Of approximately 300 cultures of Salmonella, representing 75 different serotypes, none was found to be as heat-resistant as S. senftenberg 775W. However, S. blockley 2004 was 5 times more heat-resistant and S. senftenberg 775W was 30 times more heat-resistant than S. typhimurium Tm-i, the reference strain in this study. All other strains of Salmonella tested, including 19 strains of S. senftenberg and 7 strains of S. blockley, had decimal reduction times at 57 C of about 1 min, equivalent to that of the reference organism, Tm-1. As observed in other bacterial species, strain 775W is more heat-sensitive in the log phase than in the stationary phase of growth. Cells from cultures grown at 44 C were more heat-resistant than those grown at either 35 or 15 C; the medium of growth, whether minimal or complex, made no appreciable difference in heat resistance. Cells from cultures limited by a carbon source were killed at a much slower rate than those limited by a nitrogen source and exhibited a 1-hr lag at 55 C before a significant rate of kill was attained. For any given set of growth conditions, strain 775W was always more heat-resistant than another strain of S. senftenberg, 197B, which has normal heat resistance.

Bean (Phaseolus vulgaris L. cv. Dwarf) roots were inoculated with Rhizobium phaseoli and colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomusfasciculatum Gerd. and Trappe or left uncolonized as controls. The symbiotic associations were grown in an inert substrate using 0, 25, 50, 100, or 200 milligrams hydroxyapatite (HAP) (CaioIPO4161OH12) per pot as a P amendment. Plant and nodule dry weights and nodule activity increased for both VAM and control plants with increasing P availability, but values for VAM plants were significantly lower in all parameters than for controls. Inhibition of growth and of N2 fixation in VAM plants was greatest at the lowest and highest P regimes. It was smallest at 50 milligrams HAP, where available P at harvest (7 weeks after planting) was 5 micrograms P per gram substrate. At this level of P availability, the association apparently benefited from increased P uptake by the fungal endophyte. Percent P values for shoots, roots, and nodules did not differ significantly (p > 0.05) between VAM and control plants. The extent of colonization, fungal biomass, and the fungus/association dry weight ratio increased several fold as HAP was increased from 0 to 200 milligrams. It is concluded that intersymbiont competition for P and photosynthate was the prunary cause for the inhibition of growth, nodulation, and nodule activity in VAM plants. Impaired N2 fixation resulted in N stress which contributed to inhibition of host plant growth at all levels of P availability.Enhancement of N2 fixation by root nodules as a result of improved P nutrition is well documented (12,18,20), and appears to depend on the high P requirement of the bacteriods (3). When the availability of P is low, increased P uptake in legumes colonized by VAM2 fungi enhances host plant growth and also stimulates N2 fixation (2,12,14). This has been defined as mycotrophic growth (15 significant sink for carbohydrates (6). Nodulation and N2 fixation have also been shown to depend directly on the availability of carbohydrates (9). Competition for photosynthates by the microsymbionts of the tripartite legume/Rhizobium/VAM fungal association therefore appears likely.The benefits of enhanced nutrient uptake by VAM fungi may be counteracted by the loss of carbohydrates from the host to the fungal endophyte (21). The concentration of P in the substrate appears to be crucial in determining whether VAM colonization will be beneficial, detrimental, or will occur at all. When P is extremely limiting, growth of both symbionts is inhibited (12). When P availability is low, enhanced growth of the host occurs (mycotrophy; Ref. 15). At intermediate levels of P, fungal proliferation may be at the expense of the host without enhancing P uptake ( 13), while at the high levels of P fungal growth is inhibited (21). In a nonsorbing medium, such as the sand-perlite culture with HAP as the P source used in the present investigation, P concentration at the absorbing root or fungal surface depends on the distance between absorbi...

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