bacteria that stain purple and arrange in grape-like clusters. - *Bacillus anthracis* is a **rod-shaped bacterium**, not a coccus. *Gram positive bacilli with spherical ends* - While *Bacillus anthracis* is a **gram-positive bacillus**, its ends are distinctly **square-cut**, not spherical. - Bacteria with spherical or rounded ends would include some other *Bacillus* species or members of the **Clostridium** genus.

Q: The causative agent for duodenal ulcer is:

H. Pylori. ***H. pylori*** - ***H. pylori*** is a **Gram-negative spiral-shaped bacterium** that colonizes the stomach and duodenum, causing inflammation and increasing the risk of peptic ulcers, including duodenal ulcers. - It produces **urease**, which neutralizes stomach acid by converting urea to ammonia, protecting the bacteria and contributing to mucosal damage. *Lactobacilli* - **Lactobacilli** are **Gram-positive, rod-shaped bacteria** commonly found in the digestive tract and fermented foods. - They are generally considered **beneficial probiotics** and do not cause duodenal ulcers. *E. coli* - ***E. coli*** is a **Gram-negative, rod-shaped bacterium** typically found in the intestines of humans and animals. - While some strains can cause **gastrointestinal infections** (e.g., traveler's diarrhea), *E. coli* is not a known cause of duodenal ulcers. *Shigella* - **Shigella** is a **Gram-negative, rod-shaped bacterium** that causes **shigellosis**, an intestinal infection characterized by severe diarrhea (dysentery). - It primarily affects the **large intestine** and does not cause duodenal ulcers.

Question 1

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 2

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 3

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 4

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

Question 5

Whipple's disease is caused by:

Accepted Answer

Bacteria

Answer

Virus

Answer

Helminths

Answer

Protozoa

Question 6

What is the causative agent for malignant otitis externa?

Accepted Answer

Pseudomonas

Answer

Streptococcus

Answer

Staphylococcus aureus

Answer

Influenza

Question 7

Bacterial vaginosis is caused by all except

Accepted Answer

Proteus

Answer

Coccobacillus

Answer

Mobiluncus

Answer

Gardnerella

Question 8

Most common cause of bacterial meningitis in post-neonatal period:

Accepted Answer

Streptococcus pneumoniae

Answer

Mycobacterium tuberculosis

Answer

Staphylococcus aureus

Answer

Klebsiella

Question 9

Bacillus anthracis is:

Accepted Answer

Gram positive rods with square ends

Answer

Gram negative cocci in cluster

Answer

Gram positive cocci in cluster

Answer

Gram positive bacilli with spherical ends

Question 10

The causative agent for duodenal ulcer is:

Accepted Answer

H. Pylori

Answer

Lactobacilli

Answer

E. coli

Answer

Shigella

Bacteriology — MCQs

Bacteriology — MCQs

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