2023 年 58 巻 2 号 p. 48-59
This study aimed to elucidate the genetic characteristics of lincomycin (LCM) resistance in the absence of erythromycin (EM) resistance in fish pathogenic Lactococcus garvieae serotype I. Two novel lsa(D) variants were found in clinically LCM-sensitive and resistant strains. The amino acid sequences of the two Lsa(D) variants shared 94.77% identity with that of Lsa(D) in L. garvieae serotype II. One of the lsa(D) variants, designated as lsa(D)36A, conferred cross-resistance to lincosamides, streptogramins A, and pleuromutilin, which was defined as the LSAP-resistant phenotype. This phenotype was confirmed by the minimum inhibitory concentrations of the lsa(D)36A-disruption mutant Δlsa(D)36A, and its complementation strain. However, a single-point mutation, which led to an amino acid substitution, was found in lsa(D)36D in clinically LSAP-sensitive strains. In addition, single-nucleotide replacement of lsa(D)36D alleles was identified in laboratory-induced LCM-resistant mutants. Meanwhile, the lsa(D)36D or lsa(D)36A was also detected in EM-resistant strains carrying erm(B). These strains carried lsa(D)36D with erm(B) or lsa(D)36A with erm(B), which conferred the EM- and LCM-resistant (ML-resistant) or MLSAP-resistant phenotype, respectively. In conclusion, lsa(D)36D was an intrinsic in the LSAP-sensitive L. garvieae serotype I, and its mutated allelic variant was named as lsa(D)36A, which explains the characteristic resistance of this strain to the LSAP-resistance phenotype.