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

Document Type : Full paper (Original article)

Authors

1 Ph.D. Student in Food Hygiene, Department of Food Hygiene and Public Health, School of Veterinary Medicine, Shiraz University, Shiraz, Iran

2 Department of Pathobiology, School of Veterinary Medicine, Shiraz University, Shiraz, Iran

3 Department of Food Hygiene and Public Health, School of Veterinary Medicine, Shiraz University, Shiraz, Iran

Abstract

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 three
P. fluorescensisolates were examined for phenotypic characteristics and 16S-23S PCR ribotyping.
Phylogenetic analysis was conducted on the phenotypic and genotypic characteristics. TheP. fluorescens
isolates 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.

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