title: A large potentiation effect of serum on the in vitro potency of tulathromycin against Mannheimia haemolytica and Pasteurella multocida.
abstract: The antimicrobial properties of tulathromycin were investigated for M. haemolytica and P. multocida. Three in vitro indices of antimicrobial activity, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and time-kill curves, were established for six isolates of each organism. Each index was measured in two growth media: Mueller-Hinton broth (MHB) and calf serum. It was shown that MICs and MBCs were markedly lower in serum than in MHB. MHB:serum ratios for MIC were 47:1 (M. haemolytica) and 53:1 (P. multocida). For both serum and MHB, adjustment of pH led to greater potency at alkaline compared to acid pH. Tulathromycin MIC was influenced by size of inoculum count, being 4.0- to 7.7-fold greater for high compared to low initial counts. It was concluded that for the purpose of determining dosages for therapeutic use, pharmacodynamic data for tulathromycin should be derived in biological fluids such as serum. It is hypothesized that in vitro measurement of MIC in broth, conducted according to internationally recommended standards, may be misleading as a basis for estimating the in vivo potency of tulathromycin.
authors.affiliation: The Royal Veterinary College, Hatfield, Herts, UK.
authors.forename: P
authors.initials: P
authors.lastname: Lees
authors.affiliation: The Royal Veterinary College, Hatfield, Herts, UK.
authors.forename: J
authors.initials: J
authors.lastname: Illambas
authors.affiliation: The Royal Veterinary College, Hatfield, Herts, UK.
authors.forename: T J
authors.initials: TJ
authors.lastname: Potter
authors.affiliation: The Royal Veterinary College, Hatfield, Herts, UK.
authors.forename: L
authors.initials: L
authors.lastname: Pelligand
authors.affiliation: The Royal Veterinary College, Hatfield, Herts, UK.
authors.forename: A
authors.initials: A
authors.lastname: Rycroft
authors.affiliation: UMR 1331 Toxalim INRA-INPT, École Nationale Vétérinaire de Toulouse, Toulouse, France.
authors.forename: P-L
authors.initials: PL
authors.lastname: Toutain
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002417:Cattle; D003470:Culture Media; D004187:Disaccharides / Q000494:pharmacology*; D006571:Heterocyclic Compounds / Q000494:pharmacology*; D016978:Mannheimia haemolytica / Q000187:drug effects* / Q000254:growth & development; D008826:Microbial Sensitivity Tests; D016979:Pasteurella multocida / Q000187:drug effects* / Q000254:growth & development; D018942:Macrolides
chemical_list: D000900:Anti-Bacterial Agents; D003470:Culture Media; D004187:Disaccharides; D006571:Heterocyclic Compounds; C485204:tulathromycin; D018942:Macrolides