The Influence of Recovery Media on Thermal Resistance Values of Spores of a Putrefactive Anaerobic Bacterium

Yesair's pork infusion medium, as modified by Wynne and Foster (1948), has been extensively employed in thermal resistance studies of spores of Clostridium spe- cies.While this medium is excellent for the recovery of spores of these organisms, it is expensive and laborious to prepare.Recent unpublished data in this laboratory have demonstrated that counts of heat-activated spores of Putrefactive Anaerobe (P.A.) No. 3679 made in Eu- gonagar (B.B.L.) are higher than those made in Yesair's medium.In view of these results, a comparative study of various media for recovery of severely heat-treated spores of Putrefactive Anaerobe 3679 was carried out.Since this organism is widely used in the food industry as an experimental organism in evaluating process requirements for low-acid canned foods, a more com- plete determination of its thermal resistance has con- siderable practical significance. EXPERIMENTAL METHODS AND RESULTSPreparation of Spore Suspension Putrefactive Anaerobe (P.A.) No. 3679 was obtained from Dr. C. R. Stumbo.Spores were produced according to the method of Stumbo, Murphy, and Cochran (1950) in a medium containing Bacto liver infusion.After incubation at 29.4 C for 13 days, the suspension was homogenized in a Waring blendor and filtered through a glass-wool pad.The suspension was centrifuged, washed 5 times with sterile distilled water, resuspended in sterile distilled water, and placed in bottles containing about 1 in of glass beads.The suspension was stored at 4.4 C. Determination of Thermal ResistanceAn estimation of the spore concentration of the sus- pension was made with the thermoresistometer method of Stumbo et al. (1950).The average of four individual spore-count determinations made in Eugonagar was 1. 9 X 106 per milliliter.The spore suspension was diluted 1:10 in sterile distilled water and 0.01-ml ali- quots measured into thermoresistometer cups.Sets of I Scientific Paper No. 1379, Washington Agricultural Ex- periment Stations, Pullman.Project No. 1046.