Objective: To determine the heat stability of antibiotics commonly used in food animals and agriculture in Thailand.

Materials and Methods: Seventeen commonly used antibiotic formulations for food animals and agriculture in Thailand were included. The antimicrobial activity of the tested antibiotics was determined by the minimum inhibitory concentration (MIC) of the antibiotics against S. aureus ATCC 29213, E. coli ATCC 25922 and B. subtilis ATCC 6633. The MIC of each antibiotic against each bacterium was performed by the broth microdilution method. The MICs of untreated antibiotic samples and heat-treated antibiotic samples after exposure to heat at 100°C for 30 minutes and/or after heating at 121°C for 15 minutes were determined. A heat-stable antibiotic was defined as one for which the MIC of the heat-treated antibiotic was not more than twofold the MIC of the untreated antibiotic. A heat-labile antibiotic was defined as one for which the MIC of the heat-treated antibiotic was more than twofold the MIC of the untreated antibiotic.

Results: Bacitracin and neomycin sulphate were heat-labile at 100°C and 121°C, whereas amoxicillin and amoxicillin/clavulanate seemed to be heat-stable at 100°C but were heat-labile at 121°C. The MICs of heated tetracycline samples at 121°C were two- to fourfold the MICS of untreated tetracycline samples. Enrofloxacin, gentamicin, kanamycin, tiamulin, tilmicosin, tylvalosin and colistin were quite stable to heat at 121°C.

Conclusion: Many antibiotics commonly used in food animals and agriculture in Thailand are heat-stable. Therefore, ingestion of heat-cooked foods containing heat-stable antibiotic residues could still present a threat for the emergence of antibiotic-resistant bacteria in humans.

Keywords: Heat Stability, Antimicrobial Agents, Food Animals, Agriculture