title: Comparison of standardised versus non-standardised methods for testing the in vitro potency of oxytetracycline against Mannheimia haemolytica and Pasteurella multocida.
abstract: The in vitro pharmacodynamics of oxytetracycline was established for six isolates of each of the calf pneumonia pathogens Mannheimia haemolytica and Pasteurella multocida. Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and bacterial time-kill curves were determined in two matrices, Mueller Hinton broth (MHB) and calf serum. Geometric mean MIC ratios, serum:MHB, were 25.2:1 (M. haemolytica) and 27.4:1 (P. multocida). The degree of binding of oxytetracycline to serum protein was 52.4%. Differences between serum and broth MICs could not be accounted for by oxytetracycline binding to serum protein. In vitro time-kill data suggested a co-dependent killing action of oxytetracycline. The in vitro data indicate inhibition of the killing action of oxytetracycline by serum factor(s). The nature of the inhibition requires further study. The outcome of treatment with oxytetracycline of respiratory tract infections in calves caused by M. haemolytica and P. multocida may not be related solely to a direct killing action.
authors.affiliation: Royal Veterinary College, Hawkshead Campus, Hatfield, Hertfordshire AL9 7TA, United Kingdom.
authors.forename: P
authors.initials: P
authors.lastname: Lees
authors.affiliation: Royal Veterinary College, Hawkshead Campus, Hatfield, Hertfordshire AL9 7TA, United Kingdom.
authors.forename: J
authors.initials: J
authors.lastname: Illambas
authors.affiliation: Royal Veterinary College, Hawkshead Campus, Hatfield, Hertfordshire AL9 7TA, United Kingdom.
authors.forename: L
authors.initials: L
authors.lastname: Pelligand
authors.affiliation: Toxalim, Université de Toulouse, INRA, ENVT, 23, Chemin des Capelles-BP 87614, 31076 Toulouse, France. Electronic address: pltoutain@wanadoo.fr.
authors.forename: P-L
authors.initials: PL
authors.lastname: Toutain
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002417:Cattle; D002418:Cattle Diseases / Q000188:drug therapy* / Q000382:microbiology; D016978:Mannheimia haemolytica / Q000187:drug effects*; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D010118:Oxytetracycline / Q000494:pharmacology*; D016979:Pasteurella multocida / Q000187:drug effects*; D016871:Pasteurellaceae Infections / Q000188:drug therapy / Q000382:microbiology / Q000662:veterinary*
chemical_list: D000900:Anti-Bacterial Agents; D010118:Oxytetracycline
keywords: Bovine; Mannheimia haemolytica; Oxytetracycline; Pasteurella multocida; Pharmacodynamics