Enumeration of lactic acid bacteria: lacuna and improvement areas highlighted by proficiency testing

Abstract

During the 2011 to 2018 food microbiology proficiency testing (PT) schemes organized by REQUASUD, a systematic analytical issue was detected. For the enumeration of the lactic acid bacteria (LAB), laboratories carrying out the same analytical method (ISO 15214) yielded completely incoherent results while analysing naturally contaminated samples (raw milk, smoked salmon and ham sausage), leading to an atypically large distribution of the PT results. A comprehensive study of these atypical PT results provided insight into the source of systematic errors affecting the LAB enumeration. The main sources of analytical variability turned out to be linked to the ISO 15214:1998 standard. To better harmonize the analytical results, practical improvements can be proposed for the revision of this standard, mainly: the addition of precise criteria for the colony count, a restriction of the allowed incubation conditions and the use of a more selective culture medium (e.g. supplemented with sorbate) to limit the growth of interfering colonies. PT is a privileged observation post to highlight such analytical issues, which would otherwise remain unnoticed. Whenever an unusual distribution of PT results is observed (e.g. high dispersion or non-normal distribution of the values), it is the responsibility of the PT provider to try to identify the causes of non-normality, in a perspective of method improvement. The statistical analysis of atypical PT results requires specific attention: ignoring the non-normality of PT results when it occurs can lead to set an erroneous assigned value, unfairly penalizing participants.