Specific detection of Pseudomonas spp. in milk by fluorescence in situ hybridization using ribosomal RNA directed probes

Aims: Pseudomonas spp. are considered the most important milk spoilage organisms. Here we describe development of a fluorescence in situ hybridization (FISH) probe specific for detection and enumeration of Pseudomonas spp. in milk. Methods and Results: 16S rRNA sequences were analysed to develop specific oligonucleotide probe for the genus Pseudomonas . Twenty different Pseudomonas spp. and 23 bacterial species from genera other than Pseudomonas (as negative controls) were tested. All tested Pseudomonas spp. yielded a positive FISH reaction, whereas negative controls showed no FISH reaction except for Burkholderia cepacia that showed a relatively weak FISH reaction. The FISH assay specifically stains Pseudomonas in milk when the milk contains a mixture of other bacterial species. The FISH assay takes 2 h and compares favourably with current culturing methods, which take a minimum of 48 h. Specificity of the probe was validated using polymerase chain reaction to selectively amplifying the Pseudomonas rDNA gene and sequencing the gene products. Conclusions: The method presented in this study allows simultaneously detection, identification and enumeration of Pseudomonas spp. in milk. Significance and Impact of the Study: Rapid and accurate enumeration of Pseudomonas facilitates the identification of specific contamination sources in dairy plants, the accurate validation of pasteurization treatments and the prediction of shelf life of processed milk.