The in vitro activity of the aminoglycoside antibiotics tobramycin, sisomicin, amikacin, gentamicin, and netilmicin (SCH 20569) were compared against 26 gentamicin-resistant isolates of Pseudomonas aeruginosa cultured from hospitalized children. Tobramycin had the greatest activity on a weight basis, followed by sisomicin, gentamicin, amikacin, and netilmicin. All isolates were resistant to achievable concentrations of netilmicin and gentamicin, but 23% were inhibited by achievable concentrations of tobramycin, 8% by amikacin, and 4% by sisomicin. The combinations carbenicillin/tobramycin, carbenicillin/sisomicin, and carbenicillin/amikacin were synergistic for 92% of strains; antagonism was not encountered. These in vitro results suggest that tobramycin, sisomicin, or amikacin in combination with carbenicillin would be the safest initial regimen in the therapy of gentamicin-resistant Pseudomonas infections pending susceptibility studies.Pseudomonas aeruginosa resistant to currently available antibiotics, including gentamicin, are becoming more prevalent. A study was undertaken to investigate the activity of several new aminoglycosides against a population of gentamicin-resistant P. aeruginosa to determine the extent of cross-resistance between tobramycin, sisomicin, amikacin, netilmicin (SCH 20569), and gentamicin. The possibility of carbenicillin/aminoglycoside synergism against this resistant population of P. aeruginosa was also examined. MATERIALS AND METHODSTwenty-six strains of P. aeruginosa were isolated from 23 patients at the Montreal Children's Hospital and identified by using standard microbiological techniques. At least 14 different types ofPseudomonas were represented among the isolates studied, as determined by serological and phage-typing methods (kindly performed by N. Hinton, Toronto). All strains were screened for resistance to gentamicin by a standardized disk diffusion test (16) and were considered resistant if the zone diameter was <13 mm and the agar dilution minimum inhibitory concentration (MIC) was -6.25 lAg/ml. Bacteria were stored in sheep blood at -20°C until tested.Tobramycin sulfate was supplied as a solution of 1,000 ,ig/ml and was stored at 4°C. Gentamicin, sisomicin, amikacin, netilmicin, and carbenicillin were supplied as dry powders and were reconstituted immediately before use. Agar dilution MICs were performed according to the method of Steers et al. (19) using Mueller-Hinton agar (Ca2+, 7 mg/100 ml, Mg2+, 3.6 mg/100 ml) and an inoculum of 106 to 107 bacteria/ml. The MIC was defined as the lowest concentration of drug allowing no colonial growth on the surface of the agar.Plates for single antibiotic agar dilution studies were prepared 48 h in advance and stored at 4°C. Plates for combination studies were prepared 2 h before use. Synergism was tested by adding either 25 or 50 ,ug of carbenicillin per ml to each dilution of four of the aminoglycosides listed above. The combination was considered synergistic if there was a decrease of four double dilutions or greater in the ami...

Sulfamethoxazole-trimethoprim and three oral cephalosporins, cefaclor, cephalexin, and cephradine, were evaluated in vitro as possible alternatives to chloramphenicol in the treatment of non-central nervous system infections due to ampicillin-resistant Haemophilus influenzae. Sixty-four isolates of H. influenzae, including 31 ,f-lactamase-positive strains, were tested by the agar dilution method.All strains were inhibited by 0.78/0.039 jig sulfamethoxazole-trimethoprim per ml and by 0.78 jig of chloramphenicol per ml. At 6.25 jig/ml, 100, 11, and 3% of all strains were inhibited by cefaclor, cephalexin, and cephradine, respectively. Thus, on the basis of drug concentrations presumably achievable in serum, 100% of strains were susceptible to sulfamethoxazole-trimethoprim, chloramphenicol, and cefaclor. However, a considerable inoculum effect was noted with both ,B-lactamase-positive and -negative strains, when tested with sulfamethoxazole-trimethoprim; the miniInal inhibitory concentrations of cefaclor were only slightly affected. Also, synergistic effects of sulfamethoxazole-trimethoprim, sulfamethoxazole-erythromycin, and sulfamethoxazole-cefaclor were seen when combinations were tested against both ,8-lactamase-positive and -negative strains, as determined by minimal inhibitory concentrations measured by the broth dilution method and by killing curve analyses. These results support further evaluation of these combinations and of cefaclor alone for the treatment of non-central nervous system infections due to H. influenzae. were adjusted to contain 2 x 105 CFU/ml, and 0.05 ml was added to an equal volume of drug suspension. The plates were incubated at 35°C for 18 h. The lowest concentration of drug with no visible turbidity was used to define the minimal inhibitory concentration (MIC) of ampicillin, chloramphenicol, erythromycin, TMP, and cefaclor, whereas a sudden reduction in growth was used for SMZ MIC determinations. Effect of inoculum concentration. The effect of inoculum size on MIC was tested (i) by the agar dilution technique, with all strains using inocula of 10' and 107 CFU/ml, or (ii) by the microtiter broth dilution technique with one strain each of ,B-lactamasepositive and -negative strains, using final inocula of 103, 104, 105, and 106 CFU/ml. The effect of inoculum size on the synergism of antimicrobials used in combination was also tested by the microtiter broth dilution technique using the four different inocula described above.Tests for synergism. Synergism was measured by two techniques against one strain each of,B-lactamasepositive and -negative strains: (i) by checkerboard microtiter broth dilution and (ii) hibited by 0.78/0.039 ig of SMZ-TMP per ml and by 0.78 ,ug of chloramphenicol per ml. At 6.25 ug/ml, 100, 11, and 3% of all strains were inhibited by cefaclor, cephalexin, and cephradine, respectively. With ampicillin, 100% of the /3-lactamase-negative strains were inhibited by a concentration of 0.78 ug/ml; in contrast, f8-lactamase-positive strains exhibited a wide range of susceptib...

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...