SUMMARY: Four Gram-negative, non-sporulating, aerobic, obligate thermophilic bacteria, isolated from non-thermal environments by enrichment with -heptadecane as substrate, utilized -alkanes, carbohydrates and organic acids as sole source of carbon and energy and also grew on complex media. The growth rate of these organisms, when utilizing -heptadecane as substrate, was markedly increased by adding a low concentration (7·5 mg 1-1) of yeast extract. They grew optimally between 55 and 65 °C, and at a pH between 6·2 and 7·5. The mol% G + C for all was between 51 and 58. On the basis of the amino acid and amino sugar compositions of their peptidoglycan, these organisms and other Gram-negative thermophilic bacteria can be divided into four distinct groups. Group A includes the newly isolated hydrocarbon-utilizing bacteria which have nearly equimolar amounts of glutamic acid, alanine, diaminopimelic acid and glucosamine. Group B consists of obligate hydrocarbon-utilizing microbes that have lower molar ratios of glutamic acid and diaminopimelic acid, and contain either ornithine or lysine. The previously isolated non-hydrocarbon-utilizing thermophiles (k-2, YT-1, x-1) and a newly isolated organism from a hot spring comprise group C and contain glycine, ornithine, no diaminopimelic acid, and much lower molar ratios of glutamic and muramic acids than in groups A and B. lacked peptidoglycan and is placed separately in group D.


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