THE PERFORMANCE OF AN EGG YOLK-TELLURITE MEDIUM IN PRACTICAL USE

Listeria monocytogenes in foods. Isolation, characterization and control

The intent for examining foods for Listeria monocytogenes, i.e. for surveying, for epidemiological purposes, or to inspect consignments of foods for microbiological safety, determines which analytical method is to be used. For instance resuscitation of debilitated cells may be required, and the degree of accuracy and precision necessary should be considered. Moreover, in the case of acceptance-or-rejection monitoring target values for 'absence' of the pathogen have been amply but not always effectively discussed. Recommendations are given for assessing adequate repair of sublethally damaged populations of L. monocytogenes and for performance testing of selective enrichment and isolation media to be used for the isolation and enumeration of L. monocytogenes. An approach to the empirical assessment of reference values for L. monocytogenes in foods processed for safety is also presented. This relies on a data base of results obtained when examining foods manufactured and distributed according to practices previously validated by longitudinally integrated ('holistic') quantitative risk analysis.

Egg yolk-free Baird-Parker medium for the accelerated enumeration of foodborne Staphylococcus aureus

A simplified procedure is described for the accelerated enumeration of foodborne Staphylococcus aureus. This involves the replacement of egg yolk in the Baird-Parker medium with Tween 80 and MgCl2. These compounds, along with pyruvate, allow the recovery of stressed cells of S. aureus on a medium which contains potassium tellurite, LiCl, and glycine as selective agents. Black colonies are identified as S. aureus by the simplified thermonuclease test.

Liquid modification of Baird-Parker's medium for selective enrichment of Staphylococcus aureus

A liquid modification of Baird-Parker's medium is suggested for the detection of low (less than 20/g) numbers of Staphylococcus aureus. Model experiments showed that the medium had an acceptable level of selectivity and that it was non-inhibitory to injured cells. Practical evaluation demonstrated the advantage of selective enrichment procedures over both non-selective enrichment technique and direct plating methods.