| Summary: | Mammary pathogenic <i>E. coli</i> (MPEC) is one of the main pathogens of environmental origin responsible for causing clinical mastitis worldwide. Even though <i>E. coli</i> are strongly associated with transient or persistent mastitis and the economic impacts of this disease, the virulence factors involved in the pathogenesis of MPEC remain unknown. Our aim was to characterize 110 MPEC isolates obtained from the milk of cows with clinical mastitis, regarding the virulence factor-encoding genes present, adherence patterns on HeLa cells, and antimicrobial resistance profile. The MPEC isolates were classified mainly in phylogroups A (50.9%) and B1 (38.2%). None of the isolates harbored genes used for diarrheagenic <i>E. coli</i> classification, but 26 (23.6%) and 4 (3.6%) isolates produced the aggregative or diffuse adherence pattern, respectively. Among the 22 genes investigated, encoding virulence factors associated with extraintestinal pathogenic <i>E. coli</i> pathogenesis, <i>fimH</i> (93.6%) was the most frequent, followed by <i>traT</i> (77.3%) and <i>ompT</i> (68.2%). Pulsed-field gel electrophoresis analysis revealed six pulse-types with isolates obtained over time, thus indicating persistent intramammary infections. The genes encoding beta-lactamases detected were as follows: <i>bla<sub>TEM</sub></i> (35/31.8%); <i>bla<sub>CTX-M-2</sub></i>/<i>bla<sub>CTX-M-8</sub></i> (2/1.8%); <i>bla<sub>CTX-M-15</sub></i> and <i>bla<sub>CMY-2</sub></i> (1/0.9%); five isolates were classified as extended spectrum beta-lactamase (ESBL) producers. As far as we know, <i>papA</i>, <i>shf</i>, <i>ireA</i>, <i>sat</i> and <i>bla<sub>CTX-M-8</sub></i> were detected for the first time in MPEC. In summary, the genetic profile of the MPEC studied was highly heterogeneous, making it impossible to establish a common genetic profile useful for molecular MPEC classification. Moreover, the detection of ESBL-producing isolates is a serious public health concern.
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