Isolation, antimicrobial resistance, and virulence genes of Pasteurella multocida strains from swine in China

Abstract

A total of 233 isolates of Pasteurella multocida were obtained from 2,912 cases of clinical respiratory disease in pigs in China, giving an isolation rate of 8.0%. Serogroup A P. multocida isolates were isolated from 92 cases (39.5%), and serogroup D isolates were isolated from 128 cases (54.9%); 12 isolates (5.2%) were untypeable. P. multocida was the fourth most frequent pathogenic bacterium recovered from the respiratory tract, after Streptococcus suis, Haemophilus parasuis, and Escherichia coli. All isolates were characterized for their susceptibilities to 20 antibiotics and the presence of 19 genes for virulence factors (VFs). The frequency of antimicrobial resistance among P. multocida isolates from swine in China was higher than that reported among P. multocida isolates from swine in from other countries, and 93.1% of the isolates showed multiple-drug resistance. There was a progressive increase in the rate of multiresistance to more than seven antibiotics, from 16.2% in 2003 to 62.8% in 2007. The resistance profiles suggested that cephalosporins, florfenicol, and fluoroquinolones were the drugs most likely to be active against P. multocida. Use of PCR showed that colonization factors (ptfA, fimA, and hsf-2), iron acquisition factors, sialidases (nanH), and outer membrane proteins occurred in most porcine strains. The VFs pfhA, tadD, toxA, and pmHAS were each present in <50% of strains. The various VFs exhibited distinctive associations with serogroups: concentrated in serogroup A, concentrated in serogroup D, or occurring jointly in serogroups A and D. These findings provide novel insights into the epidemiological characteristics of porcine P. multocida isolates and suggest that the potential threat of such multiresistant bacteria in food-producing animals should not be neglected.

FIG. 1.

Distribution by year of P. multocida isolates that showed phenotypes of multidrug resistance to the antimicrobial agents tested. All determinations of antimicrobial MICs with the P. multocida isolates were performed by broth microdilution methods recommended by the CLSI (29). The breakpoint used for ciprofloxacin was those previously used by Aarestrup et al. (1) for Danish strains of H. parasuis. For the other antimicrobials tested, breakpoint values were taken from the CLSI guidelines (29). Isolates that were nonsusceptible to at least three different antibiotic classes were classified as multidrug resistant. The percentage of isolates resistant to at least three antimicrobials was equally high between 2003 and 2007. The proportion of isolates resistant to more than five antimicrobials became more frequent, with a progressive increase from 47.8% in 2003 to 97.1% in 2007. The proportion of isolates resistant to more than seven antimicrobials increased approximately fourfold between the years 2003 and 2007, from 16.2% to 62.8% (P < 0.05).