*5.6. Moderate to Extreme Hot or Cold Environments*

Myxobacteria are mesophilic and grow well at 30 ◦C, although their temperature range is much wider. For most myxobacterial strains, the growth temperatures is between 4 ◦C and 44 ◦C. Usually vegetative cells cannot survive temperatures above 45 ◦C, but myxospores suspended in water tolerate 58–60 ◦C. This property can be used to be purify myxobacteria from mesophilic accompaniment organisms [12]. A moderate terrestrial habitat was investigated by Brockman in 1976 [114], who isolated strains of *Archangium*, *Chondromyces, Cystobacter*, *Myxococcus*, *Polyangium,* and *Stigmatella* from arid Mexican soils. He reported a greater species diversity from regions with higher annual rainfall (400–800 mm compared to 200–400 mm). Moderate thermophilic myxobacteria of Cystobacterineae and Sorangiineae-suborders, which grew very fast at temperatures of 42 ◦C–44 ◦C, were isolated from soil samples of semiarid and warm climates by Gerth and Müller [115] (Figure 12). One strain even grew at 48 ◦C, whereas the majority of the described species grows best at 30 ◦C.

**Figure 12.** Moderately thermophilic strains of *Sorangium* on VY/2 agar. (**a**) GT47 and (**b**) GT 41, isolated by Dr. K. Gerth.

Recently, a new *Nannocystis* species, *N. konarekensis*, was isolated from an Iranian desert [111]. The strain shows an optimal growth temperature at 37 ◦C, in contrast to the other known *Nannocystis* species *N. pusilla* and *N. exedens*, which show optimal growth at 30 ◦C. Iizuka et al. reported about enrichment and phylogenetic analysis of moderately thermophilic myxobacteria. During their search for thermophilic myxobacteria in geothermal environments, four strains that grew at temperatures up to 50 ◦C (optimum 45 ◦C–49 ◦C) could be isolated from various hot springs in Japan [116]. Three of the cultures were from fresh water hot springs and one was from a coastal saline spring. Even after repeated enrichment procedures, other thin film-like spreading bacteria accompanied the strains. PCR, cloning, and sequencing of 16S genes revealed that all cultivated bacteria belong to the order Myxococcales and showed between 89–99% homology to strains of myxobacteria. Therefore, some of these cultures represent new undescribed but cultivable species, genera, and perhaps even families (Figure 13).

Although numerous (cultivation-independent) studies about bacterial diversity of hot springs/geothermal sources are published, the NCBI search for sequences of uncultured thermophilic myxobacteria or myxobacteria from hot springs revealed only very few matches. Hot springs are probably not the most suitable habitat for the mainly mesophilic myxobacteria. But, based on the cultivation success mentioned by Iizuka et al. [116], it is certainly worth investigating these habitats more precisely to isolate new myxobacteria.

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**Figure 13.** Neighbour joining tree of some myxobacterial type strains (16S rRNA-genes) and cultures (in bold) from hot springs (AB246767-AB246770, AB246772) [117] and alkaline hot spring, all from Japan. Suborders of the order Myxococcales, origin of samples, and accession numbers are shown. Bar, 0.1 substitutions per nucleotide position.

Some publications deal with myxobacteria from cold environments like Arctic soils. However, in the study of Brockman who tried to isolate myxobacteria from Alaskan and Canadian Arctic soils, myxobacterial growth was only observed when soil plates were incubated at 24 ◦C–26 ◦C, but not at 6 ◦C–8 ◦C [118]. In contrast, Dawid described psychrophilic myxobacteria which grow at 4 ◦C but not under mesophilic conditions between 18 ◦C and 30 ◦C (after 7–9 month of incubation) on samples of Antarctic soils [117].

Due to long incubation times of psychrophilic strains, their biotechnological use in large scale fermentation would be expensive and time consuming and would only be worthwhile if a highly promising antibiotic was detected in such a psychrophilic strain.
