Key Findings

The study identifies three macrolide resistance determinants, erm(42), msr(E), and mph(E), in field isolates of Mannheimia haemolytica and Pasteurella multocida. These genes confer resistance to macrolides, lincosamides, and streptogramin B, with erm(42) mediating resistance through 23S rRNA methylation, msr(E) through efflux, and mph(E) through phosphorylation.