The importance of inorganic ions for the heat stability of thermophilic bacteria was investigated. Cells of Bacillus stearothermophilus, strain 1503, were incubated at elevated temperatures in various media and the number of surviving organisms was determined at suitable intervals. The bacteria rapidly died at temperatures ordinarily employed for their cultivation if the surrounding medium lacked calcium ions. Besides calcium ions, potassium and phosphate ions and glucose, or some other energy source, seemed to be required for the heat stability of the cells. A chemically defined stabilizing medium with these components was developed for the above‐mentioned strain. When any component of this medium was excluded, the heat resistance of this organism was lost. This medium had a stabilizing effect also on the cells of three other strains of B. stearothermophilus. These requirements suggest that the heat stability of thermophilic bacteria is attributable to an active transport of calcium ions from the environment into the cells.

When cells of Bacillus stearothermophilus, strain NCA 1503, were grown in tryptone starch broth and subsequently transferred to tris buffer, a fraction of tbe celis: rapidly died in the buffer. This fraction increased with increasing content of calciiina chloride in the growth medium. The addition of sodiom, potassium or magnesium chloride to the growth medium had no such effect. The rapid dying of the cells in tris buffer "was associated with a leakage of organic material and calcium ions from the cells. The results obtained are probably caused by a damage to the osmotic barrier of the cells during their contact with the buffer. Observations: made during the present investigation and a previous one (Ljunger 1970) indicate that the heat resistance of thermophilic bacteria depends on the maintenance of a liigh intracellular concentration of free calcium ions.

The permeability of a psychrophilic Achromohacter strain to the chlorides of Na, K, Mg and Ca was investigated with light-scattering technique. Comparisons, were made with cells of Escherichia coli B. Cells of both strains suspended in w,ater were plasmolyzed by 0.1 or 0.2 M solutions of MgCfe or CaCl^ "without subsequent deplasmolysis. NaCI or KCI aiso plasmolyzed the cells, but deplasmolysis foliowed. When suspended in growth medium, E. coli became completely deplasmolyzed,, whereas the psychrophile still excluded MgClj and CaClg to a great extent. The plasnaolysis and deplasmolysis were reversible. Electron micrographs of the psychrophile exposed to CaCl2 confirmed the presence of plasnaolysis.

In an investigation of the oceitrrence oi' coliform haeteria on fresh vegetables a ntmiber of strains were isolated whicb sbowed a delayed gas production in ordinary lactose brotb at 'M'-('.. Several otber .strains did not produce gas al all uitder tbe same conditiotis, Wben grown in brilliant green bile lacto.se btoth at 'M C. all but a few strains produced ga.s within 24 hours. Sitice in botb nie R./l.) with the addition nf Iho carbohydrale at a h'vol of 1.0 |iei-eent. nrl pur|)le was used as indicator. Other media employed wore prepared according to flic recommendalions in Manual of Microhiol ogical Methods (1957). All media were sterilized by autoclavinf.; at 110°f or 20 minutes.Inoculum. With IVw exceptions test lubes containing' 10 ml. medinni were used hi ttie rxpt'iinifiits. Cells fiom a culliire tiept on Dit'co nutrient ajiar for ut least throe week.s at 10° were distributed in a medium identical to that nsed in the main experiment hul with the exclusion of the compound under examination. Each tube was inoculated with 0.1 nit. of this susponsi()n. No turhidity could he ohserved in the tuhes hel'oro inciihnlioii, tncitlKititin. In all ixpeririu'iits t!ie cullures were held in therniostaticiilty controlled wuler baths f±0.05") iu darkness, Men.'iiiremcnts. Tho pnxhictlon of visible gas was measured in percentafje nf gas in Durham tuhes. This inettiod proved to be sufficiently accurate for its purpose riiil.%h,!. Plant., 15, HICJ [148]

The inability of several obligately thermophilic bacteria to grow at or below 37°C in common laboratory media was shown to be caused by inhibitors present in these media. The inhibitors could be removed by extraction with chloroform and by adsorption on ion exchange resins or starch. It was concluded that they were of organic nature, probably bile acids. Four laboratory strains of Bacillus stearothcrtno‐philus grew readily at 37°C in detoxified media. With one of them (NCA 1503) growth was obtained at 34–35°C. A fresh soil isolate having a minimum growth temperature of 40°C in glucose nutrient broth grew at 30°C in this medium after the inhibitors had been removed.