Biological indicators for low temperature steam and formaldehyde sterilization: effect of variations in recovery conditions on the response of spores of Bacillus stearothermophilus NCIMB 8224 to low temperature steam and formaldehyde

Abstract

Spores of a Bacillus stearothermophilus strain thought to be a good biological monitor for low temperature steam and formaldehyde (LTSF) sterilization have been subjected to different recovery environments following exposure to relevant LTSF treatments and prior to enumerating the numbers of surviving spores. The recovery treatments essentially consisted of holding samples at a temperature of 90°C for different time periods prior to plating out following exposure to 12 μg ml-1 formaldehyde at temperatures between 63° and 83°C The data indicate that LTSF-exposed spores show a marked recovery when kept at 90°C prior to plating out; the extent of the recovery increases with increasing inactivation temperature between 73° and 83°C. The data bring into question the sporicidal activity of LTSF.