The Resistance of Bacillus Spores: Implications for the Strain-Specific Response to High-Performance Disinfectants

Bacterial spores in materials and equipment pose significant biosecurity risks, making effective disinfection crucial. This study evaluated Ortho-phthalaldehyde (OPA) and a quaternary ammonia-glutaraldehyde solution (AG) for inactivating spores of Bacillus thuringiensis (BT), B. cereus (BC), and two strains of B. velezensis (BV1 and BV2). Spores of BV1 and BT were treated with 22.5 mg/m³ OPA by dry fumigation or 1 mg/mL AG by spray for 20 min, according to the manufacturer's recommendation. As no sporicidal effect was observed, OPA was tested at 112.5 mg/m³ for 40 min, showing effectiveness for BT but not for BV1. Minimum bactericidal concentration (MBC) tests revealed higher MBC values for glutaraldehyde, prompting an overnight test with 112.5 mg/m³ OPA by dry fumigation and 50 mg/mL AG by spray, using formaldehyde as a control. AG reduced all Bacillus strains, but with limited sporicidal effect. OPA was sporicidal for BT and BV1 but not for BC and BV2, indicating a strain-dependent effect. Formaldehyde performed better overall but did not completely inactivate BV2 spores. Our findings suggest that OPA and AG have potential as formaldehyde replacements in wet disinfection procedures.