Evaluation of the Robustness Under Alkanol Stress and Adaptability of Members of the New Genus <i>Halopseudomonas</i>

Many species of the genus <i>Pseudomonas</i> are known to be highly tolerant to solvents and other environmental stressors. Based on phylogenomic and comparative genomic analyses, several <i>Pseudomonas</i> species were recently transferred to a new genus named <i>Halopseudomonas</i>. Because of their unique enzymatic machinery, these strains are being discussed as novel biocatalysts in biotechnology. In order to test their growth parameters and stress tolerance, five <i>Halopseudomonas</i> strains were assessed regarding their tolerance toward different <i>n</i>-alkanols (1-butanol, 1-hexanol, 1-octanol, 1-decanol), as well as to salt stress and elevated temperatures. The toxicity of the solvents was investigated by their effects on bacterial growth rates and presented as EC50 concentrations. Hereby, all <i>Halopseudomonas</i> strains showed EC50 values up to two-fold lower than those previously detected for <i>Pseudomonas putida</i>. In addition, the activity of the <i>cis-trans</i> isomerase of unsaturated fatty acids (Cti), which is an urgent stress response mechanism known to be present in all <i>Pseudomonas</i> species, was monitored in the five <i>Halopseudomonas</i> strains. Although several of the tested species were known to contain the <i>cti</i> gene, no significant phenotypic activity could be detected in the presence of the assayed stressors. A bioinformatic analysis of eight <i>cti</i>-carrying <i>Halopseudomonas</i> strains examining promotor binding sites, binding motifs and signal peptides showed that most of the <i>cti</i> genes have a lipoprotein signal peptide and promotor regions and binding motifs that do not coincide with those of <i>Pseudomonas</i>. These insights represent putative reasons for the absence of the expected Cti activity in <i>Halopseudomonas</i>, which in turn has always been observed in <i>cti</i>-carrying <i>Pseudomonas</i>. The lack of Cti activity under membrane stress conditions when the <i>cti</i> gene is present has never been documented, and this could represent potential negative implications on the utility of the genus <i>Halopseudomonas</i> for some biotechnological applications.