Phenotypic characterizationand PCR-ribotyping of Pseudomonas fluorescensisolates, in tracking contamination routes in the production line of pasteurized milk

Contamination sites of Pseudomonas fluorescenswere traced in the production line for milk pasteurization in a large dairy plant in Shiraz, Iran. Samples of raw and pasteurized milk were collected at six sites along the line. All milk samples were incubated at 7°C until the aerobic plate count had reached 10 6 -10 7 cfu mL-1. Colonies were picked randomly and identified. No growth of gram negative psychrotrophic bacteria (GNP) was detected in the immediately pasteurized milk samples (just after the pasteurization), during long incubation at 7°C.Recontamination most often occurred in the filling step. In this study 34.1 and 4.9% of the milk packages showed contamination with GNP and P. fluorescens, respectively. Twenty threeP. fluorescensisolates were examined for phenotypic characteristics and 16S-23S PCR ribotyping. Phylogenetic analysis was conducted on the phenotypic and genotypic characteristics. TheP. fluorescensisolates were shown to belong to 6 biotypes (B1-B6). The predominance of a particular ribotype was often observed for a given biotype,although there were two ribotypes in eachof the B2 and B6 biotypes. The 16S-23S PCR-ribotyping technique allowed differentiation between the isolates. Based on this method, the isolates belonged to 5 subtypes. Phylogenetic analysis based on 16S-23S PCR-ribotyping and phenotypic characterization could be helpful in tracking contamination routes in the production line for milk pasteurization.