Antimicrobial Susceptibility of Bacillus Strains Isolated from Primary Starters for African Traditional Bread Production and Characterization of the Bacitracin Operon and Bacitracin Biosynthesis

Abstract

ABSTRACTBacillusspp. are widely used as feed additives and probiotics. However, there is limited information on their resistance to various antibiotics, and there is a growing concern over the transfer of antibiotic resistance genes. The MIC for 8 antibiotics was determined for 85Bacillusspecies strains,Bacillus subtilissubsp.subtilis(n= 29),Bacillus licheniformis(n= 38), andBacillus sonorensis(n= 18), all of which were isolated from starters for Sudanese bread production. All the strains were sensitive to tetracycline (8.0 mg/liter), vancomycin (4.0 mg/liter), and gentamicin (4.0 mg/liter) but resistant to streptomycin. Sensitivity to clindamycin, chloramphenicol, and kanamycin was species specific. The erythromycin resistance genesermDandermKwere detected by PCR in all of the erythromycin-resistant (MIC, ≥16.0 mg/liter)B. licheniformisstrains and one erythromycin-sensitive (MIC, 4.0 mg/liter)B. licheniformisstrain. Several amino acid changes were present in the translatedermDandermKnucleotide sequences of the erythromycin-sensitive strain, which could indicate ErmD and ErmK protein functionalities different from those of the resistance strains. TheermDandermKgenes were localized on an 11.4-kbp plasmid. All of theB. sonorensisstrains harbored the bacitracin synthetase gene,bacA, and the transporter genebcrA, which correlated with their observed resistance to bacitracin. Bacitracin was produced by all the investigated species strains (28%), as determined by ultra-high-definition quadrupole time-of-flight liquid chromatography-mass spectrometry (UHD-QTOF LC/MS). The present study has revealed species-specific variations in the antimicrobial susceptibilities ofBacillusspp. and provides new information on MIC values, as well as the occurrence of resistance genes inBacillusspp., including the newly described speciesB. sonorensis.