Detection of propionic acid bacteria in cheese.

Mesophilic lactic starters and thermophilic lactobacilli but not Streptococcus salivarius subsp. thermophilus grew on the sodium lactate agar (SLA) used for estimating the numbers of propionic acid bacteria (PAB) in cheese. The addition of cloxacillin (4 micrograms/ml) to SLA inhibited the starter bacteria but had no effect on the PAB. It was possible to count low numbers of PAB in the presence of high numbers of starter bacteria. A correlation coefficient of 0.9 was obtained between the level of propionic acid and the counts of PAB in cheese (n = 40). A disadvantage of the medium is that other bacteria found in cheese (mesophilic lactobacilli, enterococci, Clostridium tyrobutyricum) also grow on it; however, these bacteria are easily distinguishable from PAB on the basis of size, colour and absence of catalase.