Microbiology Food Testing - Presence/Absence Test Methods

... General E. coli testing is one of the more common procedures that a laboratory conducts. These general E. coli are a useful indicator of food quality/safety given that its presence in food indicates some route of faecal contamination, e.g. contaminated water. The vast majority of E. coli are harmless, however certain strains of the organism can cause food poisoning and the most severe is that caused by E. coli O157 (this number refer to its serology). Infection by E. coli O157 can result in serious complications such as kidney failure and heart problems, and in elderly people it can be fatal. Britain’s biggest outbreak occurred in Scotland in the mid 1990’s and was associated with contaminated meat products manufactured in a butchers shop in Lanarkshire.

Even in the laboratory E. coli O157 has to be treated with particular caution and the procedure requires isolation from general testing. The method is an example of a presence/absence technique and draws on both convention enrichment procedures and serology by way of a process referred to as immuno-magnetic separation. A pre-enrichment stage is conducted on a 25g sample. This pre-enrichment may be 6 to 24 hours in duration at 41.5°C. An aliquot of the pre-enrichment is then combined with a suspension of magnetic beads that are supplied coated with antibodies sensitive to E. coli O157. Should E. coli O157 be present in the pre-enrichment broth, the cells will bind to the antibody beads. These are then separated from the pre-enrichment broth by use of a magnet. Resuspended washed beads are plated out onto selective agar and following incubation, resulting characteristic colonies are examined further.

...despite being the most frequent causative agent of bacterial food borne illness in the UK are relatively unknown to the public at large. In 2007, 51½ thousand cases were reported; more than four times that of confirmed Salmonella infections. In the laboratory Campylobacter are somewhat fastidious in their growth requirements. Indeed it is impossible for their numbers to multiply on contaminated food regardless of storage temperature.

Campylobacter food poisoning can result from the ingestion of very low numbers of viable cells (10 or less), so for food safety purposes a direct plating method is not sufficiently sensitive to provide assurance of an absence of the organism, hence the need for a presence/absence method. This procedure as with other presence/absence techniques standardises with a 25g weighed food portion added to 225mL of enrichment broth (such as Preston or Bolton broth) which is heavily enriched with blood and other growth factors to coax this very fastidious organism into growth.

...widespread in the environment and can be isolated from the digestive tracts of animals and birds. The most notable of Listeria species is L. monocytogenes. This species has potential for food borne infection if present at greater than 100 organisms per gram in ready-to-eat foods. A surprisingly common organism in ready to eat foods, Listeria monocytogenes is predicted to become an increasingly problematic food pathogen. Unlike virtually all other food pathogens, it is happy to grow at refrigeration temperatures (<5°C) albeit slowly. Therefore in ready-to-eat foods able to support its growth, manufacturers have to ensure that the organism is either absent in the first place or will not exceed 100cfu/g by the end of life.

Laboratory testing can be either by plate count to provide enumeration, or via presence/absence testing. The latter is particularly useful for environmental testing whereby processors may wish to validate their hygiene procedures in cases where Listeria is known to be present within the factory environment.

... and in 2007 was responsible for 12,000 reported cases of food poisoning. In the past decade significant work has gone in to reducing the incidence of this organisms, particularly when associated with poultry. Vaccination programmes and tighter biosecurity measures have dramatically decreased Salmonellosis associated with egg consumption since the days of Edwina Curry in the 1990’s. However, the organism is still a significant cause of food poisoning, with notable recent links to a well known brand of chocolate as well as being responsible for major outbreaks traced back to imported Spanish eggs.

The recommended laboratory method follows the current ISO standard 6579, and is suitable for general foods and environmental swabs. The method follows a pre-enrichment stage leading onto subbing into two different enrichment broths before being plated out as streak plates onto two different agars. Salmonella like E. coli O157 is a notifiable organism and its presence in ready to eat foods must be reported to the Foods Standards Agency or local authority by the manufacturer.