The minimum inhibitory concentration (MIC) of 16 antibiotics for 36 strains of Lactobacillus brevis, L. paracasei, L. paraplantarum and L. plantarum was determined by microdilution. Strains suspected to be resistant to particular antibiotics were further assayed by the Etest method. Taking as a cut-off presently defined breakpoints, atypical MICs compatible with acquired resistance were recorded by for tetracycline (in 2 strains), clindamycin (in 1 strain), chloramphenicol (in 3 strains) and trimethoprim (in 2 strains). However, MICs of these antibiotics were only slightly higher than the breakpoints, suggesting the absence of dedicated resistance mechanisms. In addition, resistance genes encoding ribosome protection proteins were not identified by PCR among strains displaying the highest MICs of tetracycline. In contrast, genes of this class were detected when total microbial DNA from the cheeses was used as the template. Resistant and doubtful strains should be excluded when selecting candidates for making new adjunct cultures.

This research was founded by projects from the Vice Chancellor for Research, University of Tehran, lran (Grant no. 61 22-TNS2007), and the Spanish Ministry of Science and lnnovation (Reference, AGL2007-61869-ALI).

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