Bacteriology Practice Questions

Q: A 45-year-old construction worker suffers a penetrating wound of the left leg, which is cleaned and sutured. Three days later, the patient presents with sudden onset of severe pain at the site of injury. Physical examination shows darkening of the surrounding skin, hemorrhage, and cutaneous necrosis. The wound shows a thick serosanguinous discharge with gas bubbles and a foul-smelling odor. Which of the following is the most likely etiology of this patient's wound infection?

Clostridium perfringens. ***Clostridium perfringens*** - The sudden onset of severe pain, **darkening of skin**, **hemorrhage**, **cutaneous necrosis**, and especially the presence of **gas bubbles** and a **fragrant odor** (often described as sweet/putrid) point directly to **gas gangrene**, caused primarily by *Clostridium perfringens*. - This bacterium is a **gram-positive, anaerobic rod** that produces powerful **exotoxins**, leading to rapid tissue destruction and gas production. *Staphylococcus epidermidis* - This bacterium is a common **commensal skin organism** and typically causes **indolent infections** associated with foreign bodies (e.g., catheters, prosthetic joints), not rapidly progressing necrosis and gas. - While it can form **biofilms**, it does not produce the extensive tissue destruction, gas, or characteristic odor seen here. *Staphylococcus aureus* - *S. aureus* causes a variety of infections, including cellulitis, abscesses, and necrotizing fasciitis, but typically does **not produce gas** in tissues or the characteristic fragrant odor. - While it can cause rapid progression and necrosis, the specific feature of **gas bubbles** strongly differentiates it from *Clostridium perfringens*. *Clostridium botulinum* - *Clostridium botulinum* causes **botulism**, a neuroparalytic disease, through its potent neurotoxin. - It does **not typically cause wound infections** with severe local tissue destruction, gas production, or a fragrant discharge.

Q: HP bodies (Halberstaedter-Prowazek bodies) are seen in infections caused by which organism?

Chlamydia trachomatis. ***Chlamydia trachomatis*** - **Halberstaedter-Prowazek (HP) bodies** are distinctive **intracytoplasmic inclusions** found in the host cells infected with **Chlamydia trachomatis**. - These inclusions contain replicating **reticulate bodies** and are characteristic of this species. - A key feature is that HP bodies are **glycogen-rich** and **stain with iodine** (iodine-positive), which is a pathognomonic diagnostic feature distinguishing *C. trachomatis* from other Chlamydia species. *Chlamydia psittaci* - While *Chlamydia psittaci* also forms inclusions, they are generally **larger**, **less compact**, and **iodine-negative** (do not contain glycogen) compared to the HP bodies seen in *C. trachomatis*. - *C. psittaci* is primarily associated with **psittacosis**, a respiratory disease transmitted from birds. *Mycoplasma hominis* - *Mycoplasma* species are **wall-less bacteria** and do not form **intracytoplasmic inclusions** like Chlamydia. - They are distinct from Chlamydia in their cellular structure and life cycle. *Chlamydia pneumoniae* - *Chlamydia pneumoniae* forms **inclusions** that are **iodine-negative** (lack glycogen), **pear-shaped**, and morphologically different from the classical compact HP bodies. - This species is a common cause of **atypical pneumonia** and lacks the distinctive cytological features of *C. trachomatis* inclusions.

Q: Which organism causes prosthetic valve endocarditis within 60 days of surgery?

Staphylococcus epidermidis. ***Staphylococcus epidermidis*** - This organism is the **most common cause of early prosthetic valve endocarditis (within 60 days of surgery)**, accounting for 30-40% of cases. - As a coagulase-negative Staphylococcus, *S. epidermidis* commonly colonizes the skin and can be introduced during the surgical procedure. - It produces **biofilms**, which allow it to adhere to prosthetic surfaces and resist antibiotic treatment. *Staphylococcus aureus* - *Staphylococcus aureus* is the **second most common cause of early prosthetic valve endocarditis** (20-25% of cases) and can cause both early and late infections. - It typically presents with a more **fulminant, aggressive course** compared to *S. epidermidis*. - While both staphylococcal species cause early PVE, *S. epidermidis* remains more frequent. *Fungus* - **Fungal endocarditis** on prosthetic valves (typically *Candida* species) accounts for 5-10% of early PVE cases. - More commonly seen in immunocompromised patients, those with prolonged antibiotic use, or after complicated cardiac surgery. - Its incidence is significantly lower than staphylococcal causes in the early post-operative period. *Streptococcus viridans* - *Streptococcus viridans* is the **most common cause of native valve endocarditis**, particularly in patients with pre-existing valvular heart disease. - It typically causes a **subacute presentation** and is more associated with **late prosthetic valve endocarditis** (>60 days post-surgery), not early PVE. - Rarely implicated in early prosthetic valve infections.

Q: All of the following are true regarding bacterial toxins EXCEPT:

Endotoxin has enzymatic action. ***Endotoxin has enzymatic action*** - This statement is **false** because **endotoxins**, which are **lipopolysaccharides (LPS)** found in the outer membrane of gram-negative bacteria, **do not typically have enzymatic activity**. - Their toxicity is primarily due to their **lipid A component**, which triggers a potent inflammatory response in the host by activating immune cells, rather than through direct enzymatic modification of host molecules. *Exotoxin has enzymatic action* - This statement is **true** as many **exotoxins** are **proteins** with specific enzymatic activities that target host cellular processes. - Examples include **diphtheria toxin** (ADP-ribosylates elongation factor 2) and **botulinum toxin** (cleaves SNARE proteins), disrupting host cell function. *Exotoxin is highly antigenic* - This statement is **true** because exotoxins, being **proteins**, are generally **potent immunogens** that elicit a strong antibody response. - This high antigenicity is exploited in vaccine production; inactivated exotoxins (**toxoids**) are used to induce protective immunity (e.g., tetanus and diphtheria vaccines). *Endotoxin is weakly antigenic* - This statement is **true** because **endotoxins (LPS)** generally elicit a **weak and T-cell-independent antibody response**. - Antibodies produced against the O-antigen component of LPS are often type-specific and do not provide broad protection against different strains.

Q: In which year was Helicobacter pylori identified?

1983. ***1983*** - **Barry Marshall** and **Robin Warren** successfully cultured *Helicobacter pylori* from stomach biopsies, challenging the long-held belief that bacteria could not survive in the acidic environment of the stomach. - Their work definitively linked the bacterium to **gastritis** and **peptic ulcers**, revolutionizing understanding and treatment of these conditions. *1976* - This year is not recognized for the identification or significant discovery of *Helicobacter pylori*. - While research into stomach health was ongoing, the specific identification of this bacterium occurred later. *1994* - By 1994, *Helicobacter pylori* was well-established as a major pathogen, and this year saw significant progress in understanding its role in **gastric cancer**, leading to its classification as a Group 1 carcinogen by the WHO. - However, its *identification* happened much earlier. *1969* - Although there were earlier observations of spiral-shaped bacteria in the stomach, particularly by **Waldo L. Stewart** in 1969, these initial findings did not lead to the successful culture and definitive identification of *Helicobacter pylori* as a pathogenic agent. - The crucial link to gastric disease was not established at this time.

Q: Mouse is used for pathogenicity testing in -

Bordetella pertussis. **Bordetella pertussis** - Mice are commonly used as an animal model for studying the **pathogenesis of pertussis** and for testing vaccine efficacy. - They allow researchers to observe the **infection's progression** in a living system and evaluate potential treatments. *Brucella* - While animal models are used for *Brucella* research, typically **guinea pigs or cattle** are preferred for pathogenicity testing due to their susceptibility and similarity to natural hosts. - Mice can be infected, but they are not the primary model for assessing *Brucella* virulence in the same way they are for *Bordetella pertussis*. *M. Tuberculosis* - **Guinea pigs** are classically used for pathogenicity testing of *Mycobacterium tuberculosis* due to their high susceptibility and ability to develop **granulomatous lesions** similar to humans. - While mice can be infected, guinea pigs are preferred for assessing virulence and vaccine efficacy in a model that closely mimics human disease. *C diphtheriae* - **Guinea pigs** are the standard animal model for testing the **toxigenicity of *Corynebacterium diphtheriae*** strains, often through the Elek test or in vivo challenge. - Mice are generally less susceptible to diphtheria toxin and are not routinely used for this specific pathogenicity test.

Q: Chancroid may be caused by:

Haemophilus ducreyi. ***Haemophilus ducreyi*** - **Chancroid** is a sexually transmitted infection characterized by painful genital ulcers and regional lymphadenopathy, and its causative agent is **Haemophilus ducreyi**. - This bacterium is a **Gram-negative coccobacillus** that leads to the formation of soft chancres. *Chlamydia Trachomatis* - **Chlamydia trachomatis** is responsible for **chlamydia**, a common STI that can cause urethritis, cervicitis, and lymphogranuloma venereum (LGV), but not chancroid. - LGV involves significant **lymph node enlargement** and draining sinuses, distinct from the typical chancres of chancroid. *T. pallidum* - **Treponema pallidum** is the spirochete responsible for **syphilis**, which presents with a painless chancre in its primary stage. - Unlike chancroid, the **syphilitic chancre** is typically firm, clean-based, and usually solitary. *G. donovani* - **Klebsiella granulomatis (formerly Calymmatobacterium granulomatis)** is the causative agent of **granuloma inguinale (donovanosis)**, another STI. - Granuloma inguinale presents with progressive, painless, beefy-red ulcerative lesions that bleed easily, which are distinct from the painful ulcers of chancroid.

Q: Most common agent causing tuberculosis in AIDS patients in tropical countries is:

Mycobacterium tuberculosis. ***Mycobacterium tuberculosis*** - Despite immune compromise, **classical *Mycobacterium tuberculosis* (MTB)** remains the most common cause of tuberculosis in AIDS patients globally, including in tropical regions. - The high prevalence of **latent TB infection** in these populations, coupled with HIV-induced immunosuppression, leads to reactivation of MTB as the primary pathology. *Atypical mycobacteria* - While atypical mycobacteria can cause disseminated disease in AIDS patients, they are generally **less common** than *Mycobacterium tuberculosis* in causing true tuberculosis in tropical countries. - Atypical mycobacterial infections typically occur in patients with **more severe immunosuppression** (lower CD4 counts) than those with primary pulmonary TB caused by M. tuberculosis. *Mycobacterium intracellulare* - *Mycobacterium intracellulare* is a species of **atypical mycobacteria** often grouped with *Mycobacterium avium* as MAC. - Although it causes disseminated disease in AIDS patients, it is not the most common agent of tuberculosis compared to *Mycobacterium tuberculosis*. *Mycobacterium avium* - *Mycobacterium avium*, part of the **Mycobacterium Avium Complex (MAC)**, frequently causes disseminated infection in AIDS patients, especially with **CD4 counts below 50 cells/mm³**. - However, it typically manifests as a **disseminated disease** rather than classic pulmonary tuberculosis, and is overall less common than *Mycobacterium tuberculosis* as the causative agent of TB in tropical settings.

Q: Which of the following is commonly responsible for toxic shock syndrome in female patients:

Staphylococcus aureus. ***Staphylococcus aureus*** - This bacterium produces **TSST-1** (toxic shock syndrome toxin-1), a superantigen that triggers a massive, non-specific T-cell activation and cytokine storm. - The association with **tampon use** in menstruating women and its specific toxins are classic for **toxic shock syndrome**. *Pseudomonas* - While *Pseudomonas aeruginosa* can cause severe infections, it is typically associated with **nosocomial infections**, **cystic fibrosis exacerbations**, or conditions in immunocompromised patients, not classic toxic shock syndrome. - It does not produce the specific superantigens (like TSST-1) linked to the characteristic presentation of toxic shock syndrome. *Streptococcus-group B* - Also known as *Streptococcus agalactiae*, Group B Strep is a common cause of **neonatal sepsis** and **meningitis**, urinary tract infections, and maternal infections during pregnancy. - Group B Streptococcus does **NOT** cause toxic shock syndrome. It is **Group A Streptococcus** (*S. pyogenes*) that causes streptococcal toxic shock syndrome (STSS), typically associated with invasive soft tissue infections. *H.influenzae* - *Haemophilus influenzae* is a significant cause of **otitis media**, **sinusitis**, **epiglottitis**, and **meningitis**, particularly in unvaccinated children. - It is not known to produce the toxins responsible for either staphylococcal or streptococcal toxic shock syndrome.

Q: Cerebrospinal fever is due to

Neisseria meningitidis. ***Neisseria meningitidis*** - **Cerebrospinal fever** is another name for **meningococcal meningitis**, which is primarily caused by *Neisseria meningitidis*. - This bacterium is highly transmissible and can cause severe inflammation of the meninges, the membranes surrounding the brain and spinal cord. *Streptococcus pyogenes* - *Streptococcus pyogenes* is known for causing **strep throat**, **scarlet fever**, and **rheumatic fever**. - While it can cause severe infections, it is not the primary cause of cerebrospinal fever or meningococcal meningitis. *Staphylococcus aureus* - *Staphylococcus aureus* is a common cause of **skin infections**, **pneumonia**, and **sepsis**. - While it can cause meningitis, it is typically seen in specific contexts (e.g., healthcare-associated infections) and not primarily associated with the term "cerebrospinal fever." *None of the options* - This option is incorrect because *Neisseria meningitidis* is a well-established cause of cerebrospinal fever.

Question 1

A 45-year-old construction worker suffers a penetrating wound of the left leg, which is cleaned and sutured. Three days later, the patient presents with sudden onset of severe pain at the site of injury. Physical examination shows darkening of the surrounding skin, hemorrhage, and cutaneous necrosis. The wound shows a thick serosanguinous discharge with gas bubbles and a foul-smelling odor. Which of the following is the most likely etiology of this patient's wound infection?

Accepted Answer

Clostridium perfringens

Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Clostridium botulinum

Question 2

HP bodies (Halberstaedter-Prowazek bodies) are seen in infections caused by which organism?

Accepted Answer

Chlamydia trachomatis

Answer

Chlamydia psittaci

Answer

Mycoplasma hominis

Answer

Chlamydia pneumoniae

Question 3

Which organism causes prosthetic valve endocarditis within 60 days of surgery?

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Fungus

Answer

Streptococcus viridans

Question 4

All of the following are true regarding bacterial toxins EXCEPT:

Accepted Answer

Endotoxin has enzymatic action

Answer

Exotoxin has enzymatic action

Answer

Exotoxin is highly antigenic

Answer

Endotoxin is weakly antigenic

Question 5

In which year was Helicobacter pylori identified?

Accepted Answer

1983

Answer

1976

Answer

1994

Answer

1969

Question 6

Mouse is used for pathogenicity testing in -

Accepted Answer

Bordetella pertussis

Answer

Brucella

Answer

M. Tuberculosis

Answer

C diphtheriae

Question 7

Chancroid may be caused by:

Accepted Answer

Haemophilus ducreyi

Answer

Chlamydia Trachomatis

Answer

T. pallidum

Answer

Klebsiella granulomatis

Question 8

Most common agent causing tuberculosis in AIDS patients in tropical countries is:

Accepted Answer

Mycobacterium tuberculosis

Answer

Atypical mycobacteria

Answer

Mycobacterium intracellulare

Answer

Mycobacterium avium

Question 9

Which of the following is commonly responsible for toxic shock syndrome in female patients:

Accepted Answer

Staphylococcus aureus

Answer

Pseudomonas

Answer

Streptococcus-group B

Answer

H.influenzae

Question 10

Cerebrospinal fever is due to

Accepted Answer

Neisseria meningitidis

Answer

Streptococcus pyogenes

Answer

Staphylococcus aureus

Answer

None of the options

Bacteriology — MCQs

Bacteriology — MCQs

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