Difference Analysis on Virulence Genes, Biofilms and Antimicrobial Susceptibility of Escherichia coli from Clinical and Subclinical Bovine Mastitis

<i>Escherichia coli</i> (<i>E. coli</i>) has the ability to induce clinical and subclinical mastitis in dairy cows, causing a huge loss for the dairy industry. In this study, 51 subclinical mastitis isolates and 36 clinical mastitis isolates from eight provinces of China between 2019 and 2021 were used to investigate the differences in their biological characteristics. The results showed that B1 (52.9%) and A (39.1%) were the predominant phylogroups; R1 (50.6%) was the predominant lipopolysaccharide (LPS) core type; and 44 STs (ST10 and ST58 were the most sequence-prevalent STs) and 2 new STs (ST14828 and ST14829) were identified; however, no significant difference was observed between the clinical and subclinical group strains. To compare the virulence gene differences between the clinical and subclinical mastitis-related isolates, 18 common virulence genes (including <i>afaE</i>, <i>eaeA</i>, <i>papC</i>, <i>saa</i>, <i>sfa</i>, <i>ompA</i>, <i>aer</i>, <i>irp2</i>, <i>iucD</i>, <i>escV</i>, <i>sepD</i>, <i>east1</i>, <i>estB</i>, <i>stx2e</i>, <i>CNF1</i>, <i>cba</i>, <i>hlyA</i> and <i>traT</i>) were determined using the PCR method. The results showed that the detection rates of <i>traT</i>, <i>irp2</i> and <i>iucD</i> in clinical mastitis isolates were significantly higher than those in subclinical mastitis isolates (<i>p</i> ˂ 0.05). Meanwhile, subclinical-group <i>E. coli</i> had stronger biofilm formation abilities than the clinical group (<i>p</i> < 0.05) in 78 (89.7%) mastitis-related <i>E. coli</i> that could form biofilms. Furthermore, 87 mastitis-related <i>E. coli</i> showed severe resistance against tetracycline (37.9%), ampicillin (36.8%), streptomycin (34.5%) and cotrimoxazole (28.7%); their most prevalent resistance genes were <i>blaCTX-M</i> (33.3%), <i>tetA</i> (27.6%), <i>sul2</i> (18.4%) and <i>strB</i> (28.7%). It was noteworthy that the clinical-group strains had a higher resistance against ampicillin and possessed higher amounts of the resistance gene <i>blaCTX-M</i> (<i>p</i> < 0.05) compared to the subclinical group. This study aims to provide references for preventing the <i>E. coli</i> isolates from inducing different types of mastitis.