University:
Swansea University
Course:
PM134 | Microbiology
Academic year:

2023
Views:

282

Pages:

2
Author:

Amal J.

Dilution Methods: The sample, along with the diluent, is added into a sterile plastic bag and kept in the stomacher. The compression and shearing forces result in a homogeneous suspension of the sample and microbes. Diluent Used: Peptone water: Easy to prepare and protects microbes during dilution and plating. Disadvantage of Phosphate Buffer: Should not be kept for long; otherwise, the organism will multiply. Not more than 20 minutes should be taken between the first dilution and the last plate pour. An increase of up to 10% can be expected in this 20-minute period. Another thing to note is the composition and pH of the diluent. Any treatment needed to allow the recovery of cells injured during food processing or preparation of samples should be taken into account. Example: The recovery of injured waterborne coliforms can be aided by adding a diluent containing Peptone or milk. Total Count Methods: SPC/APC: Viable only DMC: Both viable and non-viable MPN: Both viable and non-viable Dyed Medium: Viable only Pour Plate Method: Used for APC—plate agar/tryptone glucose extract agar. Simple, can cover a large concentration range. Spread Plate Method: Drop test or spiral plating. Higher plate count for spread than pour. Heat-sensitive organisms can be observed (e.g., psychrophiles). Obligate aerobes can be grown faster on the surface than in depth. Surface colonies are always detectable sooner and take much longer to count than colonies in pour plates.