Abstract: In order to study the drug susceptibility to sreptomycin（Str） and its drug resisitance mechanism of avian Pasteurella multocida（Pm）,a Str-resistant avian Pm was obtained via in vitro induction with gradually increasing drug concentrations to the standard strain named C48-1. The minimal inhibitory concentration（MIC）,biochemical characteristics, growth curve,stability of drug resistance,drug resistance gene site mutation and drug resistance reversal of the Str-resistant strain was analyzed. The results showed that compared with the parent strain, the biochemical characteristics and growth curve had no significant difference,the MIC of the Str-resistant strain was increased from 8 μg/mL to 1 024 μg/mL. What more,the drug resistance gene StrA and StrB were found and the amino acid sequence encoded by the rpsL gene had an amino acid exchange of Lys43→Thr. In the drug resistance reversal test, carbonyl cyanide-m-chlorophenylhydrazone （CCCP） reduced the MIC of the Str-resistant strain to 16 μg/mL. In total,it shows that gradually increasing drug concentration can induce Pm resistance to aminoglycosides in vitro. Its mechanism is the modification of aminoglycoside antibiotics by phospho-transferase,the mutation of ribosomal protein and the over expression of efflux pumps.

Key words: avian Pasteurella multocida, in vitro induction, sreptomycin, drug resistance gene, site mutation, resistance mechanism