Mechanisms of Resistance to Clinically Significant Antibiotics in Bacillus Strains Isolated from Samples Obtained from a Medical Institution

Isolates of bacterial strains dominating on the surface of medical equipment used for blood sampling have been studied. Pure cultures of these bacteria have been identified as Bacillus cereus HSA01, Bacillus cereus HSA12, Bacillus cereus HSA03, Bacillus subtilis HSA06, and Bacillus amyloliquefaciens HSA09, and their resistance to a number of β-lactam antibiotics and spectinomycin has been determined. All strains are shown to be resistant to penicillin and ampicillin with the minimum inhibitory concentration (MIC) varying from 256 to 2048 μg/mL as well as to cephalosporin antibiotics with the MIC values varying from 2 to 2048 μg/mL. Resistance to spectinomycin administered to patients with allergic reactions to penicillin and cephalosporins is within the MIC range of 16–256 μg/mL. The ampicillin and cefuroxime resistance of B. cereus HSA01 is provided by the work of efflux pumps, while ceftazidime resistance is determined by the action of metal-β-lactamases (MBL); penicillin resistance is provided by the functioning of both mentioned systems. A high ampicillin and cefuroxime resistance of B. cereus HSA12 is provided by MBL and efflux activities, respectively, whereas ceftazidime resistance is determined by both MBL and efflux pumps. In the case of B. cereus HSA03, the observed resistance to penicillin, ampicillin, cefepime, and ceftazidime is explained by the efflux activity, resistance to cefazolin and ceftazidime is provided by MBL, while ampicillin and ceftazidime resistance is provided by both MBL and efflux. The penicillin and ampicillin resistance of B. subtilis HSA06 is provided by the MBL activity only. In B. amyloliquefaciens HSA09, resistance to ampicillin is provided by MBL and the action of efflux pumps, while penicillin resistance is provided only by the efflux activity. Thus, resistance to penicillin, ampicillin, and cephalosporin derivatives in the studied Bacillus group is provided, depending on the strain and the specific antibiotic, by metal-β-lactamase and/or efflux pumps representing secondary transporters.