Quorum Sensing: An Under-Explored Phenomenon in the Phylum Actinobacteria.

Quorum sensing is known to play a major role in the regulation of secondary metabolite production, especially, antibiotics, and morphogenesis in the phylum Actinobacteria. Although it is one of the largest bacterial phylum, only 25 of the 342 genera have been reported to use quorum sensing. Of these, only nine have accompanying experimental evidence; the rest are only known through bioinformatic analysis of gene/genome sequences. It is evident that this important communication mechanism is not extensively explored in Actinobacteria. In this review, we summarize the different quorum sensing systems while identifying the limitations of the existing screening strategies and addressing the improvements that have taken place in this field in recent years. The γ-butyrolactone system turned out to be almost exclusively limited to this phylum. In addition, methylenomycin furans, AI-2 and other putative AHL-like signaling molecules are also reported in Actinobacteria. The lack of existing screening systems in detecting minute quantities and of a wider range of signaling molecules was a major reason behind the limited information available on quorum sensing in this phylum. However, recent improvements in screening strategies hold a promising future and are likely to increase the discovery of new signaling molecules. Further, the quorum quenching ability in many Actinobacteria has a great potential in controlling the spread of plant and animal pathogens. A systematic and coordinated effort is required to screen and exploit the enormous potential that quorum sensing in the phylum Actinobacteria has to offer for human benefit.

Nitrincola alkalisediminis sp. nov., an alkaliphilic bacterium isolated from an alkaline lake.

Two Gram-stain-negative, aerobic, alkaliphilic bacteria (strains MEB087T and MEB142) were isolated from sediment and water samples, respectively, collected from the alkaline Lonar Lake in Maharashtra, India. Strains MEB087T and MEB142 shared 99.8 % 16S rRNA gene sequence similarity and were 85 % related on the basis of DNA-DNA hybridization. The 16S rRNA gene sequences of both strains showed close relationship with the genus Nitrincola, and their closest neighbour was Nitrincola lacisaponensis 4CAT with 97.7 % sequence similarity. MEB087T and MEB142 exhibited only 45 % and 54 % DNA-DNA relatedness, respectively, with Nitrincola lacisaponensis DSM 16316T. Both strains were asporogenous, short, non-motile rods capable of utilizing a limited range of organic acids as sole carbon and energy sources. They were oxidase- and catalase-positive, able to reduce nitrate and nitrite; but unable to degrade DNA, urea, gelatin, casein or starch. They grew optimally at pH 9.5 (tolerating up to pH 11) and could withstand up to 0.6 M NaCl. The predominant cellular fatty acids were summed feature 8 comprising C18 : 1ω7c/C18 : 1ω6c (47-49 %) followed by summed feature 3 comprising C16 : 1ω7c/C16 : 1ω6c (28-32 %). The predominant polar lipids were diphosphatidylglycerol, phosphatidylethanolamine and phosphatidylcholine. The DNA G+C content was 49.3-49.7 mol%. On the basis of the phylogenetic analysis and chemotaxonomic characteristics, strains MEB087T and MEB142 represent a novel species in the genus Nitrincola, for which the name Nitrincola alkalisediminis sp. nov. is proposed. The type strain is MEB087T ( = KCTC 42948T = JCM 19317T) with MEB142 ( = KCTC 42949 = JCM 19318) as an additional strain.

Rufibacter immobilis sp. nov., isolated from a high-altitude saline lake.

Two pinkish-red, Gram-stain-negative, non-motile aerobic bacterial strains (MCC P1(T) and MCC P2), capable of growing at low temperatures (15 °C), were isolated from water of a saline lake located in the western Himalayas of India. The strains were capable of growth in the presence of 0-2.0% NaCl and at pH 6.5-9.0. Phylogenetic analysis based on 16S rRNA gene sequences revealed the closest similarity of 96.3% to the type strain of the only species of the genus Rufibacter , Rufibacter tibetensis CCTCC AB 208084(T). Strains MCC P1(T) and MCC P2 shared 99.0% 16S rRNA gene sequence similarity and 88.6% DNA-DNA relatedness. The major cellular fatty acids were iso-C15 : 0, C17 : 1ω6c, summed feature 3 (C16 : 1ω6c/C16 : 1ω7c) and summed feature 4 (anteiso-C17 : 1 B/iso-C17 : 1 I). Predominant polar lipids were phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol. The respiratory quinone was MK-7. The DNA G+C content of the strains was 52.6-52.8 mol%. Based on morphological, physiological, chemotaxonomical and molecular characteristics, strains MCC P1(T) and MCC P2 represent a novel species of the genus Rufibacter , for which the name Rufibacter immobilis sp. nov. is proposed. The type strain is MCC P1(T) ( =MCC 2268(T) =CCTCC AB 2013351(T)).