Heat Resistance of Bacillus Spores at Various Relative Humidities

Abstract

The thermal resistance characteristics of spores from strains of five different Bacillus species were determined in phosphate buffer and at relative humidities ranging from <0.001 to 100% in a closed-can system. Spores tested in the closedcan system showed a marked increase in heat resistance over those in phosphate buffer, with the greatest increases occurring at relative humidities between 1 and 50%. When estimates of the time to reduce the initial spore concentration 99.99% (F value) at eight different relative humidities were plotted against temperature, three different types of heat resistance profiles were obtained, with maximum resistances at relative humidities of 1, 7, and 30%. When the various strains of spores were heated at the relative humidity of their maximum heat resistance, their relative order of heat resistance was different from that seen in buffer. Spores from the soil isolate were most resistant under these conditions (FI21.1 = 99.5 h). A limited number of studies of the thernal resistance of Bacillus spores in water vapor at various relative humidities (RH) have been conducted (1, 13, 18, 21). All but one of these studies focused on a single species. In every case, maximum heat resistance occurred at intermediate RH.

Effects of Environmental Humidity and Temperature on Sterilization Efficiency of Dielectric Barrier Discharge Plasmas in Atmospheric Pressure Air

“…15) For an example, the maximum heat resistance of B. atrophaeus occurs in a certain humidity range. 20) Therefore, both changes in the stress resistance of B. atrophaeus and the DBD plasma with variations of temperature and humidity should be considered as the mechanism of the inactivation.…”

Dry heat sterilization modelling for spacecraft applications

“…Analysis of the typical bioburden present in spaceship assembly conditions was considered low, with many isolates showing sensitivity to heat shocking (at 80°C for 15 mins) alone and those remaining not unexpectedly being identified as spore‐forming Bacillus and Paenibacillus genera (Seuylemezian et al, 2018). In 1972 some strains isolated from cleanroom environments demonstrated even higher resistance profiles to established B. atrophaeus strains (Reyes et al, 1981; Schubert & Beaudet, 2011). A specific example was Bacillus sp.…”

“…One isolate, originally identified as Bacillus sp. ATCC 29669 and subsequently speciated as B. canaveralius , exhibited an estimated 30 times greater resistant profile to dry heat than that of B. atrophaeus (Puleo et al, 1978; Reyes et al, 1981). Studies conducted at Jet Propulsion Laboratory, California Institute of Technology confirmed the unique high resistance of B. canaveralius spores to dry heat inactivation (Kempf et al, 2008; Schubert & Beaudet, 2011).…”