Investigation of Macrolide Resistance Genotypes of Pasteurella multocida Isolates from Cattle and Small Ruminants.

Macrolides are commonly used to control respiratory tract infections in ruminants, but the susceptibility of Pasteurella multocida strains has shown a decrease to macrolide antibiotics in the last decade. In this work we assessed the prevalence of macrolide resistance of 100 P. multocida isolates from ruminant hosts and studied the resistance genotypes with newly designed PCRs. Susceptibility to erythromycin and tilmicosin was tested using minimal inhibitory concentration (MIC) test strips. A newly designed PCR was used for the detection of macAB genes, and a PCR plus restriction enzyme-based technique was developed for detecting a 23S rRNA gene mutation at position 2059. Five bovine isolates with notably increased MICs (≥256 μg/mL for erythromycin and ≥32 μg/mL for tilmicosin) carried resistance genes msr(E) and mph(E) or the A2059G point mutation in the 23S rRNA gene. Over 73% strains from small ruminants and all bovine isolates were MacAB PCR positive. Bovine strains were less sensitive to macrolide antibiotics than isolates from small ruminants, and an increase in the prevalence of macrolide resistance in bovine P. multocida isolates has also been observed over time.