Comparative studies on antibiotic resistance in Lactobacillus casei and Lactobacillus plantarum

The levels of resistance in 17 Lactobacillus casei isolates and 15 Lactobacillus plantarum isolates to 10 antibiotics were determined using a standardized macrodilution method and the presence/absence of 20 genes implicated in antibiotic resistance in these isolates was determined by polymerase chain reaction (PCR) using gene-specific primers; 11 isolates possessed one or more of these genes but they were not always associated with phenotypic resistance. L. plantarum isolates had the widest spectrum of MIC values for streptomycin ranging from 16 to 512 μg/mL. In particular, two isolates of L. plantarum IMAU60045 and IMAU80091 both possessed aadA and ant(6) genes implicated in resistance to streptomycin but varied in their tolerance to streptomycin as evidenced by their minimum inhibitory concentration (MIC) of 16 and 256 μg/mL, respectively. Selection of high streptomycin resistance of L. plantarum IMAU60045 was performed over a 30 day period using serial passage with regular increases in streptomycin concentration to reflect the changes in resistance levels. Final MIC value of 16,384 μg/mL was recorded which was 1024-fold higher than the original parental isolate. Furthermore, associated variable degrees of increase in the MIC value for gentamicin, kanamycin and neomycin illustrated that, under the challenge of streptomycin, cross-resistance to other structurally related antibiotics of the same class developed. The relative quantity of gene expression (RQ) for the streptomycin resistance gene aadA was 3.35 times greater after passage in increasing concentrations of streptomycin than the original parental isolate. This was greater than the increase in the RQ value for the streptomycin resistance gene ant(6) after passage.