Antimicrobial susceptibility testing for major staphylococci and E. coli Isolated from bovine mastitis cases and their zoonotic importance

Abstract

Bovine mastitis is of major economic importance and is treated by using various antimicrobials/antibiotics at the field level. However, antimicrobial resistance among the predominant mastitis causing bacterial pathogens is an issue in successful treatment. Also mastitis is increasingly becoming a public health concern due to the ability of the causative bacterial pathogens and/or their products, such as enterotoxins, to enter the food supply and cause food-borne diseases. The objective of this study was to assess the antimicrobial resistance (AMR) in Staphylococcus aureus, Staphylococcus epidermidis and Escherichia coli from bovine mastitis by employing microtiter plate based Minimal Inhibitory Concentration (MIC) of commonly used antimicrobials/antibiotics in Karnataka. In all, out of 211 bacterial isolates from bovine mastitis cases, S. aureus (n=85), S. epidermidis (n=23) and E. coli (n=47) isolates, which were confirmed by targeting species specific genes like nuc, rpoB and uspA, respectively were used. On in-vitro antimicrobial susceptibility testing of 31 S. aureus isolates for 6 different antimicrobials, 9.7 per cent (3/31) of isolates were found to be sensitive and 90.3 per cent (28/31) were resistant to penicillin, and 29 per cent (9/31) isolates were sensitive and 70.9 per cent (22/31) were resistant to cefotaxime. All of the 5, 9, 17 and 8 S. aureus isolates subjected to gentamicin, streptomycin, tetracycline and oxacillin MIC, respectively, were found resistant. Of the 10 S. epidermidis isolates used for susceptibility testing, 5 isolates each were subjected to penicillin, tetracycline and cefotaxime, 3 isolates each to gentamicin and streptomycin and 6 isolates for oxacillin susceptibility testing. All of them exhibited resistance to all the six antimicrobials. On MIC testing of 20 E. coli isolates, all of them showed resistance to penicillin, enrofloxacin and tetracycline; whereas 85 per cent (17/20) of isolates were sensitive and 15 per cent (3/20) were resistant to cefotaxime. Of 10 E. coli isolates subjected to streptomycin MIC, all of them showed resistance. Overall, the study revealed the high prevalence of AMR in S. aureus, S. epidermidis and E. coli to multiple antimicrobials used to treat bovine mastitis.