General Microbiology

General Microbiology Indian Medical PG Question 1: Which of the following is NOT a mechanism of antibiotic resistance?

• A. Efflux pump activity
• B. Inactivation by enzymes such as beta-lactamase
• C. Modification of drug target sites
• D. Increased drug absorption (Correct Answer)

General Microbiology Explanation: ***Increased drug absorption*** - **Increased drug absorption** would lead to a higher intracellular concentration of the antibiotic, making it *more potent* against the bacteria rather than contributing to resistance. - Antibiotic resistance mechanisms aim to *reduce the effective concentration* of the drug at its target site or *alter the target itself*. *Efflux pump activity* - **Efflux pumps** are bacterial membrane proteins that actively pump antibiotics out of the bacterial cell [3]. - This mechanism *reduces the intracellular concentration* of the antibiotic, preventing it from reaching its therapeutic target [3]. *Inactivation by enzymes such as beta-lactamase* - Bacteria can produce enzymes like **beta-lactamase** that *chemically modify or degrade* the antibiotic molecule, rendering it inactive [2]. - This is a common mechanism of resistance against **beta-lactam antibiotics** (e.g., penicillin, cephalosporins) [2]. *Modification of drug target sites* - Bacteria can develop mutations that *alter the structure of the antibiotic's target site*, such as a bacterial ribosome or cell wall component [1]. - This change in the target means the antibiotic can no longer bind effectively or interfere with cellular processes, thus *losing its efficacy* [1].

General Microbiology Indian Medical PG Question 2: It is true regarding normal microbial flora present on skin and mucous membrane that

• A. It is difficult to permanently eradicate by antimicrobial agents (Correct Answer)
• B. It is absent in the stomach due to acidic pH
• C. It establishes in the body only after the neonatal period
• D. The flora in the small bronchi is similar to that of the trachea

General Microbiology Explanation: ***It is difficult to permanently eradicate by antimicrobial agents*** - The **normal flora** colonizes various body sites, often forming **biofilms** or residing in protected niches, making complete eradication challenging even with potent antimicrobials. - While antimicrobials can reduce microbial populations, the remaining organisms, or recolonization from external sources, can lead to the re-establishment of the flora. *It is absent in the stomach due to acidic pH* - The stomach is not entirely sterile; **acid-tolerant bacteria**, such as *Helicobacter pylori*, can colonize the gastric mucosa. - While the **acidic pH** limits the diversity and number of microbes, some still persist, particularly in the mucus layer. *It establishes in the body only after the neonatal period* - **Colonization with normal flora** begins at birth, with exposure to maternal vaginal and fecal flora during passage through the birth canal. - Environmental exposure and feeding patterns further shape the microbial composition throughout the **neonatal period** and infancy. *The flora in the small bronchi is similar to that of the trachea* - The **lower respiratory tract**, including the small bronchi and alveoli, is generally considered **sterile or has very sparse flora** due to mucociliary clearance and immune defenses. - The **trachea** does have a transient flora, but it is distinct from and much richer than the extremely sparse flora, if any, found in the small bronchi.

General Microbiology Indian Medical PG Question 3: Cell wall-deficient organisms are

• A. Mycoplasma (Correct Answer)
• B. Anaerobes
• C. Chlamydia
• D. Streptococcus

General Microbiology Explanation: ***Mycoplasma*** - **Mycoplasma** species are unique bacteria because they naturally lack a **rigid cell wall**, which makes them pleomorphic (variable in shape). - This absence of a cell wall means they are intrinsically resistant to antibiotics that target cell wall synthesis, such as **penicillins** and **cephalosporins**. *Chlamydia* - **Chlamydia** are obligate intracellular bacteria that possess a **cell wall**, though it lacks **peptidoglycan**. - Their cell wall component, while differing from typical bacteria, still provides structural integrity, distinguishing them from truly cell wall-deficient organisms. *Anaerobes* - **Anaerobes** are a diverse group of bacteria (e.g., Clostridium, Bacteroides) that thrive in the absence of oxygen and generally possess a **typical bacterial cell wall** composed of peptidoglycan. - The defining characteristic of anaerobes relates to their oxygen requirement, not the absence of a cell wall. *Streptococcus* - **Streptococcus** is a genus of Gram-positive bacteria that possess a distinct and thick **peptidoglycan cell wall**. - This cell wall is a primary target for many common antibiotics, such as **penicillin**, making them structurally distinct from cell wall-deficient organisms.

General Microbiology Indian Medical PG Question 4: Blood culture is positive in which infection caused by Staphylococcus aureus?

• A. Infective endocarditis (Correct Answer)
• B. Impetigo
• C. Toxic Shock Syndrome (TSS)
• D. Staphylococcal Scalded Skin Syndrome (SSSS)

General Microbiology Explanation: ***Infective endocarditis*** - **Infective endocarditis** is characterized by the presence of bacteria in the bloodstream, leading to a **positive blood culture** [2]. *Staphylococcus aureus* is a common cause, particularly in intravenous drug users [1], [2]. - The infection involves the **endothelial lining of the heart**, often affecting heart valves, causing vegetations that can shed bacteria into the circulation [2], [3]. *Toxic Shock Syndrome (TSS)* - TSS is caused by toxins (e.g., **TSST-1**) produced by *Staphylococcus aureus*, not by the direct presence of bacteria in the bloodstream in high numbers that would consistently yield a positive blood culture. - While *S. aureus* is present, the systemic effects are primarily **toxin-mediated**, and blood cultures are often negative. *Impetigo* - Impetigo is a **superficial skin infection** caused by *Staphylococcus aureus* or *Streptococcus pyogenes*. - It does not involve systemic bacteremia, so **blood cultures are typically negative**. *Staphylococcal Scalded Skin Syndrome (SSSS)* - SSSS is a **toxin-mediated disease** caused by exfoliatin toxins produced by *Staphylococcus aureus*. - The bacteria usually remain localized at the site of infection (e.g., nose, throat, or skin), and **blood cultures are generally negative**.

General Microbiology Indian Medical PG Question 5: A 25-year-old woman presents with a sudden onset of high fever, chills, and rigors. Blood cultures are pending. What is the next appropriate step in her management?

• A. Administer broad-spectrum antibiotics (Correct Answer)
• B. Wait for blood culture results
• C. Start antipyretic therapy only
• D. Order a CT scan

General Microbiology Explanation: ***Administer broad-spectrum antibiotics*** - The patient presents with classic signs of **sepsis** (high fever, chills, rigors), which is a medical emergency requiring prompt intervention [2]. - **Early administration of broad-spectrum antibiotics** is crucial to improve outcomes and reduce mortality in suspected sepsis, even before culture results are available [1]. *Wait for blood culture results* - Delaying antibiotic treatment in a patient with suspected sepsis can lead to rapid clinical deterioration and increased mortality [1]. - While blood cultures are essential to guide definitive therapy, initial empiric broad-spectrum antibiotics should not be withheld [3]. *Start antipyretic therapy only* - Antipyretics only address the symptom of fever and do not treat the underlying infection causing the fever and chills. - This approach would leave the potentially life-threatening infection untreated, leading to worsening patient condition. *Order a CT scan* - A CT scan is not the immediate priority in a patient presenting with acute signs of systemic infection and suspected sepsis. - While it may be useful later to identify a source of infection, controlling the infection with antibiotics is the most urgent step.

General Microbiology Indian Medical PG Question 6: A farmer presents with pustules that show Gram-positive cocci on smear. The culture shows beta hemolysis, and the organisms are catalase-negative. To confirm that the identified organism is group A streptococcus, which of the following tests should be performed?

• A. Bacitracin sensitivity (Correct Answer)
• B. Novobiocin sensitivity
• C. Bile solubility
• D. Optochin sensitivity

General Microbiology Explanation: ***Bacitracin sensitivity*** - **Group A Streptococcus (GAS)** is uniquely sensitive to bacitracin, distinguishing it from most other beta-hemolytic streptococci. - A definitive zone of inhibition around a **bacitracin disk** on a blood agar plate indicates GAS. *Optochin sensitivity* - This test is used to identify **Streptococcus pneumoniae**, which is alpha-hemolytic, not beta-hemolytic like the organism described. - *S. pneumoniae* is **sensitive to optochin**, while other alpha-hemolytic streptococci are resistant. *Novobiocin sensitivity* - This test is primarily used to differentiate coagulase-negative staphylococci, specifically to identify **Staphylococcus saprophyticus** (resistant) from other coagulase-negative staphylococci (sensitive). - It is not applicable to the identification of streptococci. *Bile solubility* - This test is used to differentiate **Streptococcus pneumoniae** (bile soluble) from other alpha-hemolytic streptococci. - The described organism is **beta-hemolytic**, making this test inappropriate for its identification.

General Microbiology Indian Medical PG Question 7: Gram stain of CSF shows gram-negative diplococci. Culture grows on Thayer-Martin medium. Most likely organism?

• A. L. monocytogenes
• B. S. pneumoniae
• C. N. meningitidis (Correct Answer)
• D. H. influenzae

General Microbiology Explanation: ***N. meningitidis*** - The presence of **gram-negative diplococci** in CSF is a hallmark feature of *Neisseria meningitidis*. - **Thayer-Martin medium** is a selective medium specifically designed for the isolation and growth of *Neisseria* species, inhibiting other bacterial and fungal contaminants. *L. monocytogenes* - *Listeria monocytogenes* is a **gram-positive rod**, which contradicts the gram stain finding of gram-negative diplococci. - It typically causes meningitis in neonates, elderly, and immunocompromised individuals, but its morphology is distinct. *S. pneumoniae* - *Streptococcus pneumoniae* is a **gram-positive coccus**, often appearing in pairs (diplococci) or short chains, but it is gram-positive, not gram-negative. - It is a common cause of bacterial meningitis but does not grow readily on Thayer-Martin medium. *H. influenzae* - *Haemophilus influenzae* is a **pleomorphic gram-negative coccobacillus**, not a diplococcus, and it requires specific growth factors (X and V factors) found in chocolate agar. - While it can cause meningitis, its characteristic morphology and growth requirements differ from the description.

General Microbiology Indian Medical PG Question 8: All of the following statements about Staphylococcus aureus are true, except:

• A. About 30% of general population is healthy nasal carriers.
• B. Methicillin resistance is chromosomally mediated.
• C. Epidermolysin and TSS toxin are superantigens.
• D. Most common source of infection is cross infection from infected people. (Correct Answer)

General Microbiology Explanation: ***Incorrect: Most common source of infection is cross-infection from infected people.*** - The most common source of *Staphylococcus aureus* infection, particularly in community-acquired cases, is from the patient's **own endogenous flora**, especially from the **nasal passages or skin**. - While cross-infection in healthcare settings (nosocomial transmission) is a significant issue, it is NOT the primary or most common source for the majority of *S. aureus* infections overall. - **Endogenous infection from colonized sites** is the predominant mode of acquisition. *Correct: About 30% of the general population is healthy nasal carriers.* - This statement is accurate; approximately **20-40%** (with an average often cited as 30%) of healthy individuals carry *Staphylococcus aureus* asymptomatically in their **anterior nares**. - This asymptomatic carriage is a significant reservoir for both self-infection and transmission to others. *Correct: Epidermolysin and TSS toxin are superantigens.* - This statement is correct. **Toxic Shock Syndrome Toxin-1 (TSST-1)** and the **exfoliative toxins (Epidermolysins)** are both classified as **superantigens**. - Superantigens bypass normal antigen processing and directly bind to MHC class II molecules and T-cell receptors, causing **widespread T-cell activation** (up to 20% of T cells) and massive cytokine release, leading to systemic symptoms like fever, rash, and shock. *Correct: Methicillin resistance is chromosomally mediated.* - This statement is true. Methicillin resistance in *Staphylococcus aureus* (MRSA) is mediated by the ***mecA* gene**, which encodes for an altered penicillin-binding protein (**PBP2a** or **PBP2'**). - The *mecA* gene is located on a mobile genetic element called the **staphylococcal cassette chromosome *mec* (SCCmec)**, which integrates into the bacterial **chromosome** (hence chromosomally mediated). - PBP2a has low affinity for β-lactam antibiotics, allowing cell wall synthesis to continue despite antibiotic presence.

General Microbiology Indian Medical PG Question 9: A patient presented with meningitis, and the CSF sample was subjected to gram staining and microscopy and appears as shown in the microscopic image given below. Which of the following features / tests will be characteristic of the organism?

• A. Catalase negative, optochin sensitive. (Correct Answer)
• B. Ferments glucose and maltose but not inulin
• C. Oxidase and catalase negative, does not ferment inulin
• D. Oxidase and catalase positive and does not ferment maltose or glucose

General Microbiology Explanation: ***Catalase negative, optochin sensitive*** - The image shows **Gram-positive diplococci** (lancet-shaped, occurring in pairs), indicative of *Streptococcus pneumoniae*. - *S. pneumoniae* is known for being **catalase-negative** and **optochin-sensitive**, which are key diagnostic features that distinguish it from other α-hemolytic streptococci. - Additional characteristic: *S. pneumoniae* **ferments inulin** (bile soluble, quellung positive). *Ferments glucose and maltose but not inulin* - The phrase "does not ferment inulin" would suggest a streptococcus species OTHER than *S. pneumoniae*. - *S. pneumoniae* characteristically **DOES ferment inulin**, which is an important diagnostic feature. - This option is incorrect because it describes sugar fermentation that contradicts *S. pneumoniae* characteristics. *Oxidase and catalase negative, does not ferment inulin* - While *S. pneumoniae* is catalase-negative and oxidase-negative (oxidase test not routinely used for Gram-positive cocci), the statement "does not ferment inulin" is **incorrect** for *S. pneumoniae*. - *S. pneumoniae* **ferments inulin**, which is a key differentiating feature from other streptococci. - This combination of features does not match the organism shown in the microscopy. *Oxidase and catalase positive and does not ferment maltose or glucose* - This biochemical profile is typical for non-fermenting Gram-negative bacteria like *Pseudomonas aeruginosa*. - Such organisms would appear as **Gram-negative rods**, not the **Gram-positive diplococci** seen in the image. - This option is clearly incorrect based on the Gram stain morphology alone.

General Microbiology Indian Medical PG Question 10: A 9-year-old child presented to OPD with complaints of high-grade fever, vomiting, and one episode of seizure. CSF examination was done and Gram staining of the culture showed lanceolate-shaped gram-positive diplococci. What is the probable causative agent?

• A. Haemophilus influenzae
• B. Streptococcus agalactiae
• C. Neisseria meningitidis
• D. Streptococcus pneumoniae (Correct Answer)

General Microbiology Explanation: ***Streptococcus pneumoniae*** - The description of **lanceolate-shaped gram-positive diplococci** in CSF is characteristic of *S. pneumoniae*. - This bacterium is a common cause of **bacterial meningitis** in children and can present with high fever, vomiting, and seizures. *Haemophilus influenzae* - This is a **gram-negative coccobacillus**, which would appear as small, pleomorphic rods rather than lanceolate-shaped diplococci on Gram stain. - While it causes meningitis, its Gram stain morphology is distinct from *S. pneumoniae*. *Streptococcus agalactiae* - *S. agalactiae* (Group B Streptococcus) is a **gram-positive coccus**, but it typically appears in **chains** and is a major cause of neonatal meningitis, not usually in a 9-year-old child. - Its morphology on Gram stain would not be described as lanceolate diplococci. *Neisseria meningitidis* - *N. meningitidis* is a **gram-negative diplococcus** and would appear as kidney-bean shaped or flattened paired cocci, not gram-positive. - Though a common cause of meningitis, the Gram stain morphology described rules it out.

General Microbiology Flashcard 1 of 9

Question: 4 Bacterial Growth Environments_____ - environment w/ O2Obligate Anaerobes - environment w/o O2Facultative Anaerobes - either w/ or w/o O2Intracellular bacteria

Answer: Obligate Aerobes

General Microbiology Flashcard 2 of 9

Question: _____ staining procedures can be used to visualise cell walls

Answer: Differential (Simple/Differential)

General Microbiology Flashcard 3 of 9

Question: The resolving power of a light microscope, shown in the image, is limited by the _____ of the light.

Answer: wavelength

General Microbiology Flashcard 4 of 9

Question: germ theory was given by

Answer: Louise pasteur

Extra Information: 

General Microbiology Flashcard 5 of 9

Question: 3 phases of bacterial growth curve: _____ phase, growth/log phase, stationary phase

Answer: lag

General Microbiology Flashcard 6 of 9

Question: The _____ of the bacterial spore is the outermost layer that is made of a keratin-like protein

Answer: coat

General Microbiology Flashcard 7 of 9

Question: Rhinosporidium seeberi is an aquatic _____.

Answer: protista

General Microbiology Flashcard 8 of 9

Question: _____ is the entry and development or multiplication of an infectious agent in another organism.

Answer: Infection

General Microbiology Flashcard 9 of 9

Question: _____ designed the first electron microscope

Answer: Ernst Ruska