Practical Food Microbiology 3rd Edition - Part 6

The procedure used for isolation of a microorganism from a food sample will depend upon a number of factors. If the organism is expected to be found in large numbers, or its presence is only significant when there are large numbers | 6 Isolation and enrichment of microorganisms Aeromonas spp. Bacillus cereus and other Bacillus spp. Brucella spp. Campylobacter jejuni C. coli C. lari Clostridium perfringens and other sulphite-reducing clostridia Coliforms thermotolerant faecal coliforms and Escherichia coli Enterobacteriaceae Enterococci Lactobacilli and the lactic acid bacteria Listeria monocytogenes and other Listeria spp. Pseudomonas aeruginosa and other pseudomonads Salmonella spp. Shigella spp. Staphylococcus aureus and other coagulase positive staphylococci Vibrio spp. Viruses Yeasts and moulds Yersinia spp. The procedure used for isolation of a microorganism from a food sample will depend upon a number of factors. If the organism is expected to be found in large numbers or its presence is only significant when there are large numbers direct enumeration on a suitable selective solid medium will be sufficient. If however only small numbers of that organism are anticipated or if their presence is significant regardless of the number of cells . salmonellae then enrichment culture will be required. This may need to incorporate a pre-enrichment or resuscitation stage if the organism is likely to have suffered injury through freezing drying heating etc. Isolation media and procedures are often a matter of personal choice but due regard should be given to their suitability for recovery of stressed organisms which are easily inhibited by many selective agents and also by elevated incubation temperatures. In addition the recovery of spoilage organisms may require adjustments to the isolation medium such as an increase in the levels of salt or glucose in order to mimic the nature of the spoiled commodity and thus to allow recovery of the organism. The quantity of food examined is important in general for pre-enrichment or direct selective enrichment a 25 g portion should be cultured and the ratio of sample to broth .

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