Optimization of an antibiotic sensitivity assay for Mycoplasma hyosynoviae and susceptibility profiles of field isolates from 1997 to 2011

Mycoplasma hyosynoviae is a common agent responsible for polyarthritis leading to decreased production in swine herds worldwide. Antimicrobial agents are used to combat infections; however breakpoints for M. hyosynoviae have not yet been established. A number of methods have previously been utilized to analyze minimum inhibitory concentrations (MICs) for antibiotics against M. hyosynoviae; however these techniques as currently described are not easily standardized between laboratories. A dry microbroth dilution method was conducted to compare the minimum inhibitory concentrations (MICs) for 18 antibiotics, representative of different classes, against 24 recent isolates (23 field isolates and the type strain) of M. hyosynoviae. The MICs were determined using standard, commercially available 96-well Sensititre® plates containing various freeze-dried antibiotics at a range of concentrations appropriate to their potency. Clindamycin (CLI), a lincosamide antibiotic, showed the highest activity and most consistent inhibition for all isolates with an MIC50 of ≤0.12 μg/ml. Tiamulin (TIA), a pleuromutilin derivative, exhibited an MIC50 of ≤0.25 μg/ml. The isolates had similar levels of susceptibility to the quinolones, enrofloxacin (ENRO) and danofloxacin (DANO), exhibiting an MIC50 of 0.25 μg/ml and 0.5 μg/ml, respectively. For the macrolides, the MIC50 for tylosin (TYLT) and tilmicosin (TIL) was ≤0.25 μg/ml and ≤2 μg/ml respectively, but was ≤16 μg/ml for tulathromycin (TUL). For the aminoglycosides, the MIC50 for gentamicin (GEN) was ≤0.5 μg/ml, while spectinomycin (SPE) and neomycin (NEO) had an MIC50 of ≤4 μg/ml. The tetracyclines, oxytetracycline (OXY) and chlortetracycline (CTET) both had an MIC50 of ≤2 μg/ml. Florfenicol (FFN) exhibited a MIC50 of ≤1 μg/ml. All isolates were resistant to penicillin (PEN), ampicillin (AMP), ceftiofur (TIO), trimethoprim/sulfamethoxazole (SXT), and sulphadimethoxine (SDM) at all concentrations. Within the isolates tested, there was a range of sensitivity detected, with some isolates being overall more resistant while others appeared more susceptible. Further research is required to demonstrate how this MIC data correlates to clinical efficacy of the various antibiotics in the field.