Phenotypic and genotypic characterization of Pseudomonas fluorescens isolated from bulk tank milk

Pseudomonas fluorescens isolates (n = 55) isolated from farm bulk tank milk (n = 55) from dairy herds in eastern South Dakota and western Minnesota were examined for phenotypic (biotype, proteolytic, and lipolytic profiles) and genotypic (plasmid profiles and 16S-23S PCR ribotypes) characteristics. The observed phenotypic and genotypic characteristics were used to conduct phylogenetic analysis. Pseudomonas fluorescens belonged to 28 API 20 NE biotypes and 14 proteolytic and lipolytic profiles. It was observed that 80, 91, and 58% of the isolates were proteolytic at 7, 22, and 32 degrees C, respectively. Only 7, 44, and 7% of the isolates were lipolytic at the same three temperatures. Pseudomonas fluorescens was more likely to produce proteinases at 7 and 22 degrees C and lipases at 22 degrees C. Only 9 of 55 isolates of P. fluorescens harbored plasmids. This small percentage of plasmid-bearing isolates provided insufficient data for inferences related to the distribution of plasmid-bearing clonal types. Based on 16S-23S PCR ribotyping, P. fluorescens belonged to 14 subtypes. The 16S-23S PCR ribotyping technique allowed differentiation between strains; however, it did not concur with the biotypes and proteolytic and lipolytic profiles. Use of biotypes in conjunction with proteolytic and lipolytic profiles might have practical value for conducting trace-back studies related to P. fluorescens. Based on phylogenetic analysis, it was inferred that for the given geographical area and time period, P. fluorescens isolated from farm bulk tank milk consists of a large heterogeneous group of organisms.