Susceptibility of Bacteroides from swine abscesses to 13 antibiotics

The minimal inhibitory concentrations (MIC) of 129 strains of genus Bacteroides, isolated from abscesses in pigs, for 13 different antimicrobial agents were determined by an agar-dilution method under anaerobic conditions. Clindamycin, the most active antibiotic tested, had a median MIC of 0.8 micrograms/ml. Penicillin G, talampicillin, minocycline, chloramphenicol, and cefoxitin also inhibited all isolates at lower concentrations. Some isolates of B pyogenes were susceptible to gentamicin (MIC, 0.4 micrograms/ml), erythromycin (MIC, 0.8 micrograms/ml), cephalexin (MIC, 0.8 micrograms/ml), and cefoxitin (MIC, 3.2 micrograms/ml). Erythromycin, at a concentration of 3.2 micrograms/ml, was active against 70% of B suis tested, but it was less active against the other Bacteroides species. Some strains tested were resistant to streptomycin and neomycin.     

Antibiogram of staphylococcal strains isolated from milk and milk-products

A study of the resistance patterns of 248 staphylococcal isolates from milk and milk products to eight antimicrobial agents using the disc method showed that 80 (32.3%) were resistant to sulphafurazole, 75 (30.2%) to penicillin G, 63 (25.4%) to ampicillin, 23 (9.3%) to cloxacillin, 19 (7.7%) to tetracycline, 17 (6.9%) to streptomycin, 14 (5.6%) to erythromycin and 12 (4.8%) to chloramphenicol. One hundred and sixteen (46.8%) of the 248 staphylococcal isolates were sensitive to all the agents tested. A significant percentage (P less than 0.05) of the isolates from raw milk were resistant to erythromycin, sulphafurazole, cloxacillin, penicillin G and streptomycin compared to isolates from fermented milk. The minimum inhibitory concentrations (MIC) as determined by the tube method for isolates resistant by the disc method, were greater than or equal to 16 micrograms for ampicillin in 5 (11.9%) isolates; greater than or equal to 40 micrograms for cloxacillin, 9 (36.0%) and greater than or equal to 12 international units for penicillin G amongst 12 (22.2%) isolates. MIC values of greater than or equal to 40 micrograms were recorded for 9 (90.0%), 9 (69.2%), 8 (73.7%) and 7 (70.0%) isolates to chloramphenicol, streptomycin, erythromycin and tetracycline respectively. The relatively high level of resistance to antimicrobial agents is a reflection of misuse or abuse of these agents in the environment.     

Antimicrobial susceptibility of udder pathogens isolated from dairy herds in the west littoral region of Uruguay

A total of 522 strains belonging to streptococci, enterococci and staphylococci isolated from sub-clinical and clinical cases of bovine mastitis from the west littoral region of Uruguay were analysed for their susceptibility to several antimicrobial agents. The susceptibility patterns were studied by agar disk diffusion methods (ADDM) and broth micro-dilution to determine the minimum inhibitory concentration (MIC). The concentration that inhibits 90% (MIC90) of the analysed strains reported in micrograms per millilitre, for Staphylococcus aureus were > 8, 8, < or = 0.5, < or = 4, < or = 1, < or = 0.5, > 64, < or = 0.25, 0.5, < or = 1 and < or = 1 to penicillin, ampicillin, oxacillin, cephalotin, gentamicin, erythromycin, oxitetracycline, enrofloxacin, trimethoprim/sulfamethoxazole, neomycin, and clindamycin, respectively. Coagulase-negative staphylococci (CNS) had different values for penicillin (4) and ampicillin (2), while the other antimicrobial agents had the same MIC90 values as reported for S. aureus. The MIC90 values for streptococci were 0.12, 0.25, < or = 4, 16, < or = 0.25, 0.5, 0.25 for penicillin, ampicillin, cephalotin, gentamicin, erythromycin, oxytetracycline and trimethoprim-sulfamethoxazole, whereas MIC90 for enterococci were 4, 4, 4, < or = 0.5, 2, > 8 for penicillin, ampicillin, gentamicin, erythromycin, oxytetracycline and trimethoprim-sulfamethoxazole, respectively. Of 336 strains of S. aureus, 160 (47.6%) were resistant to penicillin. For 41 CNS strains, 10 (27%) presented penicillin-resistance. All the streptococcal strains were susceptible to penicillin, while 3 (7%) of the 43 enteroccocal strains were resistant. Non significant statistical differences were found between the results obtained by ADDM and broth micro-dilution for classifying bacterial isolates as susceptible or resistant according to the National Committee of Clinical Laboratory Standards.     

In vitro sensitivity of Hungarian Actinobaculum suis strains to selected antimicrobials

In vitro antimicrobial sensitivity of 12 Hungarian isolates and the type strain ATCC 33144 of Actinobaculum suis to different antimicrobial compounds was determined both by the agar dilution and by the disc diffusion method. By agar dilution, MIC50 values in the range of 0.05-3.125 micrograms/ml were determined for penicillin, ampicillin, ceftiofur, doxycycline, tylosin, pleuromutilins, chloramphenicol, florfenicol, enrofloxacin and lincomycin. The MIC50 value of oxytetracycline and spectinomycin was 6.25 and 12.5 micrograms/ml, respectively. For ofloxacin, flumequine, neomycin, streptomycin, gentamicin, nalidixic acid, nitrofurantoin and sulphamethoxazole + trimethoprim MIC50 values were in the range of 25-100 micrograms/ml. With the disc diffusion method, all strains were sensitive to penicillin, cephalosporins examined, chloramphenicol and florfenicol, tetracyclines examined, pleuromutilins, lincomycin and tylosin. Variable sensitivity was observed for fluoroquinolones (flumequine, enrofloxacin, ofloxacin), most of the strains were susceptible to marbofloxacin. Almost all strains were resistant to aminoglycosides but most of them were sensitive to spectinomycin. A strong correlation was determined for disc diffusion and MIC results (Spearman's rho 0.789, p < 0001). MIC values of the type strain and MIC50 values of other tested strains did not differ significantly. Few strains showed a partially distinct resistance pattern for erythromycin, lincomycin and ampicillin in both methods.     

Anti-microbial susceptibility of Streptococcus spp. isolated from bovine mastitis in Argentina

The in vitro susceptibility to penicillin G, erythromycin and clindamycin was determined by the disc diffusion test and by E-test for a total of 47 streptococcal strains (three Streptococcus uberis, 36 Streptococcus agalactiae, eight Streptococcus dysgalactiae spp. dysgalactiae) isolated from bovine intramammary infections in Argentina. Moreover, resistance phenotypes of erythromycin-resistant streptococcal isolates was characterized. MIC90 of penicillin G, erythromycin and clindamycin for S. agalactiae were 0.75, 8.0 and 12.0 microg/ml respectively. Resistance to erythromycin and clindamycin was detected in 13 (27.6%) and 12 (25.5%) isolates respectively. No isolate was resistant to penicillin G. Resistance against macrolides, lincosamides and streptogramin B (MLS(B)) represented by the constitutive MLS(B) phenotype was present in 11 (23.4%) erythromycin-resistant isolates and two isolates (4.3%) expressed the M phenotype. The inducible MLS(B) phenotype was not identified. Results suggest that beta-lactams are the first-line antibiotics when treating streptococcal udder infections; however, the continuous monitoring of the antibiotic resistance is essential, as the emergence of resistant strains has become a growing concern on the therapy of bovine mastitis.     

In vitro comparison of the activity of various antibiotics and drugs against new Taiwan isolates and standard strains of avian mycoplasma

Twenty-nine antibiotics or drugs were incorporated individually into mycoplasma agar to evaluate their inhibitory activity against avian mycoplasmas: 100 recent Taiwan isolates of 7 serotypes and 10 standard strains of 7 serotypes were tested. All of the standard strains were very sensitive to erythromycin, chlorotetracycline, doxycycline, minocycline, and tetracycline, but the local isolates were highly resistant to these antibiotics. The drugs or antibiotics that possessed an MIC90 of 50 micrograms/ml or less against the local isolates were tiamulin (less than 0.4 micrograms/ml), lincospectin (2.7), josamycin (2.7), lincomycin (3.0), spectinomycin (4.8), tylosin (6.0), kanamycin (6.0), chloramphenicol (6.0), gentamicin (7.5), apramycin (24.5), doxycycline (27.4), minocycline (29.0), spiramycin (30.0), colistin (44.3), leucomycin (45.0), and streptomycin (50.0). The MIC90 of the other antibiotics or drugs was greater than 50 micrograms/ml. None of the isolates or strains were sensitive to nalidixic acid, ronidazole, penicillin, ampicillin, cephalexin, carbadox, or four sulfa drugs at a concentration about 5 times the therapeutic level.     

In vitro activity of five tetracyclines and some other antimicrobial agents against four porcine respiratory tract pathogens

The minimal inhibitory concentrations (MIC) of five tetracyclines and ten other antimicrobial agents were determined for four porcine bacterial respiratory tract pathogens by the agar dilution method. For the following oxytetracycline-susceptible strains, the MIC50 ranges of the tetracyclines were: P. multocida (n = 17) 0.25-0.5 micrograms/ml; B. bronchiseptica (n = 20) 0.25-1.0 micrograms/ml; H. pleuropneumoniae (n = 20) 0.25-0.5 micrograms/ml; S. suis Type 2 (n = 20) 0.06-0.25 micrograms/ml. For 19 oxytetracycline-resistant P. multocida strains the MIC50 of the tetracyclines varied from 64 micrograms/ml for oxytetracycline to 0.5 micrograms/ml for minocycline. Strikingly, minocycline showed no cross-resistance with oxytetracycline, tetracycline, chlortetracycline and doxycycline in P. multocida and in H. pleuropneumoniae. Moreover, in susceptible strains minocycline showed the highest in vitro activity followed by doxycycline. Low MIC50 values were observed for chloramphenicol, ampicillin, flumequine, ofloxacin and ciprofloxacin against P. multocida and H. pleuropneumoniae. B. bronchiseptica was moderately susceptible or resistant to these compounds. As expected tiamulin, lincomycin, tylosin and spiramycin were not active against H. pleuropneumoniae. Except for flumequine, the MIC50 values of nine antimicrobial agents were low for S. suis Type 2. Six strains of this species showed resistance to the macrolides and lincomycin.     

Infectious nature of Clostridium spiroforme-mediated rabbit enterotoxaemia

Newly weaned rabbits had diarrhoea only if they were infected with Clostridium spiroforme. In adult rabbits exposure to both clindamycin and C. spiroforme was necessary to induce disease. All diseased animals harboured C. spiroforme and its toxin. Adult rabbits given a course of clindamycin survived when held in a protected environment as did those challenged with C. spiroforme alone. At necropsy none of these apparently healthy animals showed signs of diarrhoea or caecitis. These findings suggest that, in the development of enterotoxaemia, weaning or clindamycin treatment and infection with C. spiroforme are separate events and that disease follows infection with this organism from the environment, as opposed to overgrowth by undetectable levels of C. spiroforme resident in the gut. Our data indicate that C. spiroforme is not a normal component of the rabbit gut flora and that the normal bowel ecology of the adult must be disrupted before C. spiroforme will colonize.     

Characterisation of some Ornithobacterium rhinotracheale strains and examination of their transmission via eggs

The biochemical characteristics and antibiotic susceptibility of 12 Ornithobacterium rhinotracheale strains isolated from chickens and turkeys suffering from respiratory clinical signs and the survival of some isolates on egg-shell and within chicken eggs during hatching were examined. All O. rhinotracheale strains showed typical biochemical characteristics. Among the 16 drugs examined, penicillin G, ampicillin (MICs ranging from < or = 0.06 microgram/ml to 1 microgram/ml), ceftazidim (with MICs from < or = 0.06 microgram/ml to 0.12 microgram/ml), erythromycin, tylosin, tilmicosin (with some exceptions MICs ranged from < or = 0.06 microgram/ml to 1 microgram/ml) and tiamulin (MICs varied from < or = 0.06 microgram/ml to 2 micrograms/ml) were the most effective. Lincomycin, oxytetracycline and enrofloxacin also gave good inhibitions, but with most strains in a higher concentration (MICs ranged in most cases from 2 micrograms/ml to 8 micrograms/ml). The other antibiotics inhibited the growth of O. rhinotracheale only in very high concentrations (colistin) or not at all (apramycin, spectinomycin, polymyxin B). At 37 degrees C, O. rhinotracheale did not survive on egg-shell for more than 24 hours, while upon inoculation into embryonated chicken eggs it killed embryos by the ninth day, and from the 14th day post-inoculation no O. rhinotracheale could be cultured from the eggs at all. These results suggest that O. rhinotracheale is not transmitted via eggs during hatching.     

In vitro susceptibility of some porcine respiratory tract pathogens to aditoprim, trimethoprim, sulfadimethoxine, sulfamethoxazole, and combinations of these agents

The in vitro antimicrobial activities of aditoprim (AP), a new dihydrofolate reductase (DHFR) inhibitor, trimethoprim (TMP), sulfadimethoxine (SDM), sulfamethoxazole (SMX), and combinations of these drugs against some porcine respiratory tract pathogens were determined by use of an agar dilution method. The minimal inhibitory concentrations (MIC) of these agents were determined twice against Bordetella bronchiseptica (n = 10), Pasteurella multocida (n = 10), and Actinobacillus pleuropneumoniae (n = 20) strains isolated from pigs suffering from atrophic rhinitis or pleuropneumonia. All B bronchiseptica strains were resistant to AP and TMP. The MIC50 values of AP and TMP for P multocida were 0.25 and 0.06 microgram/ml, respectively, and for A pleuropneumoniae, 1 and 0.25 microgram/ml, respectively. The MIC50 values of SDM and SMX for B bronchiseptica were 4 and 1 micrograms/ml, respectively; for P multocida, 16 and 8 micrograms/ml, respectively; and for A pleuropneumoniae, 16 and 8 micrograms/ml, respectively. The investigated combinations of the DHFR inhibitors and the selected sulfonamides had synergism for the A pleuropneumoniae strains; the MIC90 values of the combinations were less than or equal to 0.06 microgram/ml. Potentiation was not observed for the B bronchiseptica and the P multocida isolates. The MIC of the combinations against B bronchiseptica and P multocida corresponded respectively to the concentrations of the sulfonamides and the DHFR inhibitors in the combinations. For A pleuropneumoniae, 2 types of strains were used (25% of serotype 2 and 75% of serotype 9). Type-2 strains had lower susceptibility than type-9 strains to AP and TMP as well as to SDM and SMX (at least a fourfold difference in MIC between the 2 types of strains).(ABSTRACT TRUNCATED AT 250 WORDS)     

Antimicrobial susceptibility of Arcanobacterium pyogenes isolated from cattle and pigs

The minimum inhibitory concentrations (MICs) of 18 antimicrobial agents were determined for 49 Arcanobacterium pyogenes isolates (42 bovine isolates and 7 porcine isolates). Benzylpenicillin and ampicillin were the most active antibiotics, with MIC ranges of < or = 0.0125-0.05 microgram/ml for both bovine and porcine isolates. All isolates were susceptible to penicillins and cephems. MICs for 90% of the isolates of dihydrostreptomycin, gentamicin and oxytetracycline for bovine isolates were > 100 micrograms/ml, 1.56 micrograms/ml and 25 micrograms/ml, respectively. More resistance to dihydrostreptomycin appeared among porcine isolates (85.7%) than among bovine isolates (52.4%). Resistance to gentamicin occurred in only 3 (7.1%) of the bovine isolates. Resistance to oxytetracycline also appeared more frequent among porcine isolates (85.7%) than among bovine isolates (57.1%). All bovine isolates were susceptible to erythromycin, tilmocosin and lincomycin, but two porcine isolates (28.6%) were simultaneously resistant to these antibiotics. Tiamulin was as active as tilmicosin, with an MIC for 50% of the isolates (MIC50) of 0.05 microgram/ml for both bovine and porcine isolates. The MIC50s of chloramphenicol and its derivatives florfenicol and thiamphenicol were all 1.56 micrograms/ml. The fluoroquinolones enrofloxacin and ofloxacin were not so active as penicillins and macrolides, with MIC50s of 0.78 microgram/ml and 1.56 micrograms/ml, respectively, for both bovine and porcine isolates.     

Prevalence of and resistance to anti-microbial drugs in selected microbial species isolated from bulk milk samples

The prevalence of strains of Staphylococcus aureus, coagulase-negative (CN) staphylococci, Listeria monocytogenes, Escherichia coli, Enterococcus faecalis, E. faecium and Bacillus cereus, was investigated in 111 bulk milk samples. Staphylococcus aureus was isolated from 38 samples, CN staphylococci from 63 samples, E. coli from 49 samples, E. faecalis or E. faecium from 107 samples, and L. monocytogenes from two samples. Bacillus cereus was not found in any of the samples and three samples were free of any of the selected species. Sensitivity to the anti-microbial drugs amikacin, ampicillin, ampicillin + sulbactam, cephalothin (CLT), cephotaxime, clindamycin, chloramphenicol (CMP), co-trimoxazole, erythromycin (ERY), gentamicin, neomycin, norfloxacin, oxacillin, penicillin, streptomycin (STR), tetracycline (TTC) and vancomycin was tested using the standard dilution technique. Minimum inhibitory concentration (MIC) characteristics (MIC50, MIC90, MIC range) were determined for each microbial species. Resistance against one or more anti-microbial drugs was found in 93% of S. aureus, 40% of CN staphylococci, 73% of E. coli, 88% of E.faecalis, 55% of E.faecium, and one L. monocytogenes strain. Most of the strains, particularly enterococci, were resistant to STR, TTC, and ERY (MIC50 4 microg/ml). A high percentage of staphylococci were resistant to beta-lactam antibiotics. High resistance to CLT was found in 11 strains of E. coli (MIC 256 microg/ml) and strains resistant to CMP (MIC90 16 microg/ml) were detected. The highest numbers of resistance phenotypes were found in E. coil (16) and CN staphylococci (12). Eighteen identical resistance phenotypes were demonstrated in indicator bacteria (E. coli, E. faecalis, E. faecium) and pathogens (S. aureus, CN staphylococci) isolated from the same bulk milk sample. The obtained resistance data were matched against the herd owners' information on therapeutic use of the drugs. This confrontation could not explain the findings of strains resistant to ERY or CMP. Our findings are evidence of selection of resistant strains among not only pathogenic agents, but also among indicator bacteria which can become significant carriers of transmissible resistance genes.     

Minimum inhibitory concentrations of 20 antimicrobial agents against Staphylococcus aureus isolated from bovine intramammary infections in Japan

Minimum inhibitory concentrations (MICs) of 20 antimicrobial agents were determined against 51 isolates of Staphylococcus aureus from bovine intramammary infections. Fourteen (27.4%) isolates were resistant to benzylpenicillin, but none of the isolates was resistant to cloxacillin, nafcillin, or cephems. Among aminoglycosides, gentamicin was the most active, with an MIC50 of 0.2 microg/ml, followed by kanamycin, with an MIC50 of 0.78 microg/ml. Five isolates (9.8%) were resistant to dihydrostreptomycin, three isolates (5.9%) to kanamycin and two isolates (3.9%) to gentamicin. Resistance to erythromycin was observed in two isolates (3.9%). Tylosin was less active than erythromycin, with MIC50s of 1.56 microg/ml versus 0.39 microg/ml, but none of the isolates was resistant to this antibiotic. Oxytetracycline MICs were situated in the range of 0.39-1.56 microg/ml for 48 susceptible isolates. Although 19 (37.3%) isolates were resistant to one or more antimicrobial agents, a single resistance pattern was most frequent: benzylpenicillin (12 isolates), dihydrostreptomycin (two isolates) and kanamycin (one isolate). There were no isolates resistant to antimicrobial agents such as methicillin, lincomycin, clindamycin, chloramphenicol, florfenicol and virginiamycin, which have not been approved for use in cattle husbandry in Japan.     

鲜牛奶中细菌的分离纯化及其药物敏感性分析

为了解鲜牛奶中细菌的分布情况及其对抗生素的敏感状况,实验对30份采自河北张家口地区某养殖场的新鲜无菌牛奶进行细菌分离鉴定,并分别用青霉素(16μg/ml)、氯霉素(32μg/ml)、克林霉素(4μg/ml)、红霉素(8μg/ml)、四环素(16μg/ml)、庆大霉素(16μg/ml)、环丙沙星(4μg/ml)、头孢噻吩(32μg/m)l进行药敏实验。结果显示30份牛奶样品中共分离得到111株菌,分别为葡萄球菌31株、沙雷氏菌14株、假单胞菌12株、芽孢杆菌11株、肠杆菌5株、不动杆菌4株、奇异变形杆菌3株、微球菌3株、巨球菌3株、肠球菌4株、其它菌21株。其中耐青霉素16μg/ml的为50株,耐氯霉素32μg/ml的为23株,耐克林霉素4μg/ml的为46株,耐红霉素8μg/ml的为37株,耐四环素16μg/ml的为38株,耐庆大霉素16μg/ml的是7株,耐环丙沙星4μg/ml的为8株,耐头孢噻吩32μg/ml的为40株。通过实验可知,细菌类型主要有葡萄球菌、沙雷氏菌、假单胞菌等;8种抗生素中,庆大霉素、环丙沙星、氯霉素的抑菌效果较好,而青霉素、克林霉素、红霉素、四环素、头孢噻吩的抑菌效果相对较差。     

Minimum inhibitory concentration determinations for various antimicrobial agents against 1570 bacterial isolates from turkey poults

Minimum inhibitory concentrations (MICs) were determined for 1570 bacteria from eight geographic locations (1204 Escherichia coli, 231 other enteric gram-negative bacilli [including Citrobacter spp., Enterobacter spp., Klebsiella spp., Proteus spp., and Salmonella spp.], 31 Pseudomonas spp., 18 coagulase-positive staphylococci, 26 coagulase-negative staphylococci, and 55 streptococci and enterococci) by the National Committee for Clinical Laboratory Standards broth microdilution procedure. Antimicrobial agents tested included ampicillin, ceftiofur, enrofloxacin, erythromycin, florfenicol, gentamicin, neomycin, spectinomycin, sulfamethazine, tetracycline, and trimethoprim/sulfadiazine. Against the E. coli strains tested, ceftiofur, enrofloxacin, gentamicin, and trimethoprim/sulfadiazine were the most active compounds with MIC at which 50% of the strains are at or below (MIC50) = 0.5, < or = 0.03, 0.5, and 0.13 microg/ml, respectively, and MIC at which 90% of the strains are at or below (MIC90) = 1.0, 0.13, 32.0, and 2.0 microg/ml, respectively. Ampicillin, florfenicol, neomycin, and spectinomycin were the next most active compounds against the E. coli strains, with MIC50 = 4.0, 4.0, 16.0, and 16.0 microg/ml, respectively. MIC90 values for these compounds against E. coli strains were > 32.0, 8.0, 512.0, and > 128.0 microg/ml, respectively. The remaining compounds exhibited limited, strain-dependent activity against the E. coli strains tested. As with the E. coli, enrofloxacin, ceftiofur, and trimethoprim/sulfadiazine were also the most active compounds against the 231 other enteric organisms tested, with MIC50 < or = 1.0 microg/ml for all of these genera. The remaining compounds exhibited limited activity against these genera. Against the gram-positive cocci tested, ampicillin, enrofloxacin, ceftiofur, and trimethoprim/sulfadiazine were most active, whereas the remaining compounds exhibited strain-dependent activity. When MIC data for E. coli were summarized separately, differences were observed between the geographic locations for the various antimicrobial agents. In conclusion, ceftiofur, enrofloxacin, and trimethoprim/sulfadiazine were the most active of the compounds tested against all of the bacterial strains.     

Significance of Clostridium spiroforme in the enteritis-complex of commercial rabbits

Commercial rabbits showing clinical signs of enteritis-complex were examined for the presence of Clostridium spiroforme and its iota-like toxin. The bacterium was detected by Gram stain in 52.4% of 149 cecal samples and iota-like toxin in 7.4%. From 29 strains of C. spiroforme tested, 26 were toxigenic, originating from 24 of 29 rabbitries. In 13.4% of the samples, C. spiroforme was present as the only known disease agent. Gross and microscopic lesions were similar to those described in the literature. In the other samples, C. spiroforme was associated with attaching effacing Escherichia coli (29.5%), Bacillus piliformis (10.3%), rotaviruses (25.6%), coronavirus (2.6%), Eimeria spp. (44.9%) and cryptosporidia (6.4%). In 33.3% of C. spiroforme-containing samples, more than one of these agents was present. There was no significant difference between the presence of these organisms in C. spiroforme-positive and negative samples. On the basis of these results as well as that of previous data, we suggest that C. spiroforme-mediated diarrhea is favoured by maldigestion, initiated by infectious agents and/or nutritional factors.     

Anti‐microbial Susceptibility of Streptococcus spp. Isolated from Bovine Mastitis in Argentina

The in vitro susceptibility to penicillin G, erythromycin and clindamycin was determined by the disc diffusion test and by E‐test for a total of 47 streptococcal strains (three Streptococcus uberis, 36 Streptococcus agalactiae, eight Streptococcus dysgalactiae spp. dysgalactiae) isolated from bovine intramammary infections in Argentina. Moreover, resistance phenotypes of erythromycin‐resistant streptococcal isolates was characterized. MIC₉₀ of penicillin G, erythromycin and clindamycin for S. agalactiae were 0.75, 8.0 and 12.0 μg/ml respectively. Resistance to erythromycin and clindamycin was detected in 13 (27.6%) and 12 (25.5%) isolates respectively. No isolate was resistant to penicillin G. Resistance against macrolides, lincosamides and streptogramin B (MLS_B) represented by the constitutive MLS_B phenotype was present in 11 (23.4%) erythromycin‐resistant isolates and two isolates (4.3%) expressed the M phenotype. The inducible MLS_B phenotype was not identified. Results suggest that beta‐lactams are the first‐line antibiotics when treating streptococcal udder infections; however, the continuous monitoring of the antibiotic resistance is essential, as the emergence of resistant strains has become a growing concern on the therapy of bovine mastitis.     

The effects of combinations of selected antibiotics on the growth of Corynebacterium equi

The minimal inhibitory concentrations of penicillin G, ampicillin, gentamicin, erythromycin and rifampicin were determined for nine strains of Corynebacterium equi. The effect of combinations of any two of these antibiotics on the killing of these strains was determined at antibiotic concentrations achievable in horses using recommended drug dosages (ampicillin 4.0 μg/ml, gentamicin 1.0 μg/ml and erythromycin 0.25 μg/ml). Penicillin G was used at 4.0 μg/ml and rifampicin at 0.063 μg/ml. The combinations of gentamicin with erythromycin or rifampicin gave antagonistic effects on killing compared to either drug alone. Combinations of erythromycin with rifampicin or penicillin showed synergistic effects, as did penicillin-gentamicin. All other combinations, and a triple combination of penicillin-rifampicin-erythromycin, showed additive effects only.     

In vitro antimicrobial activity of sulfonamides against some porcine pathogens

The minimal inhibitory concentrations (MIC) of sulfonamides were determined against Bordetella bronchiseptica (n = 10), Pasteurella multocida (n = 10), Haemophilus pleuropneumoniae (n = 20), and Streptococcus suis (n = 10) strains isolated from pigs with atrophic rhinitis, pneumonia, or meningitis. Sulfonamides tested in an agar dilution method were sulfachloropyridazine, sulfadiazine, sulfadimethoxine, sulfamethazine, sulfadoxine, sulfisoxazole, sulfamerazine, sulfamethoxazole, sulfamethoxypyridazine, sulfanilamide, sulfatroxazole, and sulfisomidine. Results indicated that monotherapy of S suis infections with sulfonamides should not be encouraged because the MIC50 of all sulfonamides investigated was greater than 32 micrograms/ml. The MIC50 of the sulfonamides against B bronchiseptica ranged from 0.5 to 8 micrograms/ml, against P multocida from 2 to 32 micrograms/ml, and against H pleuropneumoniae from 8 to 64 micrograms/ml. The MIC50 of sulfachloropyridazine, sulfadiazine, sulfadimethoxine, sulfamerazine, and sulfamethoxazole for the gram-negative bacteria did not exceed 16 micrograms/ml. Among these compounds, sulfamethoxazole had the highest activity. The frequently prescribed sulfamethazine had an overall low antimicrobial activity.     

Antimicrobial resistance and resistance genes in Staphylococcus aureus strains from rabbits

Fifty-six Staphylococcus aureus isolates recovered between 1998 and 2003 from 31 rabbit farms with and without problems of chronic staphylococcosis, were screened for resistance to enrofloxacin, erythromycin, gentamicin, lincomycin, neomycin, penicillin and tetracyclines using the agar dilution test. For penicillin, a disk diffusion test was also performed. The detection of tetP(B), tet(K), tet(L), tet(M), tet(O), tet(T), tet(W), erm(A), erm(B), erm(C) and mec(A) genes was done via a PCR assay. Four isolates showed resistance to erythromycin and lincomycin. These isolates were positive for the erm(C) gene in the PCR. Eleven strains were resistant to tetracyclines and all harboured the tet(K) gene. In the agar dilution test, five isolates showed resistance to penicillin, whereas in the disk diffusion test 12 isolates showed resistance. None of these 12 resistant isolates carried the mec(A) gene. Only one strain showed resistance to gentamicin, and all strains were susceptible to enrofloxacin and neomycin. This study demonstrates that resistance to antimicrobial agents in S. aureus isolates originating from rabbits is relatively rare compared to resistance in S. aureus isolates originating from other animals and humans.