| Summary: | The aim of the study was phenotypic and genotypic characterization of <i>Erysipelothrix rhusiopathiae</i> strains isolated from diseased pigs in Poland and comparison of the SpaA (Surface protective antigen A) sequence of wild-type strains with the sequence of the R32E11 vaccine strain. The antibiotic susceptibility of the isolates was assessed using the broth microdilution method. Resistance genes, virulence genes, and serotype determinants were detected using PCR. The <i>gyrA</i> and <i>spaA</i> amplicons were sequenced to determine nonsynonymous mutations. The <i>E. rhusiopathiae</i> isolates (n = 14) represented serotypes 1b (42.8%), 2 (21.4%), 5 (14.3%), 6 (7.1%), 8 (7.1%), and N (7.1%). All strains were susceptible to β-lactams, macrolides and florfenicol. One isolate showed resistance to lincosamides and tiamulin, and most strains were resistant to tetracycline and enrofloxacin. High MIC values of gentamicin, kanamycin, neomycin, trimethoprim, trimethoprim/sulfadiazine, and rifampicin were recorded for all isolates. Phenotypic resistance was correlated with the presence of the <i>tetM</i>, <i>int-Tn</i>, <i>lasE</i>, and <i>lnuB</i> genes. Resistance to enrofloxacin was due to a mutation in the <i>gyrA</i> gene. All strains contained the <i>spaA</i> gene and several other genes putatively involved in pathogenesis (<i>nanH.1</i>, <i>nanH.2</i>, <i>intl</i>, <i>sub</i>, <i>hlyA</i>, <i>fbpA</i>, ERH_1356, <i>cpsA</i>, <i>algI</i>, <i>rspA</i> and <i>rspB</i>) Seven variants of the SpaA protein were found in the tested strains, and a relationship between the structure of SpaA and the serotype was noted. <i>E. rhusiopathiae</i> strains occurring in pigs in Poland are diverse in terms of serotype and SpaA variant and differ antigenically from the R32E11 vaccine strain. Beta-lactam antibiotics, macrolides, or phenicols should be the first choice for treatment of swine erysipelas in Poland. However, due to the small number of tested strains, this conclusion should be approached with caution.
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