Introduction
Bovine mastitis is one of the most prevalent, expensive, and devastating infectious diseases in the dairy industry, which poses a potential risk to public health if inadequately treated milk is consumed. Escherichia coli is a major environmental pathogen causing bovine mastitis (Timofte et al. 2014). Different pathogenic possibilities exist among bovine mastitis E. coli strains. However, bovine mastitis caused by E. coli is often acute to the per-acute form of the infection. The clinical manifestation of which can be from mild to fatal forms, resulting in a high mortality incidence (Grohn et al. 2005).
In dairy animals, mastitis is one of the most important reasons for the frequent and persistent use of antimicrobials. Unfortunately, E. coli strains resistant to almost all antimicrobial agents in curing bovine mastitis have been increasingly reported (Eisenberger et al. 2018). Accordingly, extended-spectrum β-lactamase (ESBL)-, carbapenemase (CRE) producing E. coli is characterized by their non-susceptibility to almost all beta-lactam and non-beta-lactam antimicrobial agents. Meanwhile, colistin (polymyxin E) was considered the last therapeutic option against superbugs like CRE producing E. coli. However, Liu et al. and several other studies recently reported the emergence of plasmid-mediated colistin-resistant genes mcr-1 and its variants (Carroll et al. 2019). They characterized the colistin-resistant, ESBL- and carbapenem-encoding conjugative plasmids from pigs, cattle, poultry, and human beings (Liu et al. 2015). The associated resistance is due to the fact that the strains of the bacteria become a carrier of the factors of resistance to diverse classes of antimicrobials.
The prominent ESBL families include blaCTX-M, blaSHV, blaTEM, blaOXA, and other variants. The blaCTX-M group β-lactamases has been further divided into five subgroups (blaCTX-M-1, blaCTX-M-2, blaCTX-M-8, blaCTX-M-9, and blaCTX-M-25), and more than 120 blaCTX-M enzymes have been reported (Afema et al. 2018). The spread of blaCTX-M lineages has been credited to several factors, such as competent mobilization and distribution of blaCTX-M genes by mobile genetic elements MGEs (plasmids, transposon, and insertion elements) (D'Andrea et al. 2013). In contrast, blaCTX-M-1 and blaCTX-M-14-producing E. coli are broadly disseminated and isolated from bovine mastitis cases in Europe (Timofte et al. 2014), whereas blaCTX-M-15 was the most widespread ESBL type found in E. coli mastitis in Asia (Ohnishi et al. 2013; Ali et al. 2017).
E. coli can be classified as pathogenic or nonpathogenic; within both of these groups, there are eight recognized phylogroups: with seven belonging to E. coli sensu stricto (A, B1, B2, C, D, E, and F) , whereas the eight is corresponding to Escherichia cryptic clade I (Clermont et al. 2013). Previous studies indicated that E. coli isolated from bovine mastitis belonged to phylogroup A (Suojala et al. 2011; Ali et al. 2017; Zhang et al. 2018). In addition, various virulence genes that are important for the pathogenicity in E. coli from mastitic cows have been reported; those include genes encoding aerobactin, auto agglutinating adhesion proteins, enterohemolysin, intimin, hemagglutinin, and P-fimbria (Zhang et al. 2018).
In this study, the key objective was to determine the prevalence of ESBL genes and mcr-1 in mastitis E. coli strains. To investigate antimicrobial susceptibility, phylogenetic analysis, and the frequency distribution of various virulent factors of the E. coli isolates isolated from dairy cows with mastitis from Zhejiang, Jiangsu, and Shanghai provinces of China.