Characterization of Antibiotic-Tolerant Persister Cells in Mannheimia haemolytica

Antibiotics are the primary treatment for bacterial respiratory diseases in ruminants. Mannheimia haemolytica is the major etiological agent causing respiratory diseases in ruminants, but its tolerance to antibiotics remains poorly understood. In this study, strains of M. haemolytica belonging to the main serotypes A1, A2, and A6 were selected for analysis of their ability to form multidrug-tolerant persister cells. The results showed that M. haemolytica was able to form approximately 10⁴ CFU/mL of antibiotic-tolerant persisters during both the logarithmic phase and the stationary phase. Compared to the logarithmic phase, the ability of stationary phase cells to form persisters was significantly enhanced. Further analysis revealed that different stress conditions had varying effects on the ability of the three serotype strains to form persister cells. Under low dissolved oxygen conditions, the ability of the three serotype strains to form multidrug-tolerant persisters was significantly weakened. Oxidative stress conditions exerted the greatest effect on persister formation in the serotype A2 strain QS2-1, whereas acid stress had the greatest impact on persister formation in the serotype A6 strain Mh1128. Finally, strategies for the eradication of M. haemolytica persister cells were examined. The use of antibiotic combinations with distinct mechanisms of action can effectively and rapidly reduce the formation of persister populations in certain strains under our experimental conditions. Furthermore, L-Serine and mannitol were shown to enhance the susceptibility of most strains to florfenicol. Overall, this study examined the capacity of M. haemolytica to form persisters and explored the factors influencing the formation of these persisters, as well as potential strategies for their eradication. The findings offer a theoretical foundation for the treatment of M. haemolytica respiratory infections in ruminant species.