Analysis of brevibacteria on the surface of Gruyère cheese detected by in situ hybridization and by colony hybridization

From the surface microflora of three different mature Gruyère Suisse cheeses, only a minor part of the microscopically visible resident micro-organisms was culturable. This may be due to the selectivity of the medium and growth conditions used, but also to dead or dormant cells. However, by use of fluorescent in situ hybridization (FISH) and the bacterial probe EUB338, a major part (70% of 4'6-diamidino-2-phenylindole-stained cells) of the microbial community was detectable. The analysis revealed that, with the aid of further specific nucleotide probes, most of the detectable micro-organisms were attributable to the Actinobacteria (76–90·3%), with comparable results obtained by colony hybridization (82–88%). The percentage of culturable brevibacteria detected by colony hybridization was higher (15–35%) than the percentage determined by in situ hybridization (10·6–14·3%), indicating that the numbers of brevibacteria are overestimated by the plating technique.