The darker side of raw milk spoiling psychrotrophs

In general, the quality of raw milk and of dairy products has been considerably improved by the refrigeration on farms, and in processing plants. Unfortunately, the current practices for the collection and the storage of the raw milk favour the growth of psychrotrophic bacteria, able to grow below 7°C, regardless of their optimal growth temperature. Besides their rapid growth ability in refrigerated milk, psychrotrophs produce heat stable extracellular proteases, lipases, phospholipases. Pseudomonas spp. constitute the predominant microorganisms limiting the shelf life of processed fluid at 4°C. In our studies, 64 isolates, selected from 13 raw-milk samples in Finland (that originate from farm, lorry or silo tanks) were characterized by the determination of their spoilage and phenotypic features. Phenotypic characterization of the isolates was mainly performed with API 20NE and BIOLOG GN2 identification systems; the results were system-dependent, although the presence of representatives of the Pseudomonas genus (for the majority of the isolates) was suggested by both systems; moreover, a significant number of isolates were partially identified as members of Stenotrophomonas, Burkholderia and Acinetobacter genera. All three genera comprise species considered in recent reports as emerging pathogens, that carry innate resistance to antibiotics. There is a general consensus that the widespread use of antibiotics has imposed a strong selective pressure that contributed to the emergence of multidrug resistant microorganisms. In the context of the increasing prevalence of antibiotic resistant bacteria, we evaluated the distribution of antibiotic resistant traits among the group of psychrotrophs retrieved from the raw milk samples The results obtained, with the ATB®PSE system (composed of a panel of 17 β-lactams and non-β-lactams antibiotics, representatives of 5 classes), designed to investigate the pseudomonads susceptibility/resistance, revealed that the majority of raw milk psychrotrophs, besides exhibiting spoilage features, also harboured antibiotic multiresistant traits.