GROWTH OF PROPIONIBACTERIA IN CHEESE - CO MPARISON BETWEEN 2 ENUMERATION MEDIA

Growth of 'wild' or inoculated propionibacteria has been studied durin g ripening of experimental, Swiss-type cheeses. Two media have been co mpared regarding their ability to be used for counting propionic acid bacteria in cheeses of various complex flora: LGA (lithium-glycerol-ag ar) et YELA (yeast-extract-lactate-agar). In order to characterize the strain variety among propionic flora, DNA restriction patterns have b een established for several colonies isolated throughout ripening usin g pulsed field gel electrophoresis. Propionic flora of cheeses made wi th thermized milk was composed of at least 10 different strains (10 di stinct patterns out of 16). Generation time of this flora during warm room ripening was equal to 18.8 h. For cheeses made with microfiltered thermized milk inoculated with Pure culture of P freudenreichii subsp shermanii TL 30, only the DNA restriction pattern of the inoculated s train could be found among isolated propionibacteria. They had a gener ation time during warm room ripening of 18.1 h. In cheeses made from t he same milk without further inoculation with propionibacteria starter , propionic flora enumerated on LGA was markedly lower compared to pro pionic flora of inoculated cheeses (3 to 6 decimal logarithms less). F or all the cheeses, a higher selectivity has been observed for LGA, on which total bacterial counts are lower than total bacterial counts on YELA (1 to 2 decimal logarithms less). However, despite this higher s electivity of LGA, the difficulty in differentiating and so enumeratin g propionic acid bacteria colonies was similar for both media.