Some thoughts around managing Salmonella laboratory error.

While?most?food laboratory?Salmonella?detections in laboratories are valid, laboratory error can?occur,?resulting in the reporting of?Salmonella?detections?when no?Salmonella?is truly present in the sample under test.?

Such errors may result in significant losses to the customer which could include reputational damage, unnecessary product?disposal, disruptions in production, withdrawals and recalls. The mental stress on business owners, managers?and QA staff?cannot?be underestimated.

As we have a new Australian Standard this year, and?after?some recent NATA technical assessments, some thoughts are presented around this matter.

• External Accreditation to ISO17025; In Australia, the?Government-endorsed?accreditation body is the National Association of Testing Authorities, Australia?(NATA). Achieving NATA accreditation is a great step towards assurance.
• Australian Standard 5013.10-2022 (ISO 6579-1:2017, MOD)?Detection of?Salmonella?spp, Appendix?ZZ?lists variations for Australian conditions. These variations include?
• Mandatory serological identification by a Salmonella Reference Laboratory (there are five?laboratories?listed).?Typing isolates can?alert?the?testing laboratory?to a possible cross contamination event (whether?via?reference culture,?control culture,?PT isolate, sample cross contamination or environmental contaminant).
• The use of?Salmonella?Hofit IMVS 1799 as the reference culture. Hofit was selected because it is not found in Australia and is therefore unlikely to be mistaken for an isolate under test. The additional benefit of using Hofit for laboratory customers is that its isolation can quickly alert both the testing and the five reference laboratories of a contamination event. Use of a different?control?culture?could?take away this protection.?
• Independent, external, proficiency programme (PT).?Proficiency Testing (PT) is mandated by ISO17025 and it is best practice to use a provider who is external and independent to the laboratory that ideally has many participants in their programme for added statistical robustness. PT for non-accredited labs is a recommended way to examine more generally the techniques of laboratory staff. PTs can highlight problems in laboratory technique/processes and can be a positive step to assurance. It must be pointed out that?Salmonella?contamination events are typically sporadic or idiosyncratic and a systematic programme such as PT does not necessarily link to cross-contamination.?

Typing of?isolates.

• There seem to be two protocols that laboratories use to manage?Salmonella?contamination. One is the trigger protocol that is activated when circumstances suggest a contamination event at which time priority is given to characterisation of isolates to resolve the situation. There are seasonal times when labs find serotyping a slow process, so its role in identifying a problem can be impractical. Given the availability of molecular typing or even WGS in the larger labs, they can move faster on ‘fingerprinting’ and identification and resolution of such problems.?The other protocol is the list protocol that is ongoing,?and types (whether by serology or molecular methods) and archives all laboratory?Salmonella?isolations. Such a list provides a powerful tool to identify and manage possible cross contamination. Included within this list should be recent PT isolates,?EMP?isolates, and any isolates enriched to high levels for microbial challenge testing in the laboratory. These protocols are not mutually exclusive, and it appears that typing is?increasingly?the norm, while serotyping is the current usual mode.
• Enrichment steps.?While all steps,?from weigh-up to streaking,?are subject to contamination events (especially weigh up of fine dusts/powders that could aerosolise), the incubation and?subculturing of the wet enrichments provide the greatest risk of leak, aerosols and cross contact at a stage when low?Salmonella?levels are intended to be enriched to millions. Aseptic technique and single use equipment such as long-reach graduated serological pipettes is very important; indeed the?use of insufficiently disinfected automated pipettes and the use of such pipettes without filter tips were the most probable causes for?Salmonella?laboratory false positives in one study (1).?
• Operating environment?/ Environmental monitoring programme (EMP).?It is pleasing to see a lot more laboratories implementing EMP specifically for?Salmonella, which provides early warning of contamination. Common areas monitored include incubators used for?Salmonella?enrichment, stomachers/lab blenders, balances, pipettors, and benches. Other laboratories use the zone concept and radiate testing out from the laboratory.???Many laboratories take great care in physical and/or temporal separation with high-risk testing, for example clean-down of the environment between sample types.
• Many laboratories perform ‘additional assurance’ steps.?Laboratories?implement one or more practices, such as?running?a ‘clean Lab - no raw meat /no meat meal’ policy as a way to minimise their?Salmonella?transit, superior compartmentalisation of processes,?ongoing cleaning and sanitation, internal?Salmonella?proficiencies additional to external/independent proficiencies, enrichment of?a plethora of daily blanks,?use of?genetically labelled control cultures,?or external contamination reviews. Notably, some laboratories invest in people with above industry level pay, various flexibility schemes, internal and external training and mentoring. This enhances staff retention, reduces onboarding of new staff and minimises the ever present?‘Laboratory Error’?mantra.?Such investment in people is to be encouraged.?

As a final line that is surely applicable to much more than?Salmonella?testing, please remember that?cheaper is not always better.

Sofroni Eglezos & Julian Cox

References

1. Rasschaert G, De Reu K, Heyndrickx M, Herman L. Case report of Salmonella cross-contamination in a food laboratory. BMC Res Notes. 2016 Mar 10;9:156. doi: 10.1186/s13104-016-1969-7. PMID: 26965050; PMCID: PMC4787000.
2. Katia Rouzeau-Szynalski, Caroline Barretto, Coralie Fournier, Deborah Moine, Johan Gimonet, Leen Baert, Whole genome sequencing used in an industrial context reveals a Salmonella laboratory cross-contamination. International Journal of Food Microbiology,Volume 298, 2019, Pages 39-43.

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