Risk ranking and antimicrobial resistance of Shiga toxin-producing Escherichia coli of bovine origin

: Shiga toxin-producing Escherichia coli (STEC) are zoonotic pathogens commonly carried by cattle and responsible for severe human diseases, as bloody diarrhoea (BD) and haemolytic uraemic syndrome (HUS). In an Italian slaughterhouse, 150 dairy cows were tested by swabbing the hides after stunning and the carcasses before refrigeration. STEC prevalence was 6.7% on hides and 4.0% on carcasses. No hide-to carcass contamination in the same animal was observed. Twenty isolates belonging to the serotypes O26:H11, O55:H9, O55:H12, O109:H10, O116:H28, O127:H12, O150:H2 (hides only), O157:H7 and O177:25 (hides and carcasses), O84:H2 and O177:H11 (carcasses only) were detected and analysed by Whole Genome Sequencing. Following published risk ranking schemes, STEC isolates were classified into seven groups according to their virulence gene profiles (stx1 and stx2 variants and the presence/absence of eae), in descending order from those most frequently associated with BD and HUS (groups I-V) to those considered least pathogenic (groups VI and VII). Overall, 75% of the isolates were assigned to groups I-V, highlighting the substantial pathogenic potential of the bovine strains. Regarding antimicrobial resistance, only one isolate (5%) was resistant to four or more antimicrobial classes, while 15% of the isolates carried ESBL-genes (blaCTX-M-1, or blaTEM-1 A, or blaTEM-1B) and displayed phenotypic resistance to ampicillin. Nevertheless, current therapeutic options of STEC infections suggest the use of azithromycin, a bacteriostatic agent that can avoid the progression of BD to HUS. Notably, all STEC isolates classified within groups I-V were susceptible to azithromycin, thus suggesting its potential usefulness in patient treatments.