Phenotypic and genotypic characterization of sorbitol-negative or slow-fermenting (suspected O157) Escherichia coli isolated from milk samples in Lombardy region

Aims: To investigate phenotypic and genotypic aspects of sorbitol-negative or slow-fermenting Escherichia coli, suspected to belong to O157 serogroup, isolated in Italy. Methods and Results: Milk samples originating from goats and cows were screened for the presence of E. coli O157 with cultural methods. Sorbitol-negative or slow-fermenting strains were subjected to phenotypic characterization, antibiotic resistance profiles, PCR reactions for detection of toxins (stx1 and stx2) and intimin (eaeGEN and eaeO157) genes and clustering by pulsed field gel electrophoresis (PFGE). Only one strain revealed to be O157. Susceptibility to 11 antibiotics highlighted the high resistance to tetracycline (50%), sulfonamide and streptomycin (33%). The stX2 gene was detected in two strains; only the strain identified as O157 exhibited an amplicon for both eae genes. PFGE identified seven distinct XbaI macrorestriction patterns at a similarity level of 41%. Conclusions: The use of sorbitol fermentation as cultural method is not sufficient for STEC discrimination while PCR assay proved to be a valuable method. Significance and Impact of the Study: The study reports presence of Shiga toxin-producing E. coli in raw milk, signalling a potential risk for humans.