Bacteriology Practice Questions

Q: An 8-year-old boy complains of left leg pain for 3 days. On physical examination, his temperature is 38.9°C, and he exhibits irritability when his left leg is moved. A radiograph of the left leg shows changes suggesting acute osteomyelitis in the proximal portion of the left femur. Culture of the infected bone is most likely to grow which of the following organisms?

Staphylococcus aureus. ***Staphylococcus aureus*** - **_Staphylococcus aureus_** is the most common cause of **acute osteomyelitis** in children. - The clinical presentation with a febrile child, localized leg pain, and radiographic changes strongly points to **bacterial osteomyelitis**, with **_S. aureus_** being the predominant pathogen. *Hemophilus influenzae* - **_Hemophilus influenzae_** was once a significant cause of osteomyelitis in children before widespread **HiB vaccination**. - Its incidence has drastically decreased, making it a less likely cause compared to **_S. aureus_** in an 8-year-old. *Neisseria gonorrhoeae* - **_Neisseria gonorrhoeae_** typically causes **septic arthritis** or osteomyelitis in sexually active adolescents and adults, often in disseminated infections. - It is an **uncommon cause** of osteomyelitis in an 8-year-old child and highly unlikely without a history of sexual abuse or neonatal exposure. *Salmonella enterica* - **_Salmonella enterica_** can cause osteomyelitis, particularly in patients with **sickle cell disease** or other **hemoglobinopathies**. - Without any mention of underlying predisposing conditions like sickle cell disease, **_Salmonella_** is a less probable cause than **_S. aureus_**.

Q: Which organism, when grown on agar, is likely to show green-colored colonies?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - This bacterium produces **pyoverdine** and **pyocyanin**, pigments that give colonies a characteristic **metallic sheen** and a **greenish-blue color** on agar. - The green pigmentation is a key identifying feature on selective and differential media like **cetrimide agar**. *Staphylococcus aureus* - Typically forms colonies that are **cream-colored to golden-yellow** due to the production of carotenoid pigments. - It is known for its **beta-hemolysis** on blood agar but does not produce green colonies. *Streptococcus pneumoniae* - Characteristically forms **small, gray, mucoid colonies** with an area of **alpha-hemolysis** (partial hemolysis) on blood agar, giving a greenish discoloration around the colonies due to methemoglobin formation. - The colonies themselves are not primarily green, but the surrounding agar may appear green. *Escherichia coli* - Generally forms **large, gray, flat colonies** that are typically **non-hemolytic (gamma-hemolysis)** on blood agar. - On **MacConkey agar**, *E. coli* produces **pink colonies** due to lactose fermentation and acid production.

Q: A 7-year-old boy presented to the OPD with complaints of pain and discharge from the left ear. On examination, the patient was febrile, with neck rigidity and a positive Kernig's sign. Gram-positive cocci were visualized in the Gram-stained specimen of the discharge. What is the most likely pathogen?

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - *Streptococcus pneumoniae* is a common cause of **otitis media**, which, if untreated, can lead to complications such as **meningitis**, presenting with neck rigidity and a positive Kernig's sign. - Gram-stain revealing **Gram-positive cocci** is consistent with *Streptococcus pneumoniae*, particularly in the context of ear discharge and meningeal signs. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause ear infections, it is less commonly associated with the rapid progression to **meningitis** in children following otitis media compared to *S. pneumoniae*. - *S. aureus* typically presents with **abscess formation** and cellulitis and, while Gram-positive cocci, the clinical picture strongly favors *S. pneumoniae*. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is a common **commensal skin organism** and rarely causes serious infections in immunocompetent individuals like this child. - It is often found as a contaminant in cultures and is not a primary pathogen for otitis media leading to **meningitis**. *Haemophilus influenzae* - *Haemophilus influenzae* is a common cause of **otitis media** and historically meningitis, but it is a **Gram-negative coccobacillus**, which contradicts the Gram-stain finding of Gram-positive cocci. - Meningitis due to *H. influenzae* has significantly decreased due to widespread vaccination with the **Hib vaccine**.

Q: Fever, leukopenia, disseminated intravascular coagulation, and hypotension caused by members of the Enterobacteriaceae family are most strongly associated with which of the following structures?

Lipid A. **Correct: Lipid A** - **Lipid A** is the endotoxic component of **lipopolysaccharide (LPS)** found in the outer membrane of Gram-negative bacteria like Enterobacteriaceae. - It is responsible for triggering a severe inflammatory response, leading to systemic effects such as **fever, leukopenia, disseminated intravascular coagulation (DIC)**, and **hypotension (septic shock)**. - Lipid A stimulates macrophages to release pro-inflammatory cytokines (TNF-α, IL-1, IL-6), which mediate the clinical manifestations of endotoxic shock. *Incorrect: Polysaccharides* - This term is too general; while **LPS contains a polysaccharide component (O antigen)**, it is the **Lipid A** portion that exerts the potent endotoxic effects. - The **O antigen** is significant for serotyping and can modulate immune responses, but it is not the primary mediator of the severe systemic symptoms listed. *Incorrect: H antigens (flagellar antigens)* - **H antigens** are located on the bacterial flagella and are primarily involved in **motility** and aiding in bacterial identification and serotyping. - They do not directly cause the severe systemic symptoms like DIC or septic shock, although flagella can contribute to inflammation in other ways. *Incorrect: K antigens (capsular antigens)* - **K antigens** are associated with the bacterial **capsule**, which provides protection against phagocytosis and aids in adhesion. - While capsules contribute to virulence and immune evasion, they are not directly responsible for the endotoxic effects seen with disseminated infection.

Q: What is the primary site of action of tetanospasmin in the nervous system?

Presynaptic terminals of the spinal cord. ***Presynaptic terminals of the spinal cord*** - **Tetanospasmin** is transported via **retrograde axonal transport** to the central nervous system, specifically targeting the **presynaptic terminals** of inhibitory interneurons in the spinal cord. - It interferes with the release of **inhibitory neurotransmitters** like **GABA** and **glycine**, leading to uncontrolled muscle spasms. *Postsynaptic terminals of the spinal cord* - This is incorrect because tetanospasmin acts by preventing the release of inhibitory neurotransmitters from the **presynaptic terminal**, rather than directly affecting the postsynaptic receptor. - While the absence of inhibition is perceived at the postsynaptic terminal, the direct mechanism of action is presynaptic. *Neuromuscular junction* - This is incorrect because **tetanospasmin** does not primarily act at the neuromuscular junction; that is the site of action for toxins like **botulinum toxin**. - Tetanospasmin is transported to the central nervous system to exert its effects. *Muscle fibers* - This is incorrect as **tetanospasmin** does not act directly on muscle fibers. - Its action is on the **nervous system**, leading to altered neuronal signaling that indirectly affects muscle contraction.

Q: Early reactions of the lepromin test are read at:

48 hours. ***48 hours*** - The **early reaction** of the lepromin test, known as the **Fernandez reaction**, is read at **48-72 hours** after intradermal injection of lepromin antigen. - This represents a **delayed-type hypersensitivity (Type IV) reaction** mediated by T-cells, not humoral antibodies. - The reaction appears as an indurated papule or nodule at the injection site, with **48 hours** being the standard reading time. - It is **positive in tuberculoid leprosy** and reflects pre-existing cell-mediated immunity to *Mycobacterium leprae*. *24 hours* - At 24 hours, the reaction is typically **too early** to assess the Fernandez reaction adequately. - The peak induration of the delayed-type hypersensitivity reaction occurs at **48-72 hours**, not 24 hours. - Reading at 24 hours may miss or underestimate the early reaction. *36 hours* - While 36 hours falls within the developing phase, it is **not the standard reading time** for the Fernandez reaction. - The reaction continues to develop and is optimally assessed at **48 hours** or later. *72 hours* - 72 hours is also within the acceptable window for reading the **Fernandez reaction** (48-72 hours). - However, **48 hours** is more commonly cited as the standard reading time in most references. - The **late reaction (Mitsuda reaction)** is read much later at **3-4 weeks** and reflects stronger cell-mediated immunity, seen as granuloma formation.

Q: What is the most common cause of pedal botryomycosis?

Staphylococcus aureus. ***Correct: Staphylococcus aureus*** - **Botryomycosis** is a rare chronic **bacterial infection** (not fungal) that clinically mimics actinomycetoma, hence also called **"bacterial pseudomycetoma"** - **_Staphylococcus aureus_** is the **most common causative agent** (>40% of cases), especially in cutaneous/pedal forms - Presents with characteristic **granulomatous lesions** with sulfur granules containing bacterial colonies - Other bacterial causes include _Pseudomonas aeruginosa_, _E. coli_, and _Proteus_ *Incorrect: Streptomyces* - **_Streptomyces_** species cause **actinomycetoma** (true mycetoma), not botryomycosis - It's an aerobic actinomycete causing chronic granulomatous infection with sulfur granules - Classified as a distinct type of mycetoma, not a bacterial pseudomycetoma *Incorrect: Nocardia brasiliensis* - **_Nocardia brasiliensis_** is another significant cause of **actinomycetoma**, particularly in tropical regions - Belongs to the _Actinomycetales_ order (aerobic actinomycete) - Produces sulfur granules but is NOT a common pyogenic bacteria causing botryomycosis *Incorrect: All of these* - **Only _Staphylococcus aureus_** causes botryomycosis among the listed organisms - _Nocardia_ and _Streptomyces_ cause **actinomycetoma** (true mycetoma due to aerobic actinomycetes), which is clinically similar but etiologically distinct from botryomycosis - Botryomycosis = bacterial infection by pyogenic bacteria; Actinomycetoma = infection by filamentous bacteria (actinomycetes)

Q: Botryomycosis is caused by:

Bacteria. ***Bacteria*** - Despite its name, **botryomycosis** is a chronic deep **bacterial infection**, not fungal. - It's typically caused by **Staphylococcus aureus** or, less commonly, **Pseudomonas aeruginosa** [1]. *Fungus* - The "mycosis" in the name is misleading; it's **not a fungal infection**. - Fungal infections like **mycetoma** can cause similar granulomatous lesions but are caused by true fungi. *Virus* - **Viruses** are intracellular parasites that reproduce inside living cells and do not cause botryomycosis. - Viral infections typically present differently, often with **acute symptoms** and systemic involvement. *Protozoa* - **Protozoa** are single-celled eukaryotic organisms, responsible for diseases like malaria or giardiasis. - They do not cause botryomycosis, which is a bacterial granulomatous condition.

Q: The incubation period of gonorrhoea is

2 to 15 days. The typical incubation period for **gonorrhoea** is **2-7 days in males** and up to **14 days in females**, aligning with **CDC guidelines** for *Neisseria gonorrhoeae* infection. Many cases remain **asymptomatic**, particularly in women, making this timeframe crucial for contact tracing and screening protocols. *Less than 24 hrs* - An incubation period of less than 24 hours is **medically implausible** for bacterial STIs, as it doesn't allow sufficient time for bacterial colonization and host immune response. - Such rapid onset is more characteristic of **toxin-mediated illnesses** or **hypersensitivity reactions**, not bacterial infections. *1 to 2 days* - While theoretically possible with **very high bacterial load** or in highly susceptible individuals, this represents the **absolute minimum** timeframe and is not typical. - The **majority of gonorrhoea cases** develop symptoms beyond this very short window, making it an inadequate representation of the standard incubation period. *12 to 25 days* - This timeframe is **too prolonged** for gonorrhoea and more characteristic of **chlamydia infections**, which have an incubation period of **7-21 days**. [1] - Gonorrhoea typically manifests much earlier due to the **aggressive nature** of *Neisseria gonorrhoeae* compared to other bacterial STIs.

Question 1

An 8-year-old boy complains of left leg pain for 3 days. On physical examination, his temperature is 38.9°C, and he exhibits irritability when his left leg is moved. A radiograph of the left leg shows changes suggesting acute osteomyelitis in the proximal portion of the left femur. Culture of the infected bone is most likely to grow which of the following organisms?

Accepted Answer

Staphylococcus aureus

Answer

Hemophilus influenzae

Answer

Neisseria gonorrhoeae

Answer

Salmonella enterica

Question 2

Which organism, when grown on agar, is likely to show green-colored colonies?

Accepted Answer

Pseudomonas aeruginosa

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Escherichia coli

Question 3

A 7-year-old boy presented to the OPD with complaints of pain and discharge from the left ear. On examination, the patient was febrile, with neck rigidity and a positive Kernig's sign. Gram-positive cocci were visualized in the Gram-stained specimen of the discharge. What is the most likely pathogen?

Accepted Answer

Streptococcus pneumoniae

Answer

Staphylococcus aureus

Answer

Staphylococcus epidermidis

Answer

Haemophilus influenzae

Question 4

Fever, leukopenia, disseminated intravascular coagulation, and hypotension caused by members of the Enterobacteriaceae family are most strongly associated with which of the following structures?

Accepted Answer

Lipid A

Answer

Polysaccharides

Answer

H antigens (flagellar antigens)

Answer

K antigens (capsular antigens)

Question 5

What is the primary site of action of tetanospasmin in the nervous system?

Accepted Answer

Presynaptic terminals of the spinal cord

Answer

Postsynaptic terminals of the spinal cord

Answer

Neuromuscular junction

Answer

Muscle fibers

Question 6

Early reactions of the lepromin test are read at:

Accepted Answer

48 hours

Answer

36 hours

Answer

72 hours

Answer

24 hours

Question 7

What is the most common cause of pedal botryomycosis?

Accepted Answer

Staphylococcus aureus

Answer

Streptomyces

Answer

All of these

Answer

Nocardia brasiliensis

Question 8

All the following statements regarding cervicofacial actinomycosis are true, except:

Accepted Answer

10-20% of actinomycosis cases occur in cervicofacial region.

Answer

The discharging pus contains visible 'sulphur granules'

Answer

There is usually history of trauma such as tooth extraction or a blow to the jaw

Answer

Cultures on blood agar often produce a typical 'molar tooth' morphology

Question 9

Botryomycosis is caused by:

Accepted Answer

Bacteria

Answer

Fungus

Answer

Virus

Answer

Protozoa

Question 10

The incubation period of gonorrhoea is

Accepted Answer

2 to 15 days

Answer

Less than 24 hrs

Answer

1 to 2 days

Answer

12 to 25 days

Bacteriology — MCQs

Bacteriology — MCQs

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