Susceptibility of Lactobacillus plantarum Strains to Six Antibiotics and Definition of New Susceptibility–Resistance Cutoff Values

The minimum inhibitory concentrations (MICs) of six antibiotics with activity against gram-positive bacteria (ampicillin, clindamycin, erythromycin, gentamicin, streptomycin, and tetracycline) were determined by microdilution and the Etest in 121 Lactobacillus plantarum strains of plant and dairy origin. MIC values for all antibiotics varied widely between strains. The analysis of both absolute MICs and their distribution was used to define new susceptibility–resistance cutoff values for all antibiotics, except for streptomycin. Based on these new cutoffs, the studied strains were nearly all identified as either susceptible (ampicillin, clindamycin, erythromycin, and gentamicin) or intrinsically resistant (streptomycin). The exceptions were four strains with MICs for tetracycline higher than the cutoff point (64 µg ml⁻¹); these were suspected to harbor acquired resistance determinants.