Factors which Influence the Growth of Heat-treated Bacteria

Abstract

In many cases in which enumeration of heat-treated bacteria has been undertaken, the assuimption has been made that conditions satisfactory for the growth of unheated organisms are equally satisfactory for the growth of bacteria which have been subjected to heat treatment of sub-lethal intensity. The accuracy of this assumption is a point of great importance in the development of proper methods for the quantitative determination of bacteria in heat-processed food products and for physiological studies of the effect of heat on bacteria.In the case of certain samples of pasteurized milk and ice cream which gave plate counts that were many times higher on the new standard milk agar than on the old standard agar for milk, nearly all of the bacteria isolated from plates poured with the new medium were found to grow when inoculated into the old medium. The indications were that certain heat-treated bacteria are more exacting in their growth requirements than are normal unheated bacteria. Further knowledge of the basic factors which might influence the enumeration of heat-treated bacteria seemed desirable. The studies reported here were undertaken to demonstrate the effect of certain commonly-used agar media on the ability to initiate growth of pure cultures of bacteria which had been heat-treated with sub-lethal exposures. REVIEW OF LITERATUREThe literature contains numerous references to the appreciably higher plate counts on pasteurized dairy products obtained by the use of improved media (). In milk, the organisms in question usually are present as mixed cultures, and the increases in count usually have been attributed to the ability of an increased number of organism types to develop on the improved media (Bowers and Hucker, 1935;Bradfield, 1937;Foltz and Bushnell, 1940;Yale, 1938).A large portion of the literature concerned with the relationship of the medium to the survival of pure cultures of heat-treated bacteria deals with spore-forming organisms. Siipfle and Dengler (1916) found that the addition of 3 per cent glucose and 5 per cent horse serum to the basic beef-infusion broth used to test survival of anthrax spores markedly increased the time of heating with steam required to prevent development of the spores in the recovery medium.Contribution No. 216, Department of Bacteriology. 395 on August 1, 2020 by guest

Do different standard plate counting (IDF/ISSO or AOAC) methods interfere in the conversion of individual bacteria counts to colony forming units in raw milk?

“…According to Zoller (1923), two important properties of bacteria are readily identified when milk powder agar is used-acid formation from organic compounds and proteolytic activity; therefore, it is of great use in examination of dairy products. Nelson (1943) showed that certain types of bacteria had higher growth on milk powder agar compared with growth on conventional agar in more demanding conditions of heat treatment compared with normal temperatures. According to this author, if the bacteria count aims to determine the maximum number of microorganisms that are present in food samples, it is imperative to know the requirements for the growth of microorganisms.…”

(Symposium on Bacterial Spores: Paper VII). Recovering Spores Damaged by Heat, Ionizing Radiations or Ethylene Oxide

“…Heat treated spores have more exacting nutritional requirements than unheated spores (&man & Evans, 1937;Nelson, 1943), yet Morrison & Rettger (1930a, b) had been able to eliminate dormancy in 3 species of Bacillus by choice of a suitable medium, even aftei severe heat treatment.…”

A physicochemical model for the death‐rate of a micro‐organism

“…Other properties of the recovery medium also affect the apparent number of survivors and thus the apparent death-rate constants calculated. More survivors are observed using a rich, complex recovery medium than on a nutritionally more simple medium (Nelson, 1943); this implies that the process being observed by counting survivors on the simpler medium is the capacity to synthesize the specific nutrients present in the first, but absent in the second medium. Unless the recovery medium contains every metabolite required for all essential functions of the micro-organism, the process being studied as 'thermal death' must therefore be the thermal destruction of an enzyme system involved in the synthesis of a particular metabolite, or alternatively of the nucleic acids required for the synthesis of the enzyme.…”