Measuring antibiotic resistance in mixed cultures: Isothermal microcalorimetry as a novel analytical tool

Most Swiss cheese varieties use raw, undefined mixed cultures, which are composed of strains of Lactobacillus delbrueckii subsp. lactis and Streptococcus thermophilus. Single strains of these consortia are not always available. The aim of this study was to determine the antibiotic resistance of such consortia and propose a new approach to assess the antibiotic resistance. In this work, we developed a new method using isothermal microcalorimetry to assess antibiotic resistance in raw mixed cultures. For most of these cultures, no transferable antibiotic resistance genes were detected. In most cases, the results showed that all raw mixed cultures have lower minimal inhibitory concentration (MIC) than the European Food Safety Authority-defined levels, with the exception of the aminoglycosides kanamycin and streptomycin where the MIC was >64 mg L−1 for each raw mixed culture.