Question

Outline and describe the essential detection methods to investigate these microbial causative agents in a food sample:  

1. E. coli
2. Staphylococcus aureus
3. Salmonellae

Answer 1

detection method for e. coli- polymerase chain reaction PCR using the primer  against the uidA gene that encodes beta-D-glucuronidase . it determines the presence of the E. Coli accurately and usually the food sample is put on the agar pate which was formed by plating at least 24-h enriched broth. the available media used for detection are-

MacConkey MUG Agar,Membrane Lactose Glucuronide Agar

detection method for staphylococcus aureus- selective plating medium for Staph. aureus is the one devised by Baird-Parker is used usually for the detection of the staphylococcus species.. Lithium chloride and tellurite act as selective agents while egg yolk and pyruvate assist in the recovery of damaged cells. Reduction of the tellurite by Staph. aureus gives characteristic shiny, jet-black colonies which are surrounded by a zone of clearing, resulting from hydrolysis of the egg-yolk protein lipovitellenin. Colonies also often have an inner white margin caused by precipitation of fatty acid.

detection method for salmonella species-The selective agents used are bile salts or deoxycholate and/or brilliant green and the diagnostic reaction is usually provided by the inability of most salmonellas to ferment lactose and/or the production of hydrogen sulfide. In choosing the media to use, it is advisable to select two based on different diagnostic reactions to ensure that atypical strains, for instance lactose-positive ones, will not be missed