Microbiology Food Testing - Presence/ Absence Test Methods
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Enumeration test methods provide a numerical value as to how many microorganisms are present in a food sample or environmental swab. However the level of detection they can achieve is relatively limited and most procedures will not be able to provide a count below 10 cfu/g for solid foods. Problematically, there are some food poisoning organisms that cause serious illness at levels much below this and therefore alternative procedures are needed if we are to determine the presence of organisms like E. coli 0157, Salmonella, Listeria or Campylobacter in a food sample. In these cases a qualitative method is used which serves to recover the bacteria of concern, growing them to a level whereby they can be readily detected on selective agar plates. Such tests cannot assess the original number of bacteria in a sample, merely their presence or absence.
A presence/absence test will typically start with a 25g sample of product, which is directly incubated with a nutritious selective broth medium. This pre-enrichment stage serves to recover stressed target organisms while keeping non-target bacterial growth to a minimum. In the case of Salmonella or Listeria, following incubation, a portion of the incubated pre-enrichment broth is subbed into a more stringent enrichment broth which promotes growth of the target organism and inhibition of others. The final procedure is then to transfer some of the incubated enrichment sample onto selective agar plates which after further incubation can be examined for characteristic colonies.
This multi-stage approach takes considerable time and even if characteristic colonies are recovered, further identification work is needed before the result can be confirmed.