Mutation of the gidB gene causes intrinsic streptomycin resistance in Bacillus velezensis

Bacillus velezensis strain DMB07, isolated from the traditional fermented Korean soybean meju, exhibits resistance to streptomycin [minimum inhibitory concentration (MIC) 128 mg/L]. To shed light on the genetic background behind this phenotype, this study determined the complete genome sequence of strain DMB07 and compared it with the genomes of two B. velezensis strains that are sensitive to streptomycin. Compared with the streptomycin-sensitive strains, in strain DMB07 there was a mutation of a nucleotide (C58T) of the 16 S rRNA (guanine⁵²⁷-N⁷)-methyltransferase gene (gidB) that leads to a change in the amino acid sequence of the protein (Arg20Cys). This sequence of gidB gene was previously linked with streptomycin resistance. To test the hypothesis that this change in the gidB gene sequence of strain DMB07 confers streptomycin resistance, a temperature-sensitive plasmid, pIMAY-tgidB^T58C, was constructed for site-directed mutation (from thymine to cytosine) of nucleotide 58 of gidB in strain DMB07. The resulting strain, DMB07gidB^T58C, showed the decreased MIC value (32 mg/L) against streptomycin. Furthermore, introduction of the wild-type gidB gene into strain DMB07gidB^T58C resulted in recovery of the MIC for streptomycin to 128 mg/L. Thus, a single mutation of the nucleotide sequence of the gidB gene can confer resistance to streptomycin.