Abstract
We evaluated the minimum inhibitory concentrations of clindamycin and erythromycin toward 98 Bacillus licheniformis strains isolated from several types of fermented soybean foods manufactured in several districts of Korea. First, based on recent taxonomic standards for bacteria, the 98 strains were separated into 74 B. licheniformis strains and 24 B. paralicheniformis strains. Both species exhibited profiles of erythromycin resistance as an acquired characteristic. B. licheniformis strains exhibited acquired clindamycin resistance, while B. paralicheniformis strains showed unimodal clindamycin resistance, indicating an intrinsic characteristic. Comparative genomic analysis of five strains showing three different patterns of clindamycin and erythromycin resistance identified 23S rRNA (adenine 2058-N6)-dimethyltransferase gene ermC and spermidine acetyltransferase gene speG as candidates potentially involved in clindamycin resistance. Functional analysis of these genes using B. subtilis as a host showed that ermC contributes to cross-resistance to clindamycin and erythromycin, and speG confers resistance to clindamycin. ermC is located in the chromosomes of strains showing clindamycin and erythromycin resistance and no transposable element was identified in its flanking regions. The acquisition of ermC might be attributable to a homologous recombination. speG was identified in not only the five genome-analyzed strains but also eight strains randomly selected from the 98 test strains, and deletions in the structural gene or putative promoter region caused clindamycin sensitivity, which supports the finding that the clindamycin resistance of Bacillus species is an intrinsic property.
Highlights
To clarify the genomic background explaining clindamycin resistance of B. licheniformis, we evaluated the minimum inhibitory concentrations (MICs) of clindamycin and erythromycin toward an additional 98 B. licheniformis strains isolated in Korea and undertook comparative genomic analysis of five strains exhibiting different patterns of clindamycin and erythromycin resistance
B. paralicheniformis was separated from B. licheniformis on the basis of phylogenomic and phylogenetic studies [20]. spo0A sequence analysis separated 24 strains as B. paralicheniformis from the 98 strains previously identified as B. licheniformis
We found a report of synergistic effects between biogenic polyamines and antibiotics on the antibiotic susceptibility of clinically-relevant bacteria [25]
Summary
For the analysis of putative promoter sequences of speG, the primer set PspeG-Up/PspeG-Down was designed based on the genome sequence of strain 14DA11 (Table 1). Total DNAs of B. paralicheniformis strains 14DA11 and KJ-16T were used as the templates for PCR amplification of potential clindamycin resistance genes. If independent determinants of clindamycin and erythromycin resistance contribute to the phenotype of Bacillus strains, four combinations of phenotypes can be exhibited.
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