Escherichia coli is one of the most important causative agents of mastitis in bovine worldwide. The aim of this study was to investigate the genetic diversity and antimicrobial resistances profiles of an Escherichia coli strain collection (n=93) isolated from routine mastitis diagnostics in 2017 in Switzerland. E. coli strains were assigned to phylogenetic groups and to sequence types (ST) by multilocus sequence typing (MLST). Furthermore, the strains were screened for a range of virulence factors such as different adhesins and toxins. Phenotypic resistance testing was performed for 11 antimicrobial agents using a commercial test panel.
84 of the strains could be assigned to 36 different sequence types with ST1125 being the most frequent (10.2%). Nine strains could not be assigned to any known ST. Three strains were tested positive for stx1a and one for subAB1. None were tested positive for stx2, eae, heat-labile enterotoxin (LT) and heat-stable enterotoxin (ST).
Aggregate virulence factor (VF) score showed a broad diversity from 0 up to 8 VF tested positive. Four strains were classified as UPEC (3.7%), 11 strains as exPEC (10.2%) and one of them as both. Antimicrobial resistance testing showed that over 90% of the strains exhibit minimal inhibitory concentrations (MIC) of ≥8 mg/L ≥4 mg/L, ≥4 mg/L and ≥4 mg/L for penicillin, oxacillin, pirlimycin and erythromycin, respectively. For all other antimicrobials, most strains showed an MIC at or below the lowest concentration tested. This study indicates that E. coli causing mastitis belong to a high genetic heterogeneity and commensal as well as pathogenic E. coli can be found. For resistance testing there is still a lack of veterinary breakpoints for several antibiotics used in mastitis therapy.