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Microbiology

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General Microbiology

Question
75 out of 97
 

Pasteurized milk is tested most commonly by-(AIPG -11)



A Phosphatase test

B Coliform test

C Catalase test

D Methylene blue test

Ans. A Phosphatase test

a. Phosphatase is an enzyme normally present in milk, which gets inactivated by pasteurization of milk.

b. Its presence in milk after pasteurization indicates improper/ incomplete pasteurization. Its presence is detected by adding disodium phenyl phosphate to 1 ml of milk in a test tube and incubating in a water bath at 37°C for 2 hours. Development of yellow color indicates presence of phosphatase

EXTRA EDGE

Acquiring new DNA from external source

  1. Transformation c. Transduction
  2. Lysogenic conversion d. Conjugation

1. Transformation

  1. Acquisition of soluble DNA from environment and incorporation in its genome
  2. First demonstrated in S. pneumoniae. DNA must be derived from closely related strain
  3. Cells that can take up naked DNA are called as competent
  4. First demonstrated by Griffith 1928
  5. Transformation can be inhibited by DNAases

2. Transduction

· Transfer of a portion of DNA from one bacterium to another by bacteriophage

Generalized transduction:

a. Due to phages that lyse host cell: Virulent phages

b. During assembly, phage head is filled with host cell DNA (packaging error)

c. Such a phage infects a second bacterium, DNA enters

d. Transduce any gene. Chromosome DNA as well as plasmid DNA e.g. penicillin resistance in S. aureus

e. Size similar to phage genome

f. B/w closely related strains

Specific transduction:

  1. Due to phages incorporated into host genome: Temperate phages (Lysogenic)
  2. When infected with temperate phage: small proportion get incorporated into host cell genome: In some lysogenic cell lytic cycle is resumed The prophage (integrated phage DNA) is excised In some cells prophage is excised inaccurately so that neighboring portion of bacterial DNA is also removed Such piece of DNA is transduced into second cell and integrates to the specific site.

3. Lysogenic conversion:

  1. In lysogenic bacteria, prophage codes for new characters
  2. Corynebacterium diphtheriae; diphtheria toxin is coded by α phage
  3. S. pyogenes; Dick toxin is coded by a bacteriophage

4. Conjugation

  1. Transfer of DNA that occurs during contact b/w bacterial cells
  2. Plasmid - codes for Sex Pilus, 1-2 μm length
  3. Pilus attaches to the surface of recipient cell and holds the two cells together
  4. The plasmid DNA replicates and a copy of it passes through the sex pilus
  5. F-Factor: genetic information for synthesis of sex pilus, which is required for self transfer
  6. F-Factor has an ability to integrate in the host chromosome: Hfr cells
  7. F-Factor with host chromosomal genes: F' Factor (transferred by sexduction)
  8. R- Plasmids: RTF + r determinant Resistance to eight or more drugs may be transferred simultaneously, most common method of resistance transfer.E. coli, K. pneumoniae, Salmonella, Shigella, Pseudomonas

General Microbiology Flashcard List

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