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Methods in Microbiology (GATE - XL)

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Which of the following is produced by the alpha-hemolytic colonies on blood agar?

  1. Zone of complete clearing

  2. No zone

  3. Zone of partial clearing (green)

  4. Brown colored zone


Correct Option: C
Explanation:

Alpha-hemolytic colonies produce zone of partial clearing on blood agar. Alpha hemolysis is caused by hydrogen peroxide produced by the bacterium, oxidising hemoglobin to green methemoglobin.

Which of the following types of organisms produce colonies on Columbia nalidixic acid agar?

  1. Gram-positive cocci

  2. Gram-negative cocci

  3. Gram-positive bacilli

  4. Gram-negative bacilli


Correct Option: A
Explanation:

Gram-positive cocci produce colonies on Columbia nalidixic acid agar. Columbia nalidixic acid agar with sheep blood is a selective microbiological medium.

Cary Blair medium is used as a

  1. selective medium

  2. enriched medium

  3. differential medium

  4. transport medium


Correct Option: D
Explanation:

Cary Blair medium is a transport medium for the collection and shipment of clinical specimens. The low nutrient content of the medium and the inclusion of phosphate buffer prevent bacterial overgrowth by E. coli, Citrobacter freundii and Klebsiella aerogenes. It can occur in other transport mediums containing sodium glycerophosphate. The low oxidation-reduction potential of the medium ensures bacterial survival over long periods.

Salk polio vaccine is sterilised by

  1. vaporized hydrogen peroxide

  2. formaldehyde

  3. peracetic acid

  4. ozone


Correct Option: B
Explanation:

Salk polio vaccine is sterilised by formaldehyde (gaseous form).

The highest probability of obtaining a pure culture lies in

  1. pour plate method

  2. spread plate method

  3. streak plate method

  4. inoculating a slant


Correct Option: C
Explanation:

The most probable method of obtaining a pure culture is by streak plate method. Individual colonies can be obtained in the fourth quadrant.

Which of the following macro-nutrients provides heat resistance to endospores?

  1. Potassium

  2. Magnesium

  3. Calcium

  4. Iron


Correct Option: C
Explanation:

Calcium provides heat resistance to endospores. Calcium forms calcium dipicolinate in endospores. It may aid in resistance to heat and oxidising agents.

The presence of which of the following elements is required for the formation of leghemoglobin in N2 fixing bacteria?

  1. Cobalt

  2. Copper

  3. Molybdenum

  4. Zinc


Correct Option: A
Explanation:

Cobalt is required for the synthesis of leghemoglobin and for the growth of legumes relying on symbiotically fixed nitrogen. It is an essential micronutrient. It has been established that rhizobium and other Nfixing microorganisms have an absolute cobalt requirement whether or not they are growing within nodules and regardless of whether they are dependent on a nitrogen supply from N2 fixation or from mineral nitrogen.

Blood agar is

  1. differential and selective

  2. differential and enriched

  3. differential and used for enrichment

  4. enriched and selective


Correct Option: B
Explanation:

Blood agar is enriched with sheep blood and it also aids in identifying bacteria on the basis of different hemolysis patterns.

A fecal sample suspected to contain pathogens was plated on nutrient agar in aerobic, anaerobic and microaerophilic environment. Growth was seen only on the plate kept under microaerophilic environment. Which of the following can be the suspected organism and the medium used to confirm it?

  1. Enterobacter and McConkey agar

  2. Campylobacter and Butzler's virion

  3. Bacteriodes and brain heart infusion agar

  4. Pseudomonas and cetrimide agar


Correct Option: B
Explanation:

The suspected organism is campylobacter as it is a strict microaerophile. Confirmatory test can be done by plating on Butlezr's virion, which is selective for campylobacter.

The palladium of which of the following organisms is required as a catalyst for culture?

  1. Mycobacteria

  2. Escherichia

  3. Propionibacterium

  4. Enterococcus


Correct Option: C
Explanation:

Propionibacterium is an obligate anaerobe. For anaerobic culturing, palladium is required as a catalyst. It is only able to work if it is not dry and is not inactivated by H2S or other products of bacterial metabolism.

Which of the following ingredients in mannitol salt agar makes it selective for Staphylococci?

  1. Mannitol

  2. Sodium chloride

  3. Phenol red

  4. Beef extract


Correct Option: B
Explanation:

MSA contains a high concentration (~7.5%-10%) of salt (NaCl).

Selenite F broth is used for enrichment of Salmonella and

  1. isolation of Pseudomonas

  2. transportation of Vibrio cholerae

  3. transportation of Neisseria

  4. isolation of coliforms


Correct Option: B
Explanation:

Selenite F broth is also recommended for the transport of strains of Vibrio cholerae. These organisms can survive for 2 to 5 days in Sodium Selenite broth.

Which of the following conditions is appropriate to sterilise a sample containing 263K scrapie prion?

  1. 121-132 degree centigrade for 60 minutes

  2. 134-138 degree centigrade for 18 minutes

  3. 121 degree centigrade for 20 minutes

  4. 134 degree centigrade for 10 minutes


Correct Option: A
Explanation:

The 263K scrapie prion is inactivated at 121-132 degree centigrade for 60 minutes.

Lucas semisolid indicator is used to

  1. confirm autoclave performance

  2. check for anaerobiosis

  3. detect potability of water

  4. differentiate between LF and NLF


Correct Option: B
Explanation:

Lucas semisolid indicator is used to indicate anaerobiosis in a jar. It is based on the reduction of methylene blue. In an anaerobic environment, it is reduced from its coloured oxidised form to colourless reduced leuco compound.

Which of the following is the most important benefit of using phase contrast microscopy over contrast and electron microscopy?

  1. Greater resolution

  2. Visualising live processes

  3. Greater magnification

  4. Greater contrast


Correct Option: B
Explanation:

The phase contrast microscope makes it possible for biologists to study living cells and how they proliferate through cell division. Thus, it helps in visualising live processes.

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