Pharmacodynamics of antimicrobials against Mycoplasma mycoides mycoides small colony, the causative agent of contagious bovine pleuropneumonia

Abstract

Background: Mycoplasma mycoides subspecies mycoides Small Colony (MmmSC) is the causative agent of Contagious Bovine Pleuropneumonia (CBPP), a disease of substantial economic importance in sub-Saharan Africa. Failure of vaccination to curtail spread of this disease has led to calls for evaluation of the role of antimicrobials in CBPP control. Three major classes of antimicrobial are effective against mycoplasmas, namely tetracyclines, fluoroquinolones and macrolides. Therefore, the objectives of this study were to determine the effector kinetics of oxytetracycline, danofloxacin and tulathromycin against two MmmSC field strains in artificial medium and adult bovine serum.

Methods: Minimum inhibitory concentrations (MIC) were determined for oxytetracycline, danofloxacin and tulathromycin against MmmSC strains B237 and Tan8 using a macrodilution technique, and time-kill curves were constructed for various multiples of the MIC over a 24 hour period in artificial medium and serum. Data were fitted to sigmoid E(max) models to obtain 24 hour-area under curve/MIC ratios for mycoplasmastasis and, where appropriate, for mycoplasmacidal activity and virtual mycoplasmal elimination.

Results: Minimum inhibitory concentrations against B237 were 20-fold higher, 2-fold higher and approximately 330-fold lower in serum than in artificial medium for oxytetracycline, danofloxacin and tulathromycin, respectively. Such differences were mirrored in experiments using Tan8. Oxytetracycline was mycoplasmastatic against both strains in both matrices. Danofloxacin elicited mycoplasmacidal activity against B237 and virtual elimination of Tan8; similar maximum antimycoplasmal effects were observed in artificial medium and serum. Tulathromycin effected virtual elimination of B237 but was mycoplasmastatic against Tan8 in artificial medium. However, this drug was mycoplasmastatic against both strains in the more physiologically relevant matrix of serum.

Conclusions: Oxytetracycline, danofloxacin and tulathromycin are all suitable candidates for further investigation as potential treatments for CBPP. This study also highlights the importance of testing drug activity in biological matrices as well as artificial media.

Figure 1. Minimum inhibitory concentrations.

Minimum inhibitory concentrations for oxytetracycline (OTC), danofloxacin (DANO) and tulathromycin (TUL) against MmmSC strain B237 in artificial medium using microdilution and macrodilution techniques (inoculum size 10⁷ cfu/mL). Values were based on either just one set of doubling dilutions (micro, macro) or five overlapping sets of doubling dilutions (micro overlapping, macro overlapping).

Figure 2. Effect of inoculum size on MIC.

Ratio of MIC at different inoculum sizes to MIC at 10⁷ cfu/mL for oxytetracycline (OTC), danofloxacin (DANO) and tulathromycin (TUL) against MmmSC strain B237 in artificial medium.

Figure 3. Time-kill curves.

Representative time-kill curves for (A) danofloxacin, (B) oxytetracycline and (C) tulathromycin in artificial medium, and (D) danofloxacin, (E) oxytetracycline and (F) tulathromycin in adult bovine serum against MmmSC strain B237.

Figure 4. Sigmoid E_max models.

Sigmoid E_max relationships for antimycoplasmal effect (E, log₁₀ (cfu/mL)) versus in vitro AUC:MIC ratio, derived from data obtained from time-kill curves for (A) danofloxacin (B) oxytetracycline and (C) tulathromycin in artificial medium, and (D) danofloxacin, (E) oxytetracycline and (F) tulathromycin in adult bovine serum against MmmSC strain B237.