Serotypes, Antimicrobial Susceptibility, and Potential Mechanisms of Resistance Gene Transfer in Erysipelothrix rhusiopathiae Strains from Waterfowl in Poland

Erysipelas is a significant problem in the waterfowl farming in Poland, and information on the characteristics of the <i>Erysipelothrix rhusiopathiae</i> strains causing this disease is limited. In this study, we determined the serotypes, antimicrobial susceptibility, and potential mechanisms of resistance gene transfer in <i>E. rhusiopathiae</i> isolates (n = 60) from domestic geese and ducks. We also developed a multiplex PCR for the detection of resistance genes. The antimicrobial susceptibility of the isolates was assessed using the broth microdilution method. Resistance genes, integrative conjugative element (ICE)-specific genes, phage-specific genes, and serotype determinants were detected by PCR. Multilocus sequence typing (MLST) was performed for selected resistant strains. The comparative analyses included 260 <i>E. rhusiopathiae</i> strains whose whole genome sequences (WGSs) are publicly available. <i>E. rhusiopathiae</i> isolates represented 7 serotypes, among which serotypes 5 (38.3%) and 1b (28.3%) were the most common. All strains were susceptible to β-lactams, and the vast majority of them were resistant to tetracycline (85%) and enrofloxacin (80%). The percentages of isolates resistant to other antimicrobials used ranged from 3.3% to 16.7%. Ten isolates (16.7%) were found to be multidrug resistant (MDR). The genotypic resistance profiles of the <i>E. rhusiopathiae</i> strains corresponded to their phenotypic resistance, and the amplification patterns obtained using the 10-plex PCR developed in this study were fully consistent with the results of single PCRs. The most prevalent resistance gene was <i>tetM</i>. In enrofloxacin-resistant strains, nonsynonymous mutations in the <i>gyrA</i> and <i>parC</i> genes were identified. The presence of ICE-specific genes was confirmed in resistant strains, and in MDR isolates of serotype 8 that represented sequence type (ST) 113, prophage DNA (Javan630-like) linked to the <i>lsaE</i> gene was additionally detected. The results indicate that β-lactam antibiotics should be the first choice for the treatment of waterfowl erysipelas in Poland. ICEs, including a transposon from the Tn916/Tn1545 family, and bacteriophages are most likely responsible for the transfer of resistance genes in <i>E. rhusiopathiae</i>.