STAPHYLOCOCCI STRAINS ISOLATED FROM CATTLE MASTITIS CASES AND PEOPLE IN CONTACT, AND THEIR ANTIBIOTIC SUSCEPTIBILITY.

Antibiotic resistance of mastitis-causing organisms has been the topic of discussion in South Africa for some time. In addition, since the major organisms such as Staphylococcus species and strains (e.g. Staph. aureus) are zoonotic, the contact with humans is of concern as the possible resistance may become a major threat to human health. Of course, the reverse situation, that is the transfer from humans to cattle, is also of concern.

The present study isolated several coagulase-negative staphylococci (CNS) from mastitis cases and from humans that were in contact with the infected dairy cows. One would have thought that the authors would have targeted Staph. aureus specifically as it is highly pathogenic. However, the rationale is that CNS often exhibit greater resistance to antibiotics and also have a greater tendency to develop multidrug resistance. In addition, the CNS are believed to serve as reservoirs of antimicrobial genes, which can transfer and integrate into the genome of Staph.aureus.

The objectives of the study were (1) to examine the diversity of Staphylococcus spp. recovered from cows with intra-mammary infections and humans working in close contact with the animals; and (2) to evaluate the susceptibility of the staphylococcal isolates to different antimicrobials (antibiotics). A total of 3387 milk samples and 79 human nasal swabs were collected from 13 sampling sites in KwaZulu-Natal. In total, 146 Staph. aureus isolates and 102 CNS were recovered from clinical and sub-clinical milk samples.

Staphylococcus aureus was isolated from 15% of the human nasal swabs and 95 representative CNS were recovered for further characterization. Seven Staphylococcus species were identified among the CNS of dairy cow origin, with Staph. chromogenes (78%) predominating. The predominant CNS species recovered from the human nasal swabs was Staph. epidermidis (80%) followed by Staph. chromogenes (6.3%). Ninety-eight (67%) Staph. aureus isolates of dairy cow origin were susceptible; 39 (27%) isolates were resistant to a single class and 7 (4.8%) isolates were resistant to two classes of antimicrobials. Two Staph. aureus (1.4%) isolates were multidrug-resistant. Resistance to penicillin was common, with 29% of the cow and 75% of the human Staph. aureus isolates exhibiting resistance. A similar observation was made with the CNS, where 37% of the cow and 90% of the human isolates were resistant to penicillin. Multidrug-resistance was common among the human CNS, with 39% of the isolates exhibiting resistance to three or more classes of antimicrobials.

The antimicrobial susceptibility results suggest that resistance among staphylococci causing dairy cow intra-mammary infections in South Africa is uncommon and not a significant cause for concern. In contrast, antimicrobial resistance was frequently observed in staphylococcal isolates of human origin, highlighting a possible reservoir of resistance genes. The results suggest that continued testing of staphylococcal isolates is necessary to monitor changes in the susceptibility of isolates to different classes of antimicrobials.

 Reference:

T. Schmidt, M. M. Kock and M. M. Ehlers, 2015. Diversity and antimicrobial susceptibility profiling of staphylococci isolated from bovine mastitis cases and close human contacts. J. Dairy Sci. 98:6256–6269