In vitro effect of combinations of antimicrobial agents and EDTA-tromethamine on certain gram-positive bacteria

Combinations of EDTA-tromethamine and 7 antimicrobial agents (chloramphenicol, nalidixic acid, oxytetracycline, penicillin, polymyxin-B, streptomycin, and a triple sulfonamide preparation) were tested for synergistic activities against Staphylococcus aureus, Corynebacterium renal, Listeria monocytogenes, Erysipelothrix rhusiopathiae, and a beta-hemolytic streptococci. Two in vitro tests were used--minimal inhibitory concentrations of the drugs and a 2-dimensional Microtiter checkerboard technique. A slight synergistic action was seen when L monocytogenes was exposed to combinations of penicillin-EDTA or streptomycin-EDTA.     

In vitro susceptibility of Pseudomonas mallei to antimicrobial agents

Pseudomonas mallei was isolated from pus samples obtained from 34 mallein-positive horses. The isolates were subjected to in vitro sensitivity test using 16 different antimicrobial discs. All isolates (34) were sensitive to sulfamethizole, gentamycin, tetracycline, sulfathiazole, kanamycin, tobramycin, streptomycin and a combination of trimethoprim and sulfamethoxazole while none of them were sensitive to cephalothin, colistin, ampicillin, penicillin and nitrofurantoin. Rifapicin, chloramphenicol and carbenicillin were effective against 32, 26 and 18 isolates respectively. The minimum inhibitory concentrations (MICs) of gentamycin, tetracycline, tobramycin, sulfamethizole, streptomycin, rifampicin and a combination of trimethoprim and sulfamethoxazole were 0.28, 0.38, 0.67, 1.40, 3.40, 5.86 and 5.30 micrograms/ml, respectively.     

In vitro antimicrobial susceptibility of 183 Pseudomonas aeruginosa strains isolated from dogs to selected antipseudomonal agents

During the years from 1993 to 2000, 183 strains of Pseudomonas aeruginosa were isolated from different pathological specimens originating from dogs. Antimicrobial susceptibility patterns against 10 antipseudomonal agents were obtained on 183 P. aeruginosa strains. In vitro antimicrobial susceptibility testing was performed using the disk diffusion method (Kirby-Bauer). Antimicrobial susceptibility profiles showed that among beta-lactam antibiotics, imipenem was the most active compound. Out of the 183 strains tested, 96.7% were sensitive to imipenem. Cefoperazone showed good in vitro activity against 86.9% of the tested strains. Against ceftazidime, 77.0% of strains showed sensitivity. An old penicillin, carbenicillin, gave only 71.6% sensitive strains. Sensitivity to amikacin was 87.4% and it was 83.1% to gentamicin. Pipimedic acid, a first-generation quinolone, was the least active compound of all those tested, 47.0% were resistant. The in vitro sensitivity against enrofloxacin showed that 71.0% strains were sensitive and 26.2% showed resistance. Almost all strains tested, 93.4%, were susceptible to ciprofloxacin and marbofloxacin. Besides imipenem, the quinolone antibiotics, marbofloxacin and ciprofloxacin were the most effective against P. aeruginosa strains isolated from dogs.     

Potentiation of thein vitro activity of some antimicrobial agents against selected Gram-negative bacteria by EDTA-tromethamine

Thein vitro synergistic effects of combinations of EDTA-tromethamine and six antimicrobial agents (ampicillin, chloramphenicol, oxytetracycline, streptomycin, nalidixic acid and sulphadimethoxine) on clinically isolated strains ofPseudomonas aeruginosa, Proteus mirabilis andEscherichia coli were investigated. The antibacterial activity was assessed from the minimal inhibitory concentration for the antibiotics alone or in combination with EDTA-tromethamine. EDTA-tromethamine potentiated the antibacterial activity of ampicillin, chloramphenicol, oxytetracycline and streptomycin up to four-fold. There were no significant or consistent synergistic effects with nalidixic acid or sulphadimethoxine.Abbreviations cfu colony-forming units - EDTA ethylenediaminetetraacetic acid - MIC minimal inhibitory concentration - tromethamine tris(hydroxymethyl)aminomethane Part of this paper was communicated at the XLV Congress of the Italian Society of Veterinary Sciences, Palermo, 25–28 September 1991     

Susceptibility of clinical isolates of Pseudomonas aeruginosa to antimicrobial agents

One hundred and eighteen isolates of Pseudomonas aeruginosa obtained from clinical material were tested for susceptibility to antimicrobial agents. Regardless of species of animal from which the isolate was obtained, 75% of the isolates were inhibited by ≤ 4 μg of gentamicin/ml, 100% by ≤ 4 μg of tobramycin/ml, 97% by ≤ 128 μg of carbenicillin/ml, and 2% by ≤ 64 μg of chloramphenicol/ml.     

Antibacterial action of combinations of oxytetracycline, dimethyl sulfoxide, and EDTA-tromethamine on Proteus, Salmonella, and Aeromonas

Antibacterial effects against Proteus mirabilis, Salmonella typhimurium, and Aeromonas hydrophila were obtained with subminimal inhibitory concentrations of oxytetracycline and EDTA-tromethamine. Antibacterial effects were not observed with subminimal inhibitory concentrations of dimethyl sulfoxide plus oxytetracycline or with dimethyl sulfoxide plus EDTA-tromethamine. Using a 2-dimensional Microtiter checkerboard technique, inhibitory activities of the various combinations of solutions were studied, and isobolograms were plotted. A synergistic effect was seen with combinations of oxytetracycline and EDTA-tromethamine. The greatest synergistic effect was observed when the mixture was caused to react with P mirabilis. These findings were confirmed by kinetic studies of microbial death, using one-fourth minimal inhibitory concentrations of these preparations.     

In vitro susceptibilities of canine urinary bacteria to selected antimicrobial agents

Antimicrobial susceptibility tests were conducted on bacteria that were isolated from urine specimens collected by antepubic cystocentesis from dogs with urinary tract infections. Antimicrobics to which greater than or equal to 90% of these urinary bacteria were susceptible in vitro included trimethoprim-sulfamethoxazole (TMP-SMZ), nitrofurantoin, cephalexin, nalidixic acid, and gentamicin for isolates of Escherichia coli; ampicillin, TMP-SMZ, cephalexin, nalidixic acid, and gentamicin for isolates of Proteus mirabilis; ampicillin chloramphenicol, TMP-SMZ, nitrofurantoin, cephalexin, kanamycin, and gentamicin for isolates of coagulase-positive staphylococci; cephalexin, nalidixic acid, and gentamicin for isolates of Klebsiella pneumoniae; ampicillin, TMP-SMZ, and gentamicin for isolates of Streptococcus faecalis, Str faecium, and Str zymogenes; ampicillin, chloramphenicol, TMP-SMZ, and gentamicin for isolates of Str viridans; and ampicillin, chloramphenicol, TMP-SMZ, nitrofurantoin, cephalexin, kanamycin, and gentamicin for isolates of Str canis. No antimicrobial agent tested was effective in vitro at the 90% level for isolates of Pseudomonas aeruginosa, but gentamicin was closest, at 89%.     

In vitro susceptibility of rabbit strains of Clostridium spiroforme to antimicrobial agents

Using an agar dilution method we measured the minimum inhibitory concentration (MIC) of 12 antimicrobial agents against 11 strains of iota-toxigenic strains of Clostridium spiroforme. Each strain was isolated from a separate outbreak of toxic diarrhoea of rabbits. Vancomycin and bacitracin, both agents used to treat intestinal clostridioses of humans and other animals, had a relatively high MIC (8 micrograms/ml or more). Metronidazole was uniformly active against C. spiroforme. With MIC of 8 micrograms/ml or more, both lincomycin (11 strains) and erythromycin (9 strains) were relatively inactive against C. spiroforme, conversely, penicillin G was active (MIC for 8 strains was 0.5 micrograms/ml or less). Exposure to any one of these drugs has been implicated as a predisposing factor for C. spiroforme mediated diarrhoea of rabbits. The greatest variation in MIC was seen for erythromycin (8-fold), penicillin G (8-fold) and tetracycline (16-fold).     

In vitro activity of fifteen antimicrobial agents against methicillin-resistant and methicillin-susceptible Staphylococcus intermedius

In this study the susceptibility of 91 methicillin-resistant and -susceptible Staphylococcus intermedius strains (MRSI and MSSI, respectively) against 15 antimicrobial agents was determined. The activity of the antimicrobial agents was studied at pH 7.2 and pH 8.5. Methicillin was more active at pH7.2 (28 strains methicillin-resistant) than at pH 8.5 (55 strains methicillin-resistant). Gentamicin showed excellent activity, with only 3 strains resistant at pH 8.5. However, gentamicin would have to be administered parenterally. Oxytetracy-cline cannot be recommended for treatment of canine staphylococcal dermatitis, due to the high percentage (over 25%) of strains that were found to be resistant. Clindamycin showed little activity in inhibiting growth of the strains studied, the percent resistance at pH 7.2 was 93.4%. Rifampin behaved differently at the two pH values. However, a close relationship was noted between methicillin-resistant and rifampin-resistant strains, particularly at the lower pH. Of the fluoroquinolones, ciprofloxacin or enrofloxacin would be a good useful alternative for the treatment of methicillin-resistant strains of S. intermedius . Lastly, very high resistance to sulphamethoxypyridazine was found, as was the case with trimethoprim and a combination of trimethoprim/ sulphamethoxypyridazine, against not only MRSI but also MSSI strains.     

In vitro fluoroquinolone susceptibility of Pseudomonas aeruginosa isolates from dogs with ulcerative keratitis

OBJECTIVE: To determine the in vitro fluoroquinolone susceptibility profiles of Pseudomonas aeruginosa isolates from dogs with ulcerative keratitis. Animals-27 dogs with P. aeruginosa-associated ulcerative keratitis. PROCEDURES: P. aeruginosa isolates from dogs with ulcerative keratitis were collected during a 3-year period. Isolates were tested by use of the disk diffusion method for their susceptibility to 7 fluoroquinolones that are available as commercial ophthalmic preparations. The antimicrobials included second- (ciprofloxacin, ofloxacin, norfloxacin, and lomefloxacin), third- (levofloxacin), and fourth-generation (gatifloxacin and moxifloxacin) fluoroquinolones. Isolates were designated as susceptible, intermediate, or resistant to the various antimicrobials. The percentage of susceptible isolates was compared among individual fluoroquinolones and among fluoroquinolone generations. RESULTS: None of the dogs had received topical or systemic fluoroquinolone treatment prior to referral. Twenty-seven P. aeruginosa isolates were collected during the study period. In vitro, bacterial resistance to the tested fluoroquinolones was infrequently identified (24/ 27 isolates were susceptible to all fluoroquinolones evaluated); susceptibility percentages ranged from 88.9% to 100% for individual antimicrobials. There were no significant differences among isolate susceptibilities to the individual antimicrobials or among generations of fluoroquinolones. CONCLUSIONS AND CLINICAL RELEVANCE: On the basis of these in vitro data, none of the 7 evaluated fluoroquinolones (individually or collectively by generation) appeared to offer a clinically important advantage in the treatment of P. aeruginosa-associated ulcerative keratitis in dogs. Among the P. aeruginosa isolates collected from dogs with ulcerative keratitis in this study, the likelihood of susceptibility to the fluoroquinolones evaluated was high.     

Inhibitory effects of combinations of oxytetracycline, dimethyl sulfoxide, and EDTA-tromethamine on Escherichia coli

Antibacterial activity against Escherichia coli was obtained with subminimal inhibitory concentrations of oxytetracycline (OTC) and EDTA-tromethamine. Inhibitory effects were not observed using combinations of dimethyl sulfoxide and OTC or dimethyl sulfoxide and EDTA-tromethamine. Neither EDTA-tromethamine nor OTC used alone was capable of the same degree of inhibition. Using a 2-dimensional Microtiter checkerboard technique, the inhibitory activity of these combinations was studied and isobolograms were plotted. A synergistic effect was seen with combinations of OTC and EDTA-tromethamine. Kinetic studies of microbial death, using subminimal inhibitory concentrations of these agents, confirmed these findings.     

Comparisons of antibiotic combinations to control Pseudomonas aeruginosa in bovine semen.

Raw semen experimentally contaminated with 10(6) Pseudomonas aeruginosa cells per milliliter was processed for use in artificial insemination (AI) using three different antibiotic combinations: a) gentamicin, lincomycin, spectinomycin and tylosin (GLST) directly added to contaminated raw semen followed by dilution with whole milk or egg yolk Tris containing GLST; b) penicillin, streptomycin, lincomycin, spectinomycin and minocycline (PSLSM) in whole milk used to dilute the contaminated raw semen followed by further dilution with glycerolated milk containing PSLSM; and c) penicillin, streptomycin, lincomycin and spectinomycin (PSLS) used with egg yolk Tris diluent in the same way as PSLSM and milk. Diluted semen was incubated at 35 degrees C for 5 or 40 min before cooling commenced. To assess the efficacy of the antibiotics in controlling P. aeruginosa, diluted semen samples were cultured for the organism before and after freezing. The GLST antibiotics added to raw semen and milk reduced the counts of P. aeruginosa before or after freezing. When egg yolk Tris was used, GLST inhibited the organism as indicated by its low growth in culture before freezing and absence of growth from samples after freezing. With PSLSM and PSLS treatments, the organism was recovered in milk and egg yolk Tris processed semen both before and after freezing. However, incubation at 35 degrees C for 40 min prior to cooling, compared to incubation of 5 min, appeared to reduce the bacterial counts after freezing.     

Serotypes and antimicrobial susceptibility of Pseudomonas aeruginosa strains isolated from diseased dogs

Strains of Pseudomonas aeruginosa isolated from diseased dogs in Tokyo area during 1983 through 1986 were serotyped and assayed for antimicrobial susceptibility to obtain an epizootiological aspect of canine P. aeruginosa infection. Major sources of specimens were ear and nasal discharges and urine. The results of O-antigen typing using a monoclonal antibody kit showed that the most predominant serotype was type M. Types G and B were also major serotypes. In a yearly distribution of serotypes, type I was almost limited in 1986, and was isolated mainly from surgical wounds, which showed an episode of nosocomial infection, whereas that of type M or B was dispersed during 4 years from 1983 to 1986. As a result of antimicrobial susceptibility test, most canine strains were considered to be susceptible to 4 drugs, which were commonly used in both human and veterinary clinics, in contrast to human isolates.     

In vitro assessment of chloramphenicol and florfenicol as second‐line antimicrobial agents in dogs

The aim of this study was to evaluate the potential of chloramphenicol and florfenicol as second‐line antimicrobial agents for treatment of infections caused by methicillin‐resistant Staphyococcus pseudintermedius (MRSP) and extended‐spectrum β‐lactamase (ESBL)‐producing Escherichia coli in dogs, through a systematic in vitro assessment of the pharmacodynamic properties of the two drugs. Minimum inhibitory concentrations (MIC) and phenicol resistance genes were determined for 169 S. pseudintermedius and 167 E. coli isolates. Minimum bactericidal concentrations (MBC), time‐killing kinetics, and postantibiotic effect (PAE) of both agents against wild‐type isolates of each species were assessed. For S. pseudintermedius, the chloramphenicol MIC₉₀ was 32 μg/mL. No florfenicol resistance was detected in this species (MIC_90 = 4 μg/mL). The MIC₉₀ of both agents against E. coli was 8 μg/mL. Resistance genes found were cat_pC221 in S. pseudintermedius and catA1 and/or floR in E. coli. The phenicols displayed a time‐dependent, mainly, bacteriostatic effect on both species. Prolonged PAEs were observed for S. pseudintermedius, and no PAEs were detected for E. coli. More research into determination of PK/PD targets of efficacy is needed to further assess the clinical use of chloramphenicol and florfenicol as second‐line agents in dogs, optimize dosage regimens, and set up species‐specific clinical break points.     

In vitro activity of 10 antimicrobial agents against bacteria isolated from cows with clinical mastitis

The susceptibility of 495 strains of bacteria, recently isolated in France from cows with clinical mastitis, to 10 antimicrobial agents--penicillin G, cloxacillin, oxacillin, cephalexin, cefazolin, cephapirin, cefquinome, neomycin, ampicillin and colistin--was determined by measuring their minimum inhibitory concentrations (MICS). Overall, the levels of resistance were very low except for staphylococci and penicillin G. The 167 streptococcal strains were susceptible to all of the beta-lactams tested, but six (3-6 per cent) were highly resistant to neomycin. Of the 171 staphylococcal isolates, 36.2 per cent were resistant to penicillin G, one strain of Staphylococcus sciuri was classified as methicillin-resistant, but they were all susceptible to neomycin. None of the 122 strains of Escherichia coli was resistant to colistin, but 12 had high MIC values for one or more of the cephalosporins.