Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2521: Streptococcus pyogenes is classified as which type of bacteria?

A. Gram positive cocci (Correct Answer)
B. Gram negative cocci
C. Gram positive bacilli
D. Gram negative bacilli

Explanation: ***Gram positive cocci*** - *Streptococcus pyogenes* are **spherical-shaped (cocci) bacteria** that stain **purple** with the Gram stain, indicating a thick peptidoglycan layer characteristic of Gram-positive organisms. - They typically arrange in **chains** and are known for causing **strep throat**, scarlet fever, and rheumatic fever. *Gram negative cocci* - Gram-negative cocci like *Neisseria meningitidis* or *Neisseria gonorrhoeae* stain **pink or red** due to their thin peptidoglycan layer and outer membrane. - While they are also cocci, their Gram stain reaction is distinctly different from *Streptococcus pyogenes*. *Gram positive bacilli* - Gram-positive bacilli are **rod-shaped bacteria** that stain purple, examples include *Bacillus anthracis* or *Clostridium difficile*. - *Streptococcus pyogenes* is clearly a coccus (spherical), not a bacillus (rod). *Gram negative bacilli* - Gram-negative bacilli are **rod-shaped bacteria** that stain pink or red, such as *Escherichia coli* or *Pseudomonas aeruginosa*. - This classification differs from *Streptococcus pyogenes* in both cell shape and Gram stain reaction.

Question 2522: Which of the following statements about Streptococcus is false?

A. Streptolysin O is active in oxidized state. (Correct Answer)
B. Pyrogenic toxin A is encoded by a bacteriophage.
C. Streptodornase cleaves DNA.
D. Streptokinase is produced from serotype A, C, K.

Explanation: ***Streptolysin O is active in oxidized state.*** - This statement is **FALSE** because **Streptolysin O** is a **thiol-activated toxin** that is **inactivated by oxygen**. - It is active in the **reduced state**, not the oxidized state. - Its lytic activity on red blood cells is maximal under **anaerobic conditions**, which distinguishes it from streptolysin S (which is oxygen-stable). - This is the correct answer to the question asking for a false statement. *Streptokinase is produced from serotype A, C, K.* - This statement is somewhat misleading in its terminology, but refers to **streptokinase** production by *Streptococcus pyogenes* (Group A Streptococcus). - While the serotype classification mentioned is not standard, streptokinase is indeed produced by Group A streptococci. *Streptodornase cleaves DNA.* - **TRUE statement**: Streptodornase (also known as deoxyribonuclease or DNase) is an enzyme produced by *Streptococcus pyogenes*. - It **cleaves extracellular DNA**, helping the bacteria degrade neutrophil extracellular traps (NETs) and reduce pus viscosity, facilitating bacterial spread. *Pyrogenic toxin A is encoded by a bacteriophage.* - **TRUE statement**: **Streptococcal pyrogenic exotoxin A** (SPEA) is encoded by a **lysogenic bacteriophage**. - This phage integrates into the *Streptococcus pyogenes* chromosome, enabling toxin production. - SPEA is responsible for conditions such as **scarlet fever** and streptococcal toxic shock syndrome.

Question 2523: Which of the following conditions is not typically associated with Ureaplasma urealyticum?

A. Non gonococcal urethritis
B. Epididymitis
C. Bacterial vaginosis (Correct Answer)
D. Pelvic inflammatory disease (including salpingitis)

Explanation: ***Bacterial vaginosis*** - **Bacterial vaginosis (BV)** is primarily associated with an imbalance in the vaginal microbiota, characterized by an overgrowth of anaerobic bacteria like *Gardnerella vaginalis* and *Prevotella* species, and a decrease in protective *Lactobacillus* species. - While *Ureaplasma urealyticum* can be present in the vaginal flora, it is not considered a primary causative agent of **bacterial vaginosis**, which has a distinct microbiological profile. *Non gonococcal urethritis* - **Non-gonococcal urethritis (NGU)** is a well-established clinical condition where *Ureaplasma urealyticum* is a common cause, often presenting with dysuria, urethral discharge, and itching. - Along with *Chlamydia trachomatis*, *Mycoplasma genitalium*, and *Trichomonas vaginalis*, *Ureaplasma urealyticum* is recognized as a significant pathogen in NGU. *Pelvic inflammatory disease (including salpingitis)* - *Ureaplasma urealyticum* has been implicated in the etiology of **pelvic inflammatory disease (PID)**, including conditions like **salpingitis**, which involves inflammation of the fallopian tubes. - The organism can ascend from the lower genital tract, causing inflammation and damage to the reproductive organs, particularly in cases where typical pathogens like *Neisseria gonorrhoeae* or *Chlamydia trachomatis* are not identified. *Epididymitis* - **Epididymitis**, especially in men under 35, is often caused by sexually transmitted infections (STIs), and *Ureaplasma urealyticum* is recognized as a potential pathogen. - It can lead to inflammation of the **epididymis**, causing pain, swelling, and tenderness in the scrotum, particularly in cases of non-gonococcal, non-chlamydial epididymitis.

Question 2524: HACEK group of organisms includes all, except:

A. Cardiobacterium hominis
B. Eikenella corrodens
C. Haemophilus parainfluenzae
D. Haemophilus ducreyi (Correct Answer)

Explanation: ***Haemophilus ducreyi*** - *Haemophilus ducreyi* is the causative agent of **chancroid**, a sexually transmitted infection, and is not considered part of the HACEK group. - The **HACEK group** consists of fastidious, gram-negative bacteria known for causing **endocarditis**. *Haemophilus parainfluenzae* - *Haemophilus parainfluenzae* is one of the five genera included in the **HACEK group** acronym, specifically the 'H'. - This organism is a known cause of **infective endocarditis**, particularly in patients with pre-existing valvular disease. *Cardiobacterium hominis* - *Cardiobacterium hominis* is represented by the 'C' in the **HACEK group** acronym. - It is a significant cause of **culture-negative endocarditis** due to its fastidious nature and slow growth. *Eikenella corrodens* - *Eikenella corrodens* is the 'E' in the **HACEK group** acronym. - It is often associated with **oral cavity infections**, human bite wounds, and can cause **endocarditis** in susceptible individuals.

Question 2525: Thumb print appearance in a culture film smear is seen in which of the following organisms?

A. Bacillus anthracis (Correct Answer)
B. Brucella species
C. Bordetella pertussis
D. Clostridium perfringens

Explanation: ***Bacillus anthracis*** - The "**thumb print**" or "**boxcar**" appearance refers to the characteristic microscopic morphology seen in **Gram-stained smears** of *B. anthracis*. - The bacilli appear as large, rectangular, Gram-positive rods with **square ends** (resembling boxcars), often arranged in chains. - This distinctive appearance aids in the laboratory diagnosis of **anthrax**. - On culture plates, colonies show a "**Medusa head**" appearance with irregular edges. *Bordetella pertussis* - Colonies on **Bordet-Gengou agar** exhibit a "**mercury drop**" or "**pearl-like**" appearance—smooth, glistening, dome-shaped colonies. - This is different from the "thumb print" morphology, which refers to microscopic rather than colonial appearance. - *B. pertussis* is the causative agent of **whooping cough** (pertussis). *Brucella species* - Produce small, smooth, translucent, slightly raised colonies on enriched media. - Lack any distinctive "thumb print" morphology in smears. - Causative agents of **brucellosis** (undulant fever), a zoonotic infection. *Clostridium perfringens* - Large, Gram-positive rods that may appear boxcar-shaped but are typically associated with **double zone of hemolysis** on blood agar. - Colonies are large, flat, and spreading, not exhibiting the classical "thumb print" identification feature. - Common cause of **gas gangrene** and food poisoning.

Question 2526: What is the causative agent of scrub typhus?

A. Orientia tsutsugamushi (Correct Answer)
B. R. prowazekii
C. R. rickettsii
D. R. akari

Explanation: ***Orientia tsutsugamushi*** - This is the **causative agent** of **scrub typhus**, an acute febrile illness transmitted by the bite of infected larval mites (chiggers). - Previously classified as *Rickettsia tsutsugamushi*, it was **reclassified** to the genus *Orientia* in 1995 due to its unique genetic and antigenic characteristics differentiating it from other rickettsiae. - Scrub typhus is endemic in the Asia-Pacific region and presents with fever, eschar at the bite site, and lymphadenopathy. *R. prowazekii* - This bacterium is responsible for **epidemic typhus**, which is typically transmitted by the **human body louse**. - It causes a more severe disease compared to scrub typhus, often associated with Brill-Zinsser disease in recurrent cases. *R. rickettsii* - This is the etiologic agent of **Rocky Mountain spotted fever (RMSF)**, a severe tick-borne disease in the Americas. - RMSF is characterized by a maculopapular rash that often involves the palms and soles. *R. akari* - This bacterium causes **rickettsialpox**, a mild febrile illness transmitted by the bite of the **house mouse mite**. - Rickettsialpox is characterized by an initial papulovesicular rash and an eschar at the bite site.

Question 2527: Which of the following statements about Mycoplasma pneumonia is false?

A. The treatment of choice is erythromycin.
B. ESR may be elevated.
C. It can be diagnosed by serum cold agglutinins.
D. It can be easily cultured from sputum on routine media. (Correct Answer)

Explanation: ***It can be easily cultured from sputum on routine media.*** - *Mycoplasma pneumoniae* is a **fastidious organism** that lacks a cell wall, making it **extremely difficult to culture** using standard laboratory methods. - It requires **specialized media** (PPLO agar, Eaton's agar) and **prolonged incubation** (2-3 weeks), so **routine sputum cultures cannot detect it**. - In clinical practice, diagnosis relies on **serology (IgM antibodies)**, **PCR**, or **cold agglutinin testing** rather than culture. *The treatment of choice is erythromycin.* - **Macrolides** such as erythromycin, azithromycin, and clarithromycin are indeed the **first-line antibiotics** for *Mycoplasma pneumoniae* infections. - Due to the **absence of a cell wall**, *Mycoplasma* is naturally resistant to beta-lactam antibiotics (penicillins, cephalosporins). - **Tetracyclines** and **fluoroquinolones** are alternative options. *ESR may be elevated.* - An **elevated Erythrocyte Sedimentation Rate (ESR)** is a common non-specific indicator of inflammation. - *Mycoplasma pneumoniae* infection causes inflammation in the respiratory tract and can lead to a systemic inflammatory response, thus **ESR is typically elevated**. *It can be diagnosed by serum cold agglutinins.* - **Cold agglutinins** are IgM autoantibodies that agglutinate red blood cells at cold temperatures (4°C) and are produced in response to *Mycoplasma pneumoniae* infections. - Develop in **50-75% of cases**, usually appearing in the **second week** of illness. - A titer of **≥1:64** or a **4-fold rise** supports the diagnosis, though it is **not entirely specific** and may be seen in other conditions (EBV, lymphoma).

Question 2528: Infection in burns is most commonly due to:

A. Candida
B. Pseudomonas (Correct Answer)
C. Staphylococci
D. Streptococci

Explanation: ***Pseudomonas*** - **_Pseudomonas aeruginosa_** is the **most common cause of infection in burn wounds**, particularly in hospitalized patients with significant burns. - It thrives in the moist environment of burn wounds and produces virulence factors including elastase, exotoxin A, and biofilm that lead to tissue destruction and invasive infection. - Pseudomonas is the leading cause of **burn sepsis and burn-related mortality**, and shows resistance to many common antibiotics. - Characteristic **green pigment (pyocyanin)** and **grape-like odor** may be noted in infected wounds. *Staphylococci* - **_Staphylococcus aureus_** is a common early colonizer of burn wounds due to its ubiquitous presence on normal skin. - More prevalent in **minor, superficial burns** and in the immediate post-injury period. - While clinically significant, it is **not the most common overall** in major burn infections. *Streptococci* - **Group A Streptococcus** (e.g., **_Streptococcus pyogenes_**) can cause burn wound infections, particularly cellulitis and invasive soft tissue infections. - Generally less common than Pseudomonas or Staphylococcus in burn wound epidemiology. *Candida* - **_Candida_ species** (primarily **_Candida albicans_**) are important fungal pathogens in burn wounds. - Typically occur in patients with **extensive burns, prolonged hospitalization, or broad-spectrum antibiotic use**. - Usually represent late infections and indicate immunosuppression.

Question 2529: In differentiating pathogenic from non-pathogenic streptococcal strains in a throat culture, which of the following is the MOST important factor?

A. Beta-hemolysis pattern on blood agar plates (Correct Answer)
B. M protein presence
C. Surface protein differences
D. Lancefield grouping based on C-carbohydrate antigen

Explanation: ***Beta-hemolysis pattern on blood agar plates*** - **Beta-hemolysis** (complete lysis of red blood cells causing a clear zone around colonies) is the **PRIMARY screening method** used in clinical microbiology laboratories to differentiate pathogenic from non-pathogenic streptococci in throat cultures. - **Group A Streptococcus** (*Streptococcus pyogenes*), the most common and clinically significant pathogen in throat infections, characteristically shows **beta-hemolysis** on blood agar. - This is the **first-line practical approach** in laboratory diagnosis - beta-hemolytic colonies are then selected for further identification tests (bacitracin sensitivity, latex agglutination, or Lancefield grouping). - While some exceptions exist (e.g., *S. pneumoniae* is alpha-hemolytic but typically not isolated from throat cultures as a primary pathogen causing pharyngitis), beta-hemolysis remains the **most important initial differentiating factor** for identifying potentially pathogenic streptococci in throat cultures. *Surface protein differences* - While surface proteins (including M protein) are important **virulence factors** that contribute to pathogenicity, they are not routinely assessed as the PRIMARY differentiating factor in clinical laboratories. - Surface protein analysis requires more sophisticated techniques (serotyping, molecular methods) that are not first-line screening tests. *M protein presence* - **M protein** is indeed a critical virulence factor specific to *Streptococcus pyogenes* (Group A Strep) and inhibits phagocytosis. - However, M protein detection requires specific immunological or molecular techniques and is not the PRIMARY screening method used in routine throat culture differentiation. - M protein typing is more useful for epidemiological studies than routine clinical diagnosis. *Lancefield grouping based on C-carbohydrate antigen* - **Lancefield grouping** is an important classification system that categorizes streptococci into groups (A, B, C, G, etc.) based on cell wall carbohydrate antigens. - However, this is a **confirmatory test** performed AFTER initial screening by hemolysis pattern, not the primary differentiating factor. - It's used to confirm the identity of beta-hemolytic isolates rather than as the first screening step.

Question 2530: A 65-year-old diabetic presents with necrosis of the external auditory meatus and foul-smelling discharge. What is the probable organism associated with this condition?

A. Streptococcus pyogenes
B. E. coli
C. Pseudomonas aeruginosa (Correct Answer)
D. Haemophilus influenzae

Explanation: ***Pseudomonas aeruginosa*** - This clinical presentation, involving a **diabetic patient** with external auditory canal **necrosis** and **foul-smelling discharge**, is classic for **malignant otitis externa**. - **Pseudomonas aeruginosa** is the most common causative organism for malignant otitis externa, thriving in moist environments and common in immunocompromised individuals like diabetics. *Haemophilus influenzae* - While it can cause otitis media, **Haemophilus influenzae** is not typically associated with **malignant otitis externa** or the severe necrosis and foul-smelling discharge seen in this patient. - It primarily causes **respiratory tract infections**, including acute otitis media, and is less common in severe, necrotizing external ear infections. *Streptococcus pyogenes* - **Streptococcus pyogenes** is a common cause of pharyngitis and skin infections (e.g., cellulitis), but it is not typically linked to **malignant otitis externa**. - This organism does not usually cause the extensive tissue necrosis and foul-smelling discharge characteristic of this condition. *E. coli* - **E. coli** is a common pathogen in urinary tract infections and intra-abdominal infections, but it is rarely a cause of **otitis externa**, especially the severe, necrotizing form. - It would not typically produce the specific clinical picture of external auditory meatus necrosis and foul-smelling discharge in a diabetic.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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