Antibiotic resistance of Erysipelothrix rhusiopathiae isolated from pigs with chronic swine erysipelas.

The susceptibility of 258 isolates of Erysipelothrix rhusiopathiae from slaughtered pigs affected with chronic erysipelas in Japan to antimicrobial agents was determined. A total of 111 (43.0%) strains showed resistance to erythromycin, oleandomycin, oxytetracycline, or dihydrostreptomycin. Plasmids were not detected. This is the first report of resistance of E. rhusiopathiae to these antibiotics.

Erysipelothrix rhusiopathiae (insidiosa) is a causative agent of swine erysipelas, which causes great economic loss in pig production. Human E. rhusiopathiae infection has also been reported (2,4,12,14). Antibiotics, especially penicillins, have been widely used for the treatment of this disease. However, in Japan, pigs are usually fed food containing various antibiotics, mainly tetracyclines and macrolides, for the purpose of growth stimulation. It seems, therefore, that long-term administration of antibiotics will give a selective advantage to antibiotic-resistant strains of E. rhusiopathiae. This paper deals with the antibiotic resistance in E. rhusiopathiae strains isolated from pigs in Japan.
A total of 258 strains of E. rhusiopathiae were submitted for assay of antibiotic susceptibility. They were isolated from cases of chronic swine erysipelas in slaughterhouses from October 1980 to December 1982. Infections associated with the isolates included arthritis (148 cases), lymphadenitis (65 cases), endocarditis (30 cases), and urticaria (15 cases). Antimicrobial agents studied were penicillin G (PC-G), ampicillin (APC), erythromycin (EM), oleandomycin (OM), oxytetracycline (OTC), chloramphenicol (CP), dihydrostreptomycin (DSM), kanamycin (KM), and sulfadimethoxine (SDM). MICs were determined by an agar dilution method (5). A 10-2 dilution of an overnight tryptic soy broth culture was inoculated by microplanter (Ebara Works, Tokyo, Japan) onto Mueller-Hinton agar containing serial twofold dilutions of the test antibiotic. The plates were incubated at 37°C for 48 h. The MIC was defined as the lowest concentration of antimicrobial agent that prevented macroscopic growth. Isolates were serotyped by using a modification of the agar gel diffusion method of Kucsera (11).
The relationship between antimicrobial resistance patterns, sources, and serotypes of isolates is shown in Table 2. A total of 111 (43.0%) strains were resistant to EM, OM, OTC, or DSM. Six different resistance patterns were found. Strains resistant only to OTC (25.6%) were most frequent, followed by those resistant to OTC and DSM (10.8%), EM, OM, OTC, and DSM (4.6%), EM, OTC, and DSM (1.2%), OM, OTC, and DSM (0.4%), and DSM (0.4%). In the E. rhusiopathiae strains isolated from cases of endocarditis, the frequency of resistance was significantly (P < 0.05, by Fisher's exact test) lower than in the isolates from other infections. Triple resistance and quadruple resistance were found only in isolates from cases of arthritis and lymphadenitis. Of the 111 resistant strains, 104 (93.7%) belonged to serotype 2. Isolation of plasmid DNA was attempted by the method of Womble et al. (15) and Kado and Liu (6), except for the use of sodium laurolyl sarcosinate (6%) in the procedure of bacteriolysis. However, we could not detect any plasmid in strains showing the various resistance patterns (data not shown).
The present results on susceptibility of E. rhusiopathiae strains to PC-G, APC, CP, KM, and SDM are in general agreement with those reported previously by others (1,3,(7)(8)(9)(10)13), indicating that penicillins remain the antibiotics of choice for the treatment of swine erysipelas. It should be noted, however, that 43% of E. rhusiopathiae strains examined showed resistance to EM, OM, OTC, or DSM. This is the first report on resistant strains of E. rhusiopathiae of porcine origin. Although the detailed mechanisms of antibiotic resistance were not established, plasmid DNA in resistant strains was not found. In any case, frequent use of tetracyclines and macrolides for pig production will undoubtedly give a selective advantage to antibiotic-resistant strains of E. rhusiopathiae.
Our results also showed that most of the resistant E. rhusiopathiae strains belonged to serotype 2. Further epidemiological studies are necessary to clarify this correlation.