The Enumeration of Heated Bacterial Spores

Experiments with 13 strains of Bacillus isolated from canned foods haveshown that, like Clostridium species, the germination of heated spores is affectedby inhibitors present in the media. As with Clostridium species, susceptibilityto inhibitors increases with the amount of heating to which the spores are exposed.The Bacillus strains, however, show more variation in the extent to whichdifferent strains are inhibited in a particular medium. There is also evidenceof substantial differences in the amounts or types of inhibitors which are containedin different media. Some of the differences between media are due tovariations in the concentrations of inhibitors which are adsorbed on charcoal, starch,or serum albumin, but some of the differences are attributable to inhibitors whichare not adsorbed on charcoal. Treatment of nutrient agar with charcoal and subsequentremoval of the charcoal is as effective in removing inhibitors as incorporationof the charcoal in the medium. The medium, therefore, is the principal orsole source of inhibitors. Attempts to demonstrate inhibitors in the inoculumwere unsuccessful. It is unlikely that unsaturated fatty acids account for morethan part of the observed inhibition. A suitable adsorbent should be incorporatedin media used for evaluating the thermal destruction of Bacillus spores.