Antimicrobial Susceptibilities of Yersinia enterocolitica Biotype 4, Serotype O:3

Agar dilution antimicrobial susceptibility tests were carried out against recent clinical isolates of Yersinia enterocolitica biotype 4, serotype O:3. Aminoglycosides and co-trimoxazole were the most active drugs. All isolates were resistant to ampicillin, carbenicillin, cloxacillin, and erythromycin.

Yersinia enterocolitica is being isolated with increasing frequency from children with gastroenteritis. At the present time there are few data pertaining to antimicrobial chemotherapy and the quantitative in vitro susceptibilities of this organism to antimicrobial agents (2,3,5). The present study was undertaken to expand upon the few existing reports ofother groups (2,3,5). The comparative in vitro activity of 16 single antimicrobials and 1 drug combination of these were studied against recent clinical isolates from ill children; the effects of different media and inocula on the agar dilution minimum inhibitory concentrations (MIC) for this organism were also examined briefly.
Twenty-three isolates of Y. enterocolitica were obtained from 22 ill children at the Montreal Children's Hospital from 1974 to 1976. These strains were all cultured from stool specimens with the exception of a swab from an appendix. Identification was carried out using the differential characteristics defined by Sonnenwirth (6). Biological and serological typing were performed by S. Toma (Public Health Laboratories, Toronto). All isolates were biotype 4, serotype 0:3, the predominant serotype associated with clinical infection in humans in Canada (8).
The agar dilution replicator technique of Steers et al.. (7) was used for MIC determinations. Mueller-Hinton agar (MH; Difco) adjusted to pH 7.4 was used for all drugs, and additional sets of sulfamethoxazole (SMZ)-trimethoprim (TMP) (alone and in combination) plates were prepared with Diagnostic Sensitivity Test (DST) agar (Oxoid DST broth with 12 g of agar [Difco] per liter) for comparative purposes (4). A stock solution of each drug was prepared and serial twofold dilutions in distilled water (0.5 ml) were added to 9.5 ml of 1 Address reprint requests to: Montreal Children's Hospital, Montreal, Quebec, H3H 1P3, Canada. agar for each dilution plate. The effects of two inocula, an undiluted overnight growth in MH broth (approximately 108 colony-forming units [CFUl/ml by colony count) and a 10-3 dilution of this suspension, were compared. After inoculation, plates were incubated for 16 to 18 h at 37°C and the results were recorded. The MIC of all drugs except SMZ alone was defined as the lowest drug concentration at which no growth was visible on the surface ofthe agar. The MIC of SMZ was defined as the lowest concentration at which there was an 80% or sudden reduction in growth.
The agar dilution test results are illustrated in Table 1 and Fig. 1. When one considers the higher achievable serum concentrations of kanamycin and amikacin, all strains had similar susceptibilities to the six aminoglycosides tested: gentamicin, kanamycin, netilmicin, sisomicin, amikacin, and tobramycin. Chloramphenicol was less active than the aminoglycosides and similar to tetracycline and rifampin.
The above results were similar with both inocula tested (108 and 105 CFU/ml); however, the lighter inoculum produced more clear-cut end points than the undiluted, overnight growth.
TMP and SMZ were examined alone and in combination on both MH and DST agar with undiluted and diluted inocula. The clearest re-NOTES 567 sults were obtained with the 105-CFU/ml inoculum and are illustrated in Fig. 1. The inoculum effect for SMZ MICs performed on both DST and MH agar was extreme. All isolates had an MIC of >400 ,ug of SMZ per ml when the inoculum was from an undiluted overnight growth but 3.12 to 6.25 ,ug/ml with the 105 inoculum. MH agar proved to be superior to DST agar for TMP susceptibilities as clearly defined end points were obtained only on this medium and only with the 105-CFU/ml inoculum. The inoculum effect was sufficient to alter the susceptibility pattern from susceptible to resistant as in the case of SMZ. When SMZ and TMP were combined in a 20:1 ratio, marked synergism resulted, reducing the MIC ofthe former at least fourfold and the latter at least eightfold. All isolates were susceptible to a ratio of 1.56 ,ug of SMZ per ml and 0.078 ,ug of TMP per ml. Disk susceptibility tests carried out on 8 of 17 drugs gave results corresponding to those agar dilution MICs where an inoculum of 105 CFU/ ml was used.
The results of this study demonstrate that aminoglycosides and co-trimoxazole are the most active antimicrobials in vitro against. Y. enterocolitica. Although the isolates tested in this study were slightly more resistant to TMP alone and more susceptible to SMZ than reported by Gutman et al., the SMZ-TMP combination results were similar (2). Our results with cloxacillin, tetracycline, kanamycin, gentamicin and ampicillin correlated well with those reported by Hausnerova et al. (3) who also studied Y. enterocolitica biotype 4 strains. In contrast to their report where 12% were susceptible to cephalothin (MIC, <10 ,ug/ml), none of our strains were susceptible to this drug; our strains were also more resistant to carbenicillin. This inconsistency could be attributed to the fact that their strains were isolated from 1967 to 1972 when these two f3-lactam drugs were relatively new. The isolates from our hospital appear to be extremely homogeneous in their antibiotic susceptibility pattern as well as biotype and serotype. No other biotype or serotype has been isolated in this hospital up to the present time. We recommend the use of MH agar and a 105-CFU/ml inoculum (a 10-3 dilution of an overnight growth in MH broth) for agar dilution MICs with the drugs tested; a Steers replicator delivers approximately 0.002 ml, thus the inoculum deposited on the agar surface is 2 x 102 CFU. A standardized disk diffusion test correlates well with the quantitative test. These in vitro results suggest co-trimoxazole or aminoglycoside antibiotics may have clinical usefulness in the therapy of Y. enterocolitica infections.