Mechanism Of Aminoglycoside Resistance In Pasteurella Multocida Isolates Of Capsular Serotype A From Bovine

Pasteurella multocida is one of the most prevalent pathogen in Bovine respiratory disease(BRD) among cattle. It is an opportunistic pathogen and inhabit in the upper respiratory tract in general. Transportation over long distance, weather conditions, and many other stress reactions may play an important role in the disease development. Despite the increasing resistance, the antimicrobial agents continue used to control BRD due to P. Multocida in China. Nevertheless, the increasing antimicrobial-resistant bacteria has become a major threat on public and animal health.In this study, we investigated the MICs with six aminoglycosides against all of the 23 P. Multocida strains, all of the P. Multocida isolates were confirmed resistance to at least one of the aminoglycosides. And the resistance rates for SM, NEO, AMK, and KAN were 52.2%, 87.0%, 100%, 69.6%. The isolates were all susceptible to SPT and GEN. The gentamicin-induced strains(Pm-Q2, Pm-Q3, Pm-Q4) showed highly resistance to all the aminoglycosides except SPT and Spectinomycin-induced strains(Pm-D1, Pm-D3, Pm-D5) had no effect on other aminoglycosides.On this basis, we detected the widespread aminoglycoside resistance genes and analysed the expression levels. Three aminoglycoside resistant genes strB(aminoglycoside 6’-phosphotransferase), aphA1(aminoglycoside 3’-phosphotransferase III), aacA4(aminoglycoside adenylyltransferase) were detected in all 23 isolates of P. Multocida and the induced resistant strains. Then all the three resistant genes relative expression were detected by qPCR. The Quantitative RT-PCR experiment revealed that aminoglycoside resistant genes in the gentamicin-induced strains with 256 μg/mL GEN had higher expression levels than the gentamicin intermediate strain Pm-1 with 4μg/mL GEN. It demonstrate that the high expression of resistant genes had result in the antimicrobial resistance.We had investigated the mutations of 16 S rRNA and rpsE gene which coding for ribosomal protein S5 of the induced resistant strains. The rusult revealed that no mutations in the 16 S rRNA operons were detectable. However, a 9-bp deletion in the rpsE which result in the losses of amino acid Met, Ser, Phe at the position of 31 to 33 were present. The gentamicin-induced resistant strains had no mutations in the 16 S rRNA operon or rpsE gene. It demonstrate that the mutations in rpsE gene which coding for the ribosomal protein S5 could result in the high level resistance to spectinomycin.