[Suitability of current selective media for the detection of Staphylococcus aureus in food]

Abstract

Three solid media (Baird-Parker-Agar (BP), BP with pig plasma instead of egg-yolk (BPP) and modified BP according to Devriese (BPA) as well as three liquid media (Brain-Heart-Infusion with 7.5% NaCl (BHIS), Liquid Baird-Parker-Medium (LBP) and Giolitti-Cantoni-Medium) were tested with regard to their suitability for the detection of Staph. aureus in foods. The following criteria were assessed: productivity for vital and thermally stressed Staph. aureus; selectivity against undesired microorganisms; practicality in routine examination. The microorganisms used in these studies were: Staph. aureus (vital and thermally stressed), "non-aureus" staphylococci, Streptococcus spp., Pseudomonas spp., Proteus spp., Bacillus spp. Using vital strains of Staph. aureus no significant differences in the productivity of the three solid media could be observed. On the other hand BPA showed a remarkable lower productivity than BP and BPP when thermally stressed cultures of Staph. aureus were examined. LBP was the most productive liquid enrichment medium. The selectivity of BPA was distinctly superior to BP and BPP, whereas no differences could be observed between the latter ones. The selectivity of LBP and GC against coagulase negative staphylococci and gramnegative microorganisms tested was relatively poor. Each culture under examination grew in BHIS. The combination of LBP and BPA yielded better results for enumeration of thermally stressed Staph. aureus from dried milk products than the other possible combinations of media. In view of their practicality dehydrated culture media are commonly preferred in routine examination. The additional time required for the preparation of LBP or BPA (which are not commercially available as dehydrated media) is negligible and is compensated by better productivity and selectivity of these media. At present BPP is not recommendable due to the problems in providing pig plasma. On the other hand this medium has the great advantage that further testing of suspicious colonies is not necessary as it is in the case of BP and BPA.