Bacteriology Practice Questions

Q: A seminal smell on culture is produced by which bacterium?

Proteus. **Explanation:** The correct answer is **Proteus**. The characteristic "seminal" or "fishy/ammoniacal" odor associated with *Proteus* species (particularly *P. mirabilis* and *P. vulgaris*) is due to the production of volatile amines and the breakdown of urea. *Proteus* is a highly motile, Gram-negative rod known for its "swarming" growth on agar. **Analysis of Options:** * **Proteus (Correct):** Produces a distinct seminal or putrid odor. It is also characterized by its ability to produce urease, which increases urinary pH, leading to the formation of struvite (triple phosphate) stones. * **Pneumococci (*S. pneumoniae*):** These are Gram-positive cocci that do not produce a seminal smell. They are typically identified by their alpha-hemolytic "draughtsman" appearance on blood agar and bile solubility. * **Vibrio cholerae:** This bacterium is famously associated with a **"fishy" or "sweetish"** odor, but it is more classically described as having a "rice-water" stool association. * **Yersinia:** *Yersinia pestis* or *Y. enterocolitica* do not have a seminal odor. *Y. enterocolitica* is known for its "bull’s eye" appearance on CIN agar and psychrophilic nature. **High-Yield Clinical Pearls for NEET-PG:** * **Proteus:** Swarming growth (inhibited by 6% salt or boric acid), Dienes phenomenon, and Phenylalanine Deaminase (TDA/PPA) positive. * **Other Characteristic Odors:** * *Pseudomonas aeruginosa:* Fruity/Grape-like/Corn-tortilla odor. * *Staphylococcus aureus:* Old musty/Sock-like odor. * *Pasteurella multocida:* Musty/Mousy odor. * *Eikenella corrodens:* Bleach-like odor. * *Actinomyces:* Earthy/Musty odor.

Q: An organism grown on agar shows green colored colonies. What is the likely organism?

Pseudomonas. **Explanation:** The correct answer is **Pseudomonas aeruginosa**. This organism is a non-fermenting Gram-negative bacillus known for its ability to produce various water-soluble pigments, which impart a characteristic color to the colonies on nutrient agar or Mueller-Hinton agar. **Why Pseudomonas is correct:** The characteristic green color is primarily due to the production of two pigments: 1. **Pyocyanin:** A blue-green pigment (unique to *P. aeruginosa*). 2. **Pyoverdin (Fluorescein):** A yellow-green fluorescent pigment. When these pigments combine, they produce the classic "grassy green" appearance. Some strains may also produce Pyorubin (red) or Pyomelanin (brown/black). **Why other options are incorrect:** * **Staphylococcus:** Typically produces golden-yellow (*S. aureus*) or white (*S. epidermidis*) colonies. * **Streptococcus:** Colonies are usually small, translucent, or greyish. While they produce "alpha-hemolysis" (greenish discoloration) *around* the colony on Blood Agar, the colonies themselves are not green. * **E. coli:** On routine media like Nutrient Agar, it produces moist, greyish-white colonies. On MacConkey agar, it produces bright pink (lactose-fermenting) colonies. **Clinical Pearls for NEET-PG:** * **Odor:** *Pseudomonas* produces a characteristic **fruity, grape-like, or corn-tortilla odor**. * **Oxidase Test:** It is **Oxidase positive** (distinguishes it from Enterobacteriaceae). * **Clinical Association:** It is a leading cause of nosocomial infections, particularly in burn patients (blue-green pus) and cystic fibrosis patients. * **Culture Media:** Cetrimide agar is the selective medium used for its isolation.

Q: A patient presented with bloody diarrhea. Laboratory examination revealed a lactose-positive, glucose-fermenting, Gram-negative rod. The patient's infection is most likely due to which organism?

Escherichia coli. ### Explanation The correct answer is **Escherichia coli**. The diagnosis is based on the interpretation of biochemical characteristics provided in the clinical vignette. **1. Why Escherichia coli is correct:** The laboratory findings describe a **Gram-negative rod** that is **Lactose-positive** (pink colonies on MacConkey agar) and **Glucose-fermenting**. Among the common causes of bloody diarrhea (dysentery), *E. coli*—specifically **Enterohemorrhagic E. coli (EHEC)** and **Enteroinvasive E. coli (EIEC)**—fits this biochemical profile perfectly. EHEC (notably O157:H7) causes bloody diarrhea via Shiga-like toxins, while EIEC invades the colonic mucosa. **2. Why the other options are incorrect:** * **Salmonella enterica:** While it is a Gram-negative rod that ferments glucose, it is **Lactose-non-fermenting** (appears pale on MacConkey agar). It typically causes enteric fever or inflammatory diarrhea, but the lactose status rules it out. * **Shigella sonnei:** Although a major cause of bloody diarrhea (bacillary dysentery), *Shigella* species are **Lactose-non-fermenters** (though *S. sonnei* is a late/slow lactose fermenter, it is clinically categorized as a non-fermenter) and are non-motile. * **Helicobacter pylori:** This is a Gram-negative **curved/spiral** rod associated with peptic ulcers and gastritis, not acute bloody diarrhea. **High-Yield Clinical Pearls for NEET-PG:** * **Lactose Fermenters (CEEK):** **C**itrobacter, **E**. coli, **E**nterobacter, **K**lebsiella. * **EHEC (O157:H7):** Does **not** ferment Sorbitol (tested on SMAC agar). It is associated with **Hemolytic Uremic Syndrome (HUS)** due to Shiga toxin. * **EIEC:** Clinically indistinguishable from Shigellosis; it uses actin tails for cell-to-cell spread. * **Golden Rule:** If a question mentions "Lactose-positive" and "Diarrhea," always prioritize *E. coli*.

Q: All of the following are true of exotoxins, except:

Heat stable. **Explanation:** The correct answer is **C (Heat stable)** because exotoxins are primarily **heat-labile** (heat-sensitive). ### 1. Why "Heat stable" is the correct (false) statement: Exotoxins are proteins secreted by both Gram-positive and Gram-negative bacteria. Because they are proteins, they are typically denatured by heat (usually at temperatures above 60°C). * **Exception to remember:** The *Staphylococcal* enterotoxin and *E. coli* heat-stable toxin (ST) are notable exceptions that remain stable at high temperatures. In contrast, **Endotoxins** (Lipopolysaccharides) are characteristically heat-stable. ### 2. Analysis of Incorrect Options: * **A. Protein in nature:** Exotoxins are polypeptide chains secreted by living bacterial cells. This allows them to be neutralized by specific antibodies. * **B. Antigenic:** Due to their high molecular weight and protein structure, exotoxins are highly immunogenic. They induce the formation of high-titer antibodies called **antitoxins**. This property is exploited to create **toxoids** (detoxified toxins that retain antigenicity) for vaccines like Tetanus and Diphtheria. * **C. Highly toxic:** Exotoxins are among the most poisonous substances known. They have specific mechanisms of action (e.g., inhibiting protein synthesis or blocking neurotransmitters) and possess a very low lethal dose (LD50) compared to endotoxins. ### High-Yield Clinical Pearls for NEET-PG: * **Source:** Exotoxins are secreted by living cells; Endotoxins are released only upon cell lysis. * **Genetics:** Exotoxin genes are often carried on **plasmids** or **bacteriophages** (e.g., Diphtheria toxin is encoded by the *tox* gene of the Beta-phage). * **Mechanism:** Many exotoxins are **A-B toxins**, where the 'B' subunit binds to the cell surface and the 'A' subunit possesses the enzymatic activity. * **Key Comparison:** Endotoxins are poorly antigenic, cannot be toxoided, and are heat-stable even at 100°C.

Q: Which bacterium is known to produce superantigens?

Staphylococcus aureus. **Explanation:** **Staphylococcus aureus** is the correct answer because it is a classic producer of **Superantigens**. Unlike regular antigens that are processed and presented by MHC II molecules to specific T-cell receptors (TCR), superantigens bypass this processing. They bind directly to the **outer surface of the MHC II molecule** and the **Vβ region of the TCR**. This results in a massive, non-specific activation of T-cells (up to 20% of the body's T-cells), leading to a "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ). **Analysis of Options:** * **Staphylococcus aureus (Correct):** Produces **TSST-1** (Toxic Shock Syndrome Toxin-1) and **Staphylococcal Enterotoxins** (causing food poisoning). * **Streptococcus pneumoniae:** Its primary virulence factor is its polysaccharide capsule; it does not produce superantigens. * **Pseudomonas aeruginosa:** Produces **Exotoxin A**, which acts by ADP-ribosylation of Elongation Factor-2 (EF-2), inhibiting protein synthesis. * **Clostridium diphtheriae:** Produces **Diphtheria toxin**, which also inhibits protein synthesis via EF-2 inactivation. **High-Yield Clinical Pearls for NEET-PG:** * **Other Superantigen Producers:** *Streptococcus pyogenes* (SpeA and SpeC toxins causing Streptococcal Toxic Shock-like Syndrome). * **Mechanism:** Superantigens do **not** require processing by Antigen Presenting Cells (APCs). * **Clinical Manifestation:** TSST-1 causes high fever, hypotension, and a diffuse erythematous rash that desquamates (often associated with tampon use or wound infections). * **Food Poisoning:** Staphylococcal enterotoxins are **heat-stable** and cause rapid-onset vomiting (1–6 hours).

Q: What is the most common site where organisms are present in a typhoid carrier?

Gall Bladder. **Explanation:** The correct answer is **Gall Bladder**. **Why Gall Bladder is correct:** *Salmonella Typhi* is a Gram-negative, facultative intracellular bacillus. In chronic carriers (individuals who shed the bacilli in stools for >1 year after the initial attack), the organisms persist in the biliary tract. The **Gall Bladder** is the most common site of colonization, particularly in patients with pre-existing gallstones or cholecystitis. The bacteria survive within the bile and form biofilms on the surface of gallstones, which protects them from antibiotics and the host immune system. From the gall bladder, the organisms are intermittently discharged into the intestine and excreted in the feces. **Why other options are incorrect:** * **Spleen & Liver:** While these are part of the Reticuloendothelial System (RES) where *S. Typhi* multiplies during the incubation period and the first week of illness (causing hepatosplenomegaly), they are not the primary sites for long-term chronic carriage. * **Salivary gland:** This is not a reservoir for *S. Typhi*. Salivary glands are more commonly associated with viral infections like Mumps or bacterial infections like *Staphylococcus aureus*. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Carrier Definition:** Shedding of *S. Typhi* in feces/urine for more than **12 months**. * **Urinary Carriers:** Less common than fecal carriers; usually associated with urinary tract abnormalities like **Schistosomiasis** or kidney stones. * **Famous Case:** "Typhoid Mary" (Mary Mallon) was a classic fecal carrier. * **Diagnosis of Carrier State:** Best detected by **repeated stool cultures** or the **Vi agglutination test** (screening for Vi antibodies). * **Treatment:** The drug of choice for eradicating the carrier state is **Ciprofloxacin** (for 4–6 weeks). If gallstones are present, cholecystectomy may be required.

Q: A pus culture on chocolate agar shows gram-negative cocci. What is the most likely organism?

Nisseria gonorrhoeae. **Explanation:** The correct answer is **Neisseria gonorrhoeae**. The diagnosis is based on the integration of morphology and culture characteristics. **1. Why Neisseria gonorrhoeae is correct:** * **Morphology:** *Neisseria* species are classic **Gram-negative diplococci** (kidney-bean shaped). * **Culture:** *N. gonorrhoeae* is a fastidious organism. It does not grow on ordinary blood agar because it is inhibited by toxic substances like fatty acids. **Chocolate agar** (heated blood agar) provides the necessary growth factors (Factor V and X) and inactivates inhibitors, making it the standard medium for isolating pathogenic *Neisseria*. **2. Why the other options are incorrect:** * **Hemophilus ducreyi (Option A):** While it grows on chocolate agar, it is a **Gram-negative coccobacillus** (rod), often described as having a "school of fish" appearance on microscopy. * **Streptococcus pyogenes (Option C):** This is a **Gram-positive coccus** in chains. It typically grows well on sheep blood agar, showing beta-hemolysis. * **Streptococcus pneumoniae (Option D):** This is a **Gram-positive lancet-shaped diplococcus**. While it can grow on chocolate agar, its Gram stain reaction (positive/purple) immediately excludes it. **Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** This is a selective version of chocolate agar (containing Vancomycin, Colistin, and Nystatin) used specifically to isolate *N. gonorrhoeae* from non-sterile sites like the cervix or urethra. * **Biochemical test:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Sugar Fermentation:** *N. **g**onorrhoeae* ferments only **G**lucose, whereas *N. **m**eningitidis* ferments **G**lucose and **M**altose.

Question 1

A seminal smell on culture is produced by which bacterium?

Accepted Answer

Proteus

Answer

Pneumococci

Answer

V. cholerae

Answer

Yersinia

Question 2

Which of the following causes both cutaneous and respiratory diphtheria?

Accepted Answer

Corynebacterium diphtheria

Answer

Corynebacterium ulcerans

Answer

Corynebacterium xerosis

Answer

Corynebacterium hoffmanii

Question 3

An organism grown on agar shows green colored colonies. What is the likely organism?

Accepted Answer

Pseudomonas

Answer

Staphylococcus

Answer

Streptococcus

Answer

E. coli

Question 4

Multiple sinuses from infection of the great toe are mainly caused by which of the following?

Accepted Answer

Actinomycetes

Answer

Tuberculosis

Answer

Trichosporum

Answer

Histoplasmosis

Question 5

A patient presented with bloody diarrhea. Laboratory examination revealed a lactose-positive, glucose-fermenting, Gram-negative rod. The patient's infection is most likely due to which organism?

Accepted Answer

Escherichia coli

Answer

Salmonellae enterica

Answer

Shigella sonnei

Answer

Helicobacter pylori

Question 6

All of the following are true of exotoxins, except:

Accepted Answer

Heat stable

Answer

Protein in nature

Answer

Antigenic

Answer

Highly toxic

Question 7

Which bacterium is known to produce superantigens?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Pseudomonas aeruginosa

Answer

Clostridium diphtheriae

Question 8

What is the most common site where organisms are present in a typhoid carrier?

Accepted Answer

Gall Bladder

Answer

Spleen

Answer

Liver

Answer

Salivary gland

Question 9

A pus culture on chocolate agar shows gram-negative cocci. What is the most likely organism?

Accepted Answer

Nisseria gonorrhoeae

Answer

Hemophilus ducreyi

Answer

Streptococcus pyogenes

Answer

Streptococcus pneumoniae

Question 10

What is the most common bacterial infection associated with lymphedema?

Accepted Answer

Streptococcus

Answer

Staphylococcus

Answer

E. coli

Answer

Pseudomonas

Bacteriology — MCQs

Bacteriology — MCQs

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