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

Isolation and identification procedure for Staphylococcus aureus in laboratory mice and rats by combined use of chromogenic X-SA agar and specific polymerase chain reaction

Generally, a conventional culture-based examination procedure (detection by egg-yolk salt agar and subsequent identification by phenotypic tests) for confirmation of the presence of S. aureus (SA) in laboratory mice and rats requires approximately 4 days. To improve the culture-based examination procedure for SA in terms of rapidity and reliability, combined use of chromogenic X-SA agar (XSA) and PCR using newly designed specific primers for SA (XSA-PCR) that can shorten the examination time (25.5 hr) was compared with the conventional procedure for SA. In 425 samples from mice and rats, 193 suspected isolates were detected by egg-yolk salt agar (EYSA), and 216 suspected isolates were detected by XSA. In the subsequent identification, 189 of 193 suspected isolates detected by EYSA were identified as SA by phenotypic tests (97.9%), and all 216 suspected isolates detected by XSA were identified as SA by PCR (100%). All SA-positive samples by the conventional procedure were included in the SA-positive samples by XSA-PCR. As a result, XSA-PCR was superior to the conventional procedure in detection rate and identification rate of SA. Therefore, XSA-PCR appears to be an effective tool for examination of SA in laboratory mice and rats that improves precision and shortens the examination time.

Media used in the detection and enumeration of Staphylococcus aureus

Over the years a range of selective and diagnostic media have been developed to assist in the detection and enumeration of Staphylococcus aureus in routine food surveillance programmes and food poisoning investigations and these are reviewed here. Baird-Parker agar remains, however, the medium of choice for direct plating and enumeration of S. aureus in both Europe and the US. This paper also reports on a comparison of the productivity and selectivity of trypticase soy broth with 10% NaCl and 1% sodium pyruvate (PTSBS) with trypticase soy broth (TSB) for the isolation of S. aureus. Using three strains of S. aureus and a strain of S. hyicus the productivity ratio of PTSBS to TSBS ranged from -0.17 to 0.57. In the recovery of heat-injured cells of S. aureus, PTSBS offered little or no improvement over TSB with sodium chloride but no pyruvate in a three-tube MPN assessment. Of the seven other bacterial cultures tested, none grew well on the PTSBS. Selectivity ratios of 4.4-7 were found.

Evaluation of a mannitol-salt-oxacillin-tellurite medium for the isolation of methicillin-resistant Staphylococcus aureus from contaminated sources

A mannitol-salt medium supplemented with 0.002% tellurite (MSOT) supported the growth of 104 of 109 strains of methicillin-resistant Staphylococcus aureus (MRSA) while suppressing 74 of 102 strains of mannitol-positive, methicillin-resistant coagulase-negative staphylococci. MSOT was equally or more sensitive and more differential than a similar formulation without tellurite for the isolation of MRSA from the skin and nares of hospitalized patients. Only S. haemolyticus produced dark gray-to-black colonies on MSOT similar to those of MRSA. MSOT is a useful adjunct to other nonselective culture formulations for the isolation of MRSA from contaminated sources.

The inhibition of Staphylococcus hyicus on a selective medium for staphylococci

Growth of Staphylococcus hyicus subsp. hyicus and Staph. hyicus subsp. chromogenes strains was found to be severely inhibited when broth cultures of these organisms were streaked on Schleifer and Kramer's staphylococcal (SK) medium. Of the selective agents contained in SK medium, potassium thiocyanate was found to be inhibitory towards both subspecies of Staph. hyicus and sodium azide had an additional inhibitory effect on Staph. hyicus subsp. chromogenes. Of six different media supplements examined, sheep blood and Tween 80 were found to improve the growth of both Staph. hyicus subspecies when added to SK medium. These findings were confirmed in subsequent work where the supplemented SK media were used to isolate potential enterotoxin-producing organisms from simulated raw milk (Staph. aureus, Staph. hyicus subsp. hyicus, Staph. hyicus subsp. chromogenes, Staph. intermedius). SK medium supplemented with sheep blood proved more effective in allowing satisfactory recovery of Staph. aureus, Staph. hyicus subsp. hyicus and Staph. intermedius. However, neither supplement enabled satisfactory recovery of Staph. hyicus subsp. chromogenes to be achieved.

The toxicity of potassium tellurite to Staphylococcus aureus in rabbit plasma fibrinogen agar

The potassium tellurite concentration, 0.01% w/v, in Baird-Parker agar has been recommended for plasma coagulase media such as pig or rabbit fibrinogen agar. Comparative tests have shown that with some strains of Staphylococcus aureus this level of potassium tellurite is too inhibitory and it should be reduced four-fold to 0.0025% w/v to maximize the isolation rate. It is postulated that egg yolk in Baird-Parker agar has a protective effect on staphylococci against the inhibitory action of tellurite.