Isolation frequency and antimicrobial susceptibilities of Bacillus species in a tertiary care hospital in Korea in the past four years (2020–2024): a retrospective surveillance study

Background: Identifying Bacillus spp. and performing antimicrobial susceptibility testing (AST) is challenging because of their diversity and limited clinical laboratory resources. We investigated the isolation frequency and antimicrobial susceptibility of Bacillus spp. over a 4-year period. Methods: Bacillus isolates collected between 2020 and 2024 were identified using matrixassisted laser desorption ionization-time of flight mass spectrometry, and AST was performed using the Pos Breakpoint Combo Panel Type 28 (Beckman Coulter). Results: Species among total isolates (n = 432) were B. cereus (25.1%), B. subtilis (11.8%), B. licheniformis (10.8%), B. pumilus (7.4%), B. simplex (6.9%), B. circulans (6.4%), and B. amyloliquefaciens ssp. plantarum (5.9%). Overall, 65% of all Bacillus isolates were obtained from patients aged ≥ 60 years. The isolation ratios of sterile body fluids, including blood, to non-sterile specimens, in decreasing order, were: B. licheniformis, 4.5; B. subtilis, 2.4; B. pumilus, 2.0; B. amyloliquefaciens ssp. plantarum, 2.0; B. circulans, 1.2; B. thuringiensis, 1.0; B. cereus, 0.76; B. simplex, 0.56; and B. infantis, 0.43. The overall antimicrobial resistance rates were as follows: penicillin, 57.1%; ampicillin, 52.4%; clindamycin, 31.6%; erythromycin, 9.0%; cotrimoxazole, 6.4%; tetracycline, 2.3%; ciprofloxacin, 1.9%; rifampicin, 1.1%; levofloxacin, 0.9%; vancomycin, 0.4%; gentamicin, 0.4%; and imipenem, 0.4%. Penicillin resistance was particularly high in B. thuringiensis (100%), B. cereus (93.5%), and B. licheniformis (71.4%). Clindamycin resistance was high in B. circulans, B. licheniformis, and B. pumilus at 81.8%, 71.4%, and 64.3%, respectively. Conclusion: Accurate identification and AST of Bacillus spp. are essential when they are isolated from invasive infections, as resistance profiles and isolated species vary significantly depending on the specimens.