Agar dilution antimicrobial susceptibility tests were carried out against recent clinical isolates ofYersinia enterocolitica biotype 4, serotype 0:3. 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.Disk diffusion susceptibility tests (1) were carried out with the following antibiotics (antibiotic concentration of disk in parentheses): ampicillin (10 ,ug), kanamycin (30 ,ug), gentamicin (10 ,g), cephalothin (30 ,ug), SMZ/TMP (23.75/1.25 ,ug), sisomicin (10 ,tg), amikacin (10 ,ug), and netilmicin (10 ,ug).The aga...
