Microbiology
10 questionsGlass vessels and syringes are best sterilised by -
Which of the following is an intermediate level disinfectant?
In blood culture the ratio of blood to reagent is?
Which anticoagulant is used when blood is sent for blood culture?
Which of the following statements regarding resistance of penicillin in Staphylococcus aureus is false?
Which of the following bacteria can be identified using phage typing?
Which of the following is called Preisz-Nocard bacillus?
Darting motility which occurs in V.cholerae, also found in -
Patient came from Nagaland and shows positive test with OXK antigen. Diagnosis is?
Which of the following stimulate adenylate cyclase with G-protein coupled action ?
NEET-PG 2013 - Microbiology NEET-PG Practice Questions and MCQs
Question 771: Glass vessels and syringes are best sterilised by -
- A. Dry heat sterilization in a hot air oven (Correct Answer)
- B. Steam sterilization in an autoclave
- C. Ethylene oxide sterilization
- D. Radiation sterilization
Explanation: **Dry heat sterilization in a hot air oven** - **Dry heat sterilization** (160-180°C for 1-2 hours) is the **traditional and preferred method** for **glassware and glass syringes**. - **Hot air ovens** achieve sterilization by oxidative destruction and protein denaturation, leaving items completely **dry** and free from moisture. - **Advantages**: No corrosion, no rusting, items remain dry, ideal for **powders, oils, and glassware** that can withstand high temperatures. - **Note**: This question refers to **glass syringes** (historically used, now largely replaced by disposable plastic syringes which are pre-sterilized by radiation). *Steam sterilization in an autoclave* - **Autoclaving** uses **moist heat** (121°C at 15 psi for 15 minutes) and is highly effective for most medical instruments. - **Disadvantage for glassware**: Rapid temperature changes and steam exposure can cause **thermal shock, cracking, or etching** of delicate glassware. - Items emerge **wet** and require drying, which is undesirable for certain laboratory applications. - **Modern context**: Standard method for surgical instruments and heat-stable plastics. *Ethylene oxide sterilization* - **Ethylene oxide (ETO)** is a **low-temperature chemical sterilization** method (50-60°C) for heat-sensitive items. - Used for plastics, rubber, electronics, and endoscopes that cannot tolerate heat. - Requires **long exposure** (12-24 hours) and **aeration period** (up to 7 days) to remove toxic residues. - **Not preferred** for routine glassware sterilization when heat methods are suitable. *Radiation sterilization* - **Gamma radiation or electron beam** sterilization is used for **pre-packaged disposable medical devices**, pharmaceuticals, and plastic syringes. - Highly effective but **expensive** and requires specialized facilities. - Not practical for **routine laboratory glassware** sterilization in clinical or research settings.
Question 772: Which of the following is an intermediate level disinfectant?
- A. Ethylene oxide
- B. Hypochlorite (Correct Answer)
- C. 2% glutaraldehyde
- D. None of the options
Explanation: ***Hypochlorite*** - **Hypochlorite** (e.g., sodium hypochlorite, bleach) is an effective **intermediate-level disinfectant** commonly used for surface disinfection and water purification. - It works by **oxidizing cellular components** and disrupting membrane function in microorganisms, effective against a wide range of bacteria, viruses, and some fungi. *2% glutaraldehyde* - **2% glutaraldehyde** is a **high-level disinfectant** and **sterilant** often used for heat-sensitive medical equipment like endoscopes. - It is effective against bacterial spores, mycobacteria, fungi, and viruses, which exceeds the scope of intermediate-level disinfection. *Ethylene oxide* - **Ethylene oxide** is a **gaseous sterilant** used for heat-sensitive and moisture-sensitive medical devices, making it a high-level modality. - It works by **alkylating proteins and nucleic acids**, effectively killing all forms of microbial life, including spores. *None of the options* - This option is incorrect because **Hypochlorite** is indeed an intermediate-level disinfectant. - The classification of disinfectants is based on their ability to kill different types of microorganisms, with hypochlorite falling squarely into the intermediate category.
Question 773: In blood culture the ratio of blood to reagent is?
- A. 1:5
- B. 1:20
- C. 1:10 (Correct Answer)
- D. 1:100
Explanation: ***1:10*** - A 1:10 ratio of **blood to reagent (culture media)** is recommended for optimal growth of microorganisms in blood cultures. - This dilution minimizes the inhibitory effect of **human antimicrobial factors** present in the blood, while still providing sufficient blood volume for pathogen detection. *1:5* - A 1:5 ratio would mean a relatively **higher concentration of blood** in the culture. - This higher concentration could lead to increased inhibition of bacterial growth by **host defense mechanisms** or **antibiotics** present in the patient's blood, potentially causing false-negative results. *1:20* - A 1:20 ratio implies a **lower concentration of blood** relative to the culture media. - While it further dilutes inhibitory factors, it also reduces the total number of microorganisms sampled, possibly leading to **false-negative results** if the bacterial load is low. *1:100* - A 1:100 ratio would result in an **extremely low concentration of blood** in the culture. - This significantly reduces the likelihood of detecting microorganisms, especially when the **bacteremia is sparse**, due to insufficient bacterial inoculum, leading to a high rate of false negatives.
Question 774: Which anticoagulant is used when blood is sent for blood culture?
- A. Sodium citrate
- B. EDTA
- C. Oxalate
- D. SPS (Correct Answer)
Explanation: ***SPS*** - **SPS (Sodium Polyanethol Sulfonate)** is the preferred anticoagulant for blood cultures because it inhibits phagocytosis and complement activation, allowing microorganisms to survive and grow. - It also neutralizes the bactericidal effect of aminoglycoside antibiotics, which might be present in the patient's blood. *Sodium citrate* - **Sodium citrate** works by binding to calcium ions, preventing blood coagulation, and is typically used for coagulation studies. - It is not suitable for blood cultures as it does not have the anti-phagocytic or antibiotic-neutralizing properties of SPS. *EDTA* - **EDTA (Ethylenediaminetetraacetic acid)** is a strong chelator of calcium and is primarily used for hematology tests like complete blood counts (CBC) as it preserves cell morphology. - It can be toxic to some bacteria and would inhibit microbial growth, making it unsuitable for blood cultures. *Oxalate* - Various **oxalates** (e.g., potassium oxalate) also act as anticoagulants by precipitating calcium, and are commonly found in tubes for glucose testing when combined with a glycolytic inhibitor. - Oxalates are generally unsuitable for blood cultures because they can inhibit the growth of certain microorganisms.
Question 775: Which of the following statements regarding resistance of penicillin in Staphylococcus aureus is false?
- A. Methicillin resistance is due to alterations in penicillin-binding proteins (PBPs).
- B. Penicillinase production is mediated by plasmids.
- C. Hospital strains predominantly produce a unique type of penicillinase. (Correct Answer)
- D. Penicillinase production can be transmitted by transduction.
Explanation: ***Hospital strains predominantly produce a unique type of penicillinase*** - This statement is **false** because hospital strains do not produce a truly "unique type" of **penicillinase** compared to community strains. - **Penicillinase (beta-lactamase)** is a common resistance mechanism found across various *S. aureus* strains, not exclusive to hospital environments. *Methicillin resistance is due to alterations in penicillin-binding proteins (PBPs)* - This statement is **true** as **MRSA** resistance involves the **mecA gene** encoding **PBP2a**. - **PBP2a** has low affinity for **beta-lactam antibiotics**, allowing cell wall synthesis despite antibiotic presence. *Penicillinase production is mediated by plasmids* - This statement is **true** because **penicillinase genes** are typically located on **plasmids**. - **Plasmids** facilitate horizontal transfer of resistance genes between bacterial populations. *Penicillinase production can be transmitted by transduction* - This statement is **true** as **transduction** via **bacteriophages** can transfer resistance genes. - **Plasmid-borne penicillinase genes** can spread through this horizontal gene transfer mechanism.
Question 776: Which of the following bacteria can be identified using phage typing?
- A. Salmonella (Correct Answer)
- B. Streptococcus
- C. Shigella
- D. Pseudomonas
Explanation: ***Salmonella*** - **Phage typing** is the most classical and widely used method for subtyping *Salmonella* species based on their susceptibility to specific bacteriophages. - This technique is extensively used in **epidemiological tracking** of outbreaks, particularly for *Salmonella typhi* and *S. enteritidis*, allowing differentiation of strains within the same species. - **Most commonly associated organism** with phage typing in clinical microbiology and medical education. *Streptococcus* - While phage typing was historically used for *Streptococcus pyogenes* (Group A Streptococcus), it has largely been replaced by **molecular typing methods** and is not the primary association when phage typing is mentioned. - *Streptococcus* species are typically identified through **Gram staining**, **catalase tests**, **hemolysis patterns** on blood agar, and **serological methods** (Lancefield grouping). *Shigella* - **Phage typing** is not routinely used for *Shigella* identification or subtyping in clinical practice. - **Serotyping** and **molecular methods** like pulse-field gel electrophoresis (PFGE) or whole genome sequencing are the standard approaches for *Shigella* differentiation. *Pseudomonas* - While **phage typing** has been used for *Pseudomonas aeruginosa* in research and outbreak investigations, it is not the primary or most commonly cited example of phage typing in medical education. - Routine identification relies on **culture characteristics** (pigment production, oxidase positive), **biochemical tests**, and **molecular methods**.
Question 777: Which of the following is called Preisz-Nocard bacillus?
- A. C. diphtheriae
- B. C. pseudotuberculosis (Correct Answer)
- C. M. tuberculosis
- D. Mycoplasma
Explanation: ***C. pseudotuberculosis*** - This bacterium is historically known as the **Preisz-Nocard bacillus**, named after its discoverers. - It is an important pathogen in animals, causing conditions like **caseous lymphadenitis** in sheep and goats. *C. diphtheriae* - This bacterium causes **diphtheria** and is known for producing a potent **exotoxin** that can lead to systemic complications. - It forms a characteristic **pseudomembrane** in the throat and is not associated with the Preisz-Nocard designation. *M. tuberculosis* - This is the causative agent of **tuberculosis** in humans, primarily affecting the lungs. - It is known for its **acid-fast staining** property and a complex pathogenesis involving granuloma formation. *Mycoplasma* - This genus includes bacteria that lack a **cell wall**, making them resistant to many common antibiotics. - They are known for causing various infections, including **atypical pneumonia** and genitourinary tract infections, but are not referred to as the Preisz-Nocard bacillus.
Question 778: Darting motility which occurs in V.cholerae, also found in -
- A. Campylobacter jejuni (Correct Answer)
- B. Bacillus anthracis
- C. Shigella
- D. Pneumococcus
Explanation: ***Campylobacter jejuni*** - *Campylobacter jejuni* is well-known for its **rapid, corkscrew-like motility**, often described as **darting motility**, which is a key characteristic for its identification. - This characteristic movement is due to its **polar flagella** and helps the bacterium penetrate the intestinal mucosa. *Shigella* - *Shigella* species are notable for being **non-motile**, a crucial feature distinguishing them from other enterobacteriaceae. - Their lack of flagella and motility prevents them from exhibiting darting or any other form of self-propulsion. *Bacillus anthracis* - *Bacillus anthracis* is typically described as **non-motile** in its vegetative form. - While other *Bacillus* species can be motile, *B. anthracis* lacks the flagella necessary for darting motility. *Pneumococcus* - *Pneumococcus* (Streptococcus pneumoniae) is a **non-motile** bacterium. - It does not possess flagella and therefore cannot exhibit darting or any other type of active movement.
Question 779: Patient came from Nagaland and shows positive test with OXK antigen. Diagnosis is?
- A. Trench fever
- B. Scrub typhus (Correct Answer)
- C. Endemic typhus
- D. Epidemic typhus
Explanation: ***Scrub typhus*** - A **positive Weil-Felix test** for the **OXK antigen** is highly suggestive of scrub typhus, particularly if the patient is from an endemic region like Nagaland. - Scrub typhus is caused by **_Orientia tsutsugamushi_**, transmitted by **chiggers** (larval mites), often presenting with fever and an **eschar**. *Trench fever* - Caused by **_Bartonella quintana_** and transmitted by the **human body louse**. - It does not typically show a positive Weil-Felix test for the OXK antigen. *Endemic typhus* - Also known as **murine typhus**, it is caused by **_Rickettsia typhi_** and transmitted by **fleas** (e.g., rat flea). - While it causes a positive Weil-Felix test, it usually involves the **OX19 antigen**, not OXK. *Epidemic typhus* - Caused by **_Rickettsia prowazekii_** and transmitted by the **human body louse**. - It yields a positive Weil-Felix test for the **OX19 and OX2 antigens**, but typically not for OXK.
Question 780: Which of the following stimulate adenylate cyclase with G-protein coupled action ?
- A. Shiga toxin
- B. Cholera toxin (Correct Answer)
- C. Diphtheria toxin
- D. Pseudomonas toxin
Explanation: ***Cholera toxin*** - Cholera toxin is a **G-protein-activating toxin** that irreversibly activates **adenylate cyclase**. - This leads to increased intracellular levels of **cAMP**, causing excessive fluid secretion into the intestinal lumen and severe diarrhea. *Shiga toxin* - Shiga toxin acts by inactivating the **60S ribosomal subunit**, thereby inhibiting protein synthesis in eukaryotic cells. - Its primary effect is **cytotoxicity**, not direct stimulation of adenylate cyclase. *Diphtheria toxin* - Diphtheria toxin inhibits **protein synthesis** by inactivating **elongation factor-2 (EF-2)** through ADP-ribosylation. - This toxin specifically targets host cells, leading to cellular death and tissue damage. *Pseudomonas toxin* - **Exotoxin A** produced by *Pseudomonas aeruginosa* also inhibits **protein synthesis** by ADP-ribosylating and inactivating **EF-2**, similar to diphtheria toxin. - It does not directly affect adenylate cyclase activity.