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.

Epidemiological and PK/PD cutoff values determination and PK/PD-based dose assessment of gamithromycin against Haemophilus parasuis in piglets

“…Supporting this notion, previous studies showed markedly lower azithromycin MICs against Pseudomonas aeruginosa in eukaryotic media and biological fluids such as serum and bronchoalveolar lavage fluid, compared with the recommended CA-MHB [15]. Similar observations were also noted for the potency of tulathromycin in calf serum against M. haemolytica and P. multocida [11]. This suggested that measuring MICs in eukaryotic media could be easily implemented to phenotypically detect acquired resistance to macrolides [16].…”

“…This resulted in the underestimate of gamithromycin clinical efficacy by measuring MICs in broth medium. In fact, the serum effects on antimicrobial activity of macrolides have been reported for most fastidious respiratory pathogens [11,13]. In our study, the MICs of gamithromycin against H. parasuis in porcine serum were significantly lower than that in HTM.…”

“…Similar to other veterinary macrolides such as tulathromycin, clinical field trials have indicated that gamithromycin concentrations in serum can be below the MIC 90 for the target SRD pathogens while retaining good clinical efficacy [7,10]. It may be due to the significant differences of AST results between serum and broth media [11,12]. This resulted in the underestimate of gamithromycin clinical efficacy by measuring MICs in broth medium.…”

“…This suggested that measuring MICs in eukaryotic media could be easily implemented to phenotypically detect acquired resistance to macrolides [16]. In addition, the serum effects may involve specific antibody and complement activities [11,12]. Heat-inactivation of serum resulted in a 500-fold increase in the MIC of gamithromycin against Mycoplasma mycoides compared with normal serum [17].…”

“…Escherichia coli ATCC 25922 served as the quality control strain. The composition of serum and HTM broth differs significantly in electrolytes and albumin concentrations [11]. To investigate the effect of serum matrix on gamithromycin susceptibility, further MIC and MBC determinations were undertaken for 19 selected H. parasuis isolates in both porcine serum and HTM broth.…”