Prevalence and antimicrobial-resistance phenotypes and genotypes of Escherichia coli isolated from raw milk samples from mastitis cases in four regions of China

Objectives: The objective was to find the differences in the prevalence and resistance of Escherischia coli isolated from raw milk samples from mastitis cases in four regions of China. Methods: A total of 750 bovine raw milk samples from mastitis cases were collected from four regions of China over two seasons. Antimicrobial resistance against 29 antimicrobial agents was determined, and 27 drug-resistant genes were tested. Results: Eighty-three strains (11.1%) of E. coli were isolated and identified. No significant differences in the number of E. coli isolates were observed between the two sampling seasons in the same regions (P > 0.05). However, a significant difference in E. coli prevalence was found among the four different regions (P < 0.01). The isolates were most frequently resistant to penicillin (100%), acetylspiramycin (100%), lincomycin (98.8%), oxacillin (98.8%) and sulphamethoxazole (53%). All the E. coli strains were multiresistant to at least three antimicrobial classes, and the most frequent multidrug-resistance patterns for the isolates were resistant to three (36.1%) or four (39.8%) classes of drugs simultaneously. The blaTEM gene (n = 69; 83.1%) was the most frequently detected resistance gene. The most frequent gene combinations were a four-gene pattern of blaCTX-M–sulII–blaTEM–sulI (n = 13; 15.7%) and a three-gene pattern of blaCTX-M–aph (3)-II–blaTEM (n = 11; 13.3%). Conclusions: This study indicated that there is a high incidence of E. coli with a great variation in resistance patterns and resistance genes; this is a matter of great concern for public and animal health in China.