Cross-contamination in a laboratory was at fault for the finding of a finished product positive for Salmonella, according to a study.

In 2013, during a routine analysis of food samples, one finished chocolate product from a European factory tested positive for Salmonella Hadar. At the same time, environmental monitoring in the lab indicated a Salmonella-positive sample from the thermocouple in one incubator.

Eight months earlier, a proficiency test was completed by the unnamed lab with Salmonella Hadar spiked on a sample. This same lab analyzed the food sample for the European factory. The lab carried out regular testing for Salmonella during 2013 but had only one positive during the year.

Due to suspicion of lab cross-contamination between the Salmonella Hadar isolate used in the proficiency testing and the isolate found on the finished product by the same lab, whole genome sequencing (WGS) was used.

The analysis showed a maximum of ten single nucleotide polymorphisms (SNPs) between the isolates coming from the lab and the finished product, confirming the cross-contamination.

Results combined with the additional investigation at the factory allowed the release of finished product batches and prevented unnecessary food waste and economic losses for the factory.

As part of food safety management systems in production facilities, factories have environmental monitoring and product testing programs to detect issues in the production area and their surroundings. Samples taken can be sent to labs for analyses to control microbiological quality and safety.

During routine microbial testing in 2013, a lab found a Salmonella Hadar positive in a sample of chocolate produced at a European factory.

The plant retained the finished product batch, started cleaning and disinfection, and increased the number of analytical tests. A root cause investigation was initiated by retrieving all production and analytical data to identify the possible source of contamination.

None of the information collected indicated abnormalities at the production site. No Salmonella was detected in raw materials or environmental samples taken at the factory.

Due to the positive environmental sample at the lab and the proficiency test, the hypothesis was probable cross-contamination in the lab had resulted in a false positive result of the food product. Participation in proficiency testing demonstrates competence while performing specific microbiological examinations.

Confirmation of the lab cross-contamination was needed using a more discriminatory method than the classical phenotypic Salmonella serotyping by slide agglutination based on the Kauffmann-White-Le Minor (KW) scheme.