Thermus aquaticus, Brock & Freeze, 1969

Brock, Thomas D. & Freeze, Hudson, 1969, Thermus aquaticus gen. n. and sp. n., a Nonsporulating Extreme Thermophile, Journal of Bacteriology 98 (1), pp. 289-297 : 295-297

Thermus aquaticus sp. n.

Definition as for genus. The specific epithet was derived from the Latin noun aqua, meaning water. Thus, T. aquaticus is an aquatic thermophile.

The type strain is YT-1 . It is being deposited in the American Type Culture Collection as ATCC 25104 . Also being deposited in this collection are strains Y-VII-51B as ATCC 25105 and Y-IV-69-2 as ATCC 25106 .

DISCUSSION

For many years, microbiologists have enriched for thermophilic bacteria by incubation at 55 C. It is quite clear, however, that thermophilic bacteria do not all grow optimally at 55 C; they represent a continuum of organisms from those with optima near the mesophilic range to those with optima of 70 C or above ( 2, 4, 5). At 55 C, T. aquaticus grows slowly and hence is probably unable to compete with the sporeforming thermophiles such as B. stearothermophilus . This could explain why it has not been seen in the usual thermophilic enrichments. At 70 to 75 C, Thermus has a selective advantage and can be easily isolated. It is important to emphasize that the media for Thermus enrichments must be fairly dilute in organic constituents, since the organism is inhibited by tryptone and yeast extract at a concentration of approximately 1 %. The high content of organic constituents characteristic of most media used for isolation of thermophiles may also explain why Thermus has not been seen before. The enrichment conditions prescribed here for Thermus are sufficiently selective that the isolation of new strains is extremely easy.

We do not imply that all yellow-pigmented organisms isolated by enrichment with our method will be members of a single species. Detailed study of our strains may reveal differences sufficient to warrant creation of other species or genera. For the moment, however, it seems preferable to classify these organisms in a single species.

The ecological relationships of Thermus need further work. Yellow- and orange-pigmented filamentous organisms which resemble Thermus morphologically are seen in large numbers in most mildly alkaline hot springs. In the temperature range of 50 to 73 C, these filamentous forms produce extensive gelatinous mats within which the unicellular blue-green alga Synechococcus is embedded (T. D. Brock, Phycologia, in press). We can routinely isolate Thermus from these mats but have not as yet shown that our isolates represent the predominant filamentous organism of the mats, although the physiological and morphological properties of Thermus are in agreement with those of the predominant organisms of the mats. At temperatures above 73 C, where blue-green algae do not grow, masses of filamentous bacteria are frequently seen (2; T. D. Brock, Symp. Soc. Gen. Microbiol. 19th, in press). Spheroplast-like structures similar to those formed by Thermus are often seen in these naturally growing filamentous bacteria (T. D. Brock, Symp. Soc. Gen. Microbiol. 19th, in press).

The ease with which Thermus can be isolated from hot tap water and other thermal sources suggests that the organism might be a good indicator of thermal pollution.

The determination of the relationship of Thermus to other bacteria must wait further studies. As a yellow-pigmented, nonmotile, gram-negative rod, the organism might be considered related to Flavobacterium , except that this latter genus is itself poorly defined (M. Mandel, personal communication). Also, the formation of long filaments is a property not found in flavobacteria. If the organism could be shown to glide, it might be considered related to the Flexibacterales ( 16), since this group comprises mostly yellow-pigmented,gram-negativeorganisms,many of which show a rod-filament dimorphism. However, the DNA base composition is considerably higher than that of the flexibacteria ( 8), and in fact is quite similar to that of the fruiting myxobacteria. In this respect the sensitivity of T. aquaticus to actinomycin D is noteworthy. Martin Dworkin has recently informed us that gliding bacteria are considerably more sensitive to actinomycin D than are other gram-negative bacteria. Since T. aquaticus shows a sensitivity to actinomycin D as great or greater than the gliding bacteria, this is anotherfeature which prompts a further study of the relationship of T. aquaticus to the gliding bacteria. One suggestion is that members of the species T. aquaticus represent forms which have lost the ability to glide, yet retain structural features which are responsible for actinomycin D sensitivity.

Filamentous thermophilic bacteria have been described before in hot springs ( 9, 12, 13) and have usually been given the name Leptothrix or Chlamydothrix (see also 2). Unfortunately, few of these filamentous forms were cultivated or adequately characterized; hence, the relationship of our isolates to these earlier forms is uncertain. Since our isolates do not form either a sheath or motile swarmer cells, it is clear that they bear little relationship to the Leptothrix-Sphaerotilus group ( 14). We have not seen sheathed organisms of the Leptothrix-Sphaerotilus group in any of our collections of natural material from hot springs. Further understanding of the taxonomic relationships of Thermus to other bacteria will require a better understanding of the taxonomic relationships of the wide variety of filamentous bacteria from nonthermal environments, most of which have been poorly characterized.