Factors which Influence the Growth of Heat-treated Bacteria

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Abstract
The comparative numbers of colonies developing when pure cultures of heat-treated bacteria are plated on 4 agars (nutrient, standard milk, beef infusion and an ammonium phosphate-glucose synthetic) were studied. Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Staphylocoecus aureus, Streptococcus liquefaciens, Str. zymogenes and Str. durans were used. Essentially the same counts were obtained on all 4 media when unheated control cultures were plated, except in those cases in which the test organism failed to develop at all on the synthetic medium. When the organisms had been heated to the extent that a substantial percentage had been destroyed, the beef infusion medium usually gave the greatest count of surviving bacteria, and the new standard milk agar was better than the old nutrient agar. This medium was especially superior for some of the thermoduric lactic streptococci. The synthetic medium frequently gave counts lower than those obtained using the other media. The results from both beef infusion and synthetic media were erratic at times, indicating that factors other than gross chemical composition of the medium may be operative. The results indicate that bacteria which have been subjected to partially lethal heat treatments are more exacting in their growth requirements than are control unheated bacteria. This should be considered in formulation of media for enumeration of bacteria in heated food products and in expts. concerned with the effect of heat upon microorganisms.