The Enumeration of Heated Bacterial Spores

Abstract

Experiments with 13 strains of types A and B Cl. botulinum have shownthat the detection and enumeration of spores surviving lethal heating becomeincreasingly difficult as the period of heating is increased. Media which arevery satisfactory for enumerating unheated spores may be unsatisfactory forenumerating spores which survive the heat treatment. By comparison withunheated spores the numerical estimates of spores surviving severe heat treatmentswere more dependent on thioglycollate in the medium and on the pHand temperature during incubation. As the amount of heating was increased,the surviving spores became progressively more sensitive to inhibitors· in themedium. These inhibitory substances were present in all media used, and theireffects were counteracted under a wide range of conditions by the incorporationof approximately 0.1 per cent. starch in the medium. The starch supplementproduced no significant effect on the estimates of unheated spores. Theeffects of starch are a feature of the amylose portion of the undegraded starchmolecule which, like charcoal or serum albumin, acts by adsorbing inhibitorsfrom the medium. The probability that at least part of the inhibition is dueto unsaturated fatty acids is discussed.