EFFECT OF CELL MOISTURE ON THE THERMAL INACTIVATION RATE OF BACTERIAL SPORES

Thermal inactivation rates were determined for two strains of Bacillus subtilis var. niger spores after equilibration to various relative humidities (RH). In these tests, small thin stainless steel squares were each inoculated with a drop of spore suspension and equilibrated to 11, 33, or 85% RH. Following equilibration, the squares were placed on a hot plate preheated to 108, 125, 136, 164, or 192C for various exposure times, then assayed for surviving organisms. The results revealed that spores of the A strain of B. subtilis were least resistant if pre-equilibrated to 11% RH and most resistant if pre-equilibrated to 85% RH. The same trend was obtained at all temperatures except 192C, at which no difference was noted, probably because the rapid kill time approaches the heat-up time of the stainless steel square. The B strain of B. subtilis spores showed an opposite relative humidity effect; that is, the cells pre-equilibrated to 11% RH were the most resistant. Because the two strains of spores were grown on different media, further studies were conducted at 136C after subculturing the cells on different media. When the B strain was subcultured on the A strain medium, the pattern was reversed; the cells pre- equilibrated to low relative humidity were then least resistant. Although it was not possible to reverse these cells to the original pattern by subculturing on the original B strain medium again, the pattern was altered to the point that there was no significant difference in heat resistance of these cells regardless of the pre-equilibration relative humidity.