title: In vitro assessment of the efficacy of erythromycin in combination with oxytetracycline or spectinomycin against Pasteurella haemolytica.
abstract: The effects of combining erythromycin (Ery) with oxytetracycline (Oxy) or spectinomycin (Sp) on Pasteurella haemolytica were evaluated in vitro using the chessboard (checkerboard) technique. These combinations were selected because all are drugs widely used in bovine respiratory disease treatment, and they represent possible sequential or complementary mechanisms of action. Using the recommended breakpoints of greater than 4 micrograms/ml for Ery, 16 micrograms/ml for Oxy, and 32 micrograms/ml for Sp, of the 33 P. haemolytica isolates, 32 were resistant to Oxy, 27 to Sp, and 14 to Ery. Based on the fractional inhibitory concentration index, Ery and Oxy in combination were synergistic or additive against 32 of 33 isolates. The combination of Ery and Sp was synergistic or additive against 27 of 33 isolates. No instances of antagonism were seen. When the effects were considered within the context of therapeutically achievable serum/tissue concentrations, the effects of Ery and Oxy in combination were only marginal. Thus, against P. haemolytica isolates, Ery and Sp appeared to represent an effective antimicrobial combination, whereas Ery and Oxy were only of marginal efficacy as a combination.
authors.affiliation: Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater 74078-0353.
authors.forename: G E
authors.initials: GE
authors.lastname: Burrows
authors.forename: P
authors.initials: P
authors.lastname: Ewing
mesh_terms: D000818:Animals; D002417:Cattle; D004352:Drug Resistance, Microbial; D004357:Drug Synergism; D004359:Drug Therapy, Combination; D004917:Erythromycin / Q000494:pharmacology* / Q000627:therapeutic use; D008826:Microbial Sensitivity Tests; D010118:Oxytetracycline / Q000494:pharmacology* / Q000627:therapeutic use; D010325:Pasteurella / Q000187:drug effects*; D012766:Pasteurellosis, Pneumonic / Q000188:drug therapy; D000198:Spectinomycin / Q000494:pharmacology* / Q000627:therapeutic use
chemical_list: D004917:Erythromycin; D000198:Spectinomycin; D010118:Oxytetracycline