Co-selection of antimicrobial resistance in livestock pathogens and zoonotic agents

This dissertation investigated the effects of post milking teat disinfectants on bacterial susceptibility, prevalence of culturable teat bacteria, and cross-resistance to antibiotics. Chapter one investigated effects of a 6 days intervention with chlorhexidine (CH), lactic acid (LA), and untreated at 2 and 13 weeks after the intervention. For this, 476 bacterial isolates from 224 milk samples were identified by MALDI-ToF MS. The minimum inhibitory concentration (MIC) of CH and LA was determined for 404 isolates, and mastitis antibiotics were tested for Aerococcus (A.) viridans (n ​​= 36) and Staphylococcus (S.) haemolyticus (n = 85). The results showed, both disinfectants were effective against udder pathogens. However, selection of Corynebacterium (C.) spp. was sustained all time points compared to before treatment; simultaneously, a significant increase in LA tolerance was observed in S. haemolyticus and A. viridans. Antibiotic susceptibility, showed minimal changes. These results underscore the potential for adaption of udder pathogens to disinfectant and shifts in the teat microbiome, possibly due to subinhibitory exposure. Chapter two investigated in vitro effects of subinhibitory LA exposure. Where, S. haemolyticus (n=5), A. viridans (n=9), and C. bovis (n=6), were exposed to a subinhibitory concentration of LA for 30 days. Tolerance to LA was continuously assessed as MIC values ​​using macrodilution. Changes in antibiotic MIC ​​were measured before and after LA exposure using microdilution. Results showed a significant increase in tolerance to LA and β-lactam antibiotics in A. viridans, with a modest change in S. haemolyticus. No such change was observed in C. bovis. This suggests that A. viridans and S. haemolyticus have the potential to develop tolerance to LA and cross-resistance to β-lactam antibiotics.

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