Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/15/1997
Publication Date: N/A
Citation: Interpretive Summary: Insect-parasitic nematodes are microscopic worms that are potential alternatives to chemical insecticides. These nematodes kill pest insects by injecting them with bacteria that are fatal to insects but completely harmless to humans. One problem with agricultural use of these nematodes is that they and their associated bacteria poorly survive cold environments. Because the types of fats in the membranes of the nematodes affect the ability of the membranes to remain fluid when exposed to cold temperatures, the fats affect the survival of the nematodes when exposed to freezing temperatures. Thus, in this research, the various kinds of fats in the membranes of the bacteria were determined, as was the fluidity of membranes from bacteria grown at different temperatures. Surprisingly, the ability of the membranes to remain fluid when exposed to cold temperatures did not change when the bacteria were grown at different temperatures, although the fat compositions changed. This discovery about the poor adaptation of bacteria to cold temperatures will be used by researchers and industrial microbiologists interested in developing better strains of nematode-associated bacteria for use in biological control of insects. The public will eventually benefit when this technology provides safe control of insect pests.
Technical Abstract: Primary and secondary forms of Photorhabdus luminescens Hm and Xenorhabdus nematophilus N2-4 were grown at 18 C and 28 C, and determinations were made of the fatty acid composition of total lipids and the fluidity of 5-doxylstearic acid embedded in vesicles made from total lipids. The levels of the unsaturated fatty acids 16:1 and 18:1 generally were higher in primary phase variants of P. luminescens grown at 18 C than at 28 C. The ratio of saturated to unsaturated fatty acids rose with prolonged culture times in both phase variants of each species. When grown at 18 C, the proportion of 16:1 in X. nematophilus was lower than in P. luminescens; the patterns of temperature-induced changes were similar in each species. Xenorhabdus nematophilus contained a greater percentage of short-chain fatty acids (i.e., chain length <14.0) than P. luminescens. Lipid vesicles from primary and secondary cultures of both bacterial species grown at 18 C were more ordered (i.e., less fluid) than those grown at 28 C. This result suggests the surprising absence of homeoviscous adaptation of membranes to temperature. Also, vesicles from primary cultures were more ordered than those from secondary cultures, and membranes from primary cultures of P. luminescens were more ordered at both culture temperatures than membranes from X. nematophilus.