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Field and laboratory observations (Frazier et al. 1935) long have indicated that the temperature of incubation of Swiss cheese starter cultures significantly influences their ability to develop following the rather severe heat exposure to which they are subjected during manufacture. If it were true that the temperature of growth had an effect on heat resistance of bacteria, this fact would be of significance in various fermentations, both commercial and natural, and would prove of general interest from the standpoint of the physiology of bacteria. Therefore, an investigation was initiated to determine the influence of incubation temperature and time on the thermal resistance of certain Swiss cheese starter cultures. For the purpose of comparison, similar studies were undertaken with a typical strain (H-52) of Escherichia coli. The results of the investigations on E. coli are presented in this paper. Because it produced less acid, was able to develop under a wider variety of environmental conditions, and could be counted fairly accurately by the plate method, E. coli gave results which were more conclusive than those obtained with the lactic starter cultures and indicated that E. coli, a favorite subject for experimentation, is an ideal organism for studies on the heat resistance of vegetative cells. Despite the apparent importance of the relationship between the growth temperature and thermal resistance of bacterial cells, iThis work has been aided by a grant from the Wisconsin Alumni Research Foundation.

TROLLER, JOHN A. (University of#Wisconsin, Madison) AND W. C. FRAZIER. Repression of Staphylococcus aureus by food bacteria. I. Effect of environmental factors on inhibition. Appl. Microbiol. 11:11-14. 1963.-The effects of environmental factors on the inhibition of an enterotoxin-producing strain of Staphylococcus aureus by food bacteria were investigated. Type of medium and temperature of incubation were important factors in determining the amount of inhibition. The pH range of maximal inhibition was found to be 7.4 to 6.2. Availability of oxygen was not a factor. As the ratios of inhibitor to staphylococcus were increased from 1:1 to 10:1 and 100:1, the amount of inhibition was markedly increased. Inhibition occurred in custard, where it increased with increasing ratios of effector to staphylococcus. The repression of the staphylococcus in all media usually was sufficient to be of practical significance.

GRAVES, R. R. (Uniiversity of Wiscoinsin,-Madison), AND W. C. IXRAZIER. Food microorganisms influencing the growth of Staphylococcus aureus. Appl. MIicrobiol. 11:513-516. 1963. Some 870 cultures of predominating microorganisms were isolated from market samples of hamburger, fresh pork sausage, fresh fish fillets, stewing beef, frozen chicken pot pie, frozen corn, frozen peas, and pasteurized and raw milk, before and after storage at different temperatures. The isolates were screened for their ability to influenice the growth of Staphylococcus aureus strain 196E by means of spot-plate tests on AlT and nutrient agars at 25 C. The 438 cultures that influeniced the growth of S. aureus were retested on spot plates at 15, 30, and 42 C. After elimination of replicates, the 143 remaining cultures were classified into species, genera, or groups, and 14 differenit cultures were tested for their influence on the growth of S. aureus in APT broth at 23 C. Over half of the effective cultures inhibited S. aureus and less than half were stimulatory. Pork sausage had the highest proportioni of inhibitory cultures, arid stewing beef had the lowest. APT agar was better than nutrient agar for screening, and incubationi at 15 C gave more effector organisms than at 30 and 42 C. .\Iost of the lactic acid bacteria were inhibitory,

Summary Pseudomonas and Achromobacteraceae cultures, mostly from foods, were tested for their effect on the growth of Staphylococcus aureus 196E in Trypticase Soy Broth at different temperatures (10 to 30 C) and with different ratios of effectors to staphylococci in inocula. Most cultures inhibited the staphylococcus, with inhibition becoming greater with decreasing proportions of S. aureus in the inoculum and decreasing temperatures of incubation, but inhibition usually was not as great as had been found with most coliform and lactic acid bacteria. Only a few of the Pseudomonas cultures could keep numbers of S. aureus below 5 × 106 cells per ml, even with an initial ratio of effectors to staphylococci of 100 : 1 and a low incubation temperature, although most cultures of Pseudomonas and Achromobacteraceae delayed the attainment of these numbers. Especially effective in inhibition of S. aureus were strains of Pseudomonas striata and P. mildenbergii or convexa, and a culture of Alcaligenes viscolactis. At 15 C Pseudomonas fluorescens, Alcaligenes faecalis, and Achromobacter xerosis stimulated S. aureus enough during early growth to hasten the attainment of hazardous numbers of staphylococci by several hours. At 15, 25 and 30 C most cultures, however, delayed the growth of S. aureus, and all kept maximal numbers of staphylococci below those reached by the coccus growing alone, although numbers usually were less by only about one- to two-thirds. Most strains of two Pseudomonas species affected S. aureus similarly, and the effects of eight species of effectors on two strains of S. aureus were, for the most part, similar.

Earlier work on the growth of Lactobacillus bulgaricus at low temperatures (Voss and Frazier, 1945) had indicated that dissociation might have been affecting the results. Therefore a study was begun of the dissociation of some of the lactobacilli and its effect on their characteristics in the hope that the information obtained would prove helpful in the production and use of starter cultures for cheese. LITERATURE Most workers agree that dissociation is the change of colony form from smooth to rough or rough to smooth, with or without the appearance of intermediate types of colonies, and whether or not other characteristic reactions of the organism are altered. Dissociation does not necessarily imply a change in other than the type of colony, but various characteristics are frequently reported as being correlated with different phases or types (Hadley, 1937). Because of the usual association of virulence and nonvirulence with smooth and rough cultures most of the reports in the literature deal with dissociation in pathogenic bacteria. Although different types of colonies of lactobacilli had been observed and reported, it was not until 1930 that definite reference to dissociation, as such, was made. At that time Hadley, Bunting, and Delves (1930) reported rough, smooth, and intermediate types of colonies, and observed dissociation from

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