Influence of pH and temperature on the growth of Enterococcus faecium and Enterococcus faecalis

A total of 70 enterococcal strains isolated from traditional Italian raw milk cheeses were identified by API 20 Strep and Biolog GP. All the identifications were confirmed by species-specific PCR. Four species of genus Enterococcus were found: E. faecalis, E. faecium, E. durans and E. hirae. E. faecalis and E. faecium were the dominant enterococcal species isolated in artisanal cheeses. According to the most representative species and the origin of isolation, 21 strains ( 11 E. faecalis and 10 E. faecium) were selected for the conductimetric analysis. The influence of pH (5.0; 5.5; 6.0 and 6.5) and temperature ( 25 degrees C and 37 degrees C) on the metabolism and the development of enterococci was evaluated by determination of Generation Time (GT), Detection Time (DT) and the maximum conductance value (Delta mu S) of the curve during a period of 48 hours. Conductance trials showed that both species were able to adapt to adverse cultural conditions ( low values of pH and temperature) during cheese-making and ripening. E. faecalis appeared to be less sensitive on the whole. In the worst conditions ( pH 5.0, 25 degrees C), after an adaptation phase ( about 10 h), these microorganisms showed GT values almost double those at 37 degrees C. The same behavior can occur in cheese, where enterococci can reach numbers of up to 10(5) - 10(8) cfu center dot g(-1) during ripening. This could explain the fact that enterococci ( in particular, E. faecalis and E. faecium) represent the typical and important microflora in raw milk cheese.