Cryptic speciation of a pelagic Roseobacter population varying at a few thousand nucleotide sites.

A drop of seawater contains numerous microspatial niches at the scale relevant to microbial activities. Examples are abiotic niches such as detrital particles that show different sizes and organic contents, and biotic niches resulting from bacteria-phage and bacteria-phytoplankton interactions. A common practice to investigate the impact of microenvironments on bacterial evolution is to separate the microenvironments physically and compare the bacterial inhabitants from each. It remains poorly understood, however, which microenvironment primarily drives bacterioplankton evolution in the pelagic ocean. By applying a dilution cultivation approach to an undisturbed coastal water sample, we isolate a bacterial population affiliated with the globally dominant Roseobacter group. Although varying at just a few thousand nucleotide sites across the whole genomes, members of this clonal population are diverging into two genetically separated subspecies. Genes underlying speciation are not unique to subspecies but instead clustered at the shared regions that represent ~6% of the genomic DNA. They are primarily involved in vitamin synthesis, motility, oxidative defense, carbohydrate, and amino acid utilization, consistent with the known strategies that roseobacters take to interact with phytoplankton and particles. Physiological assays corroborate that one subspecies outcompetes the other in these traits. Our results indicate that the microenvironments in the pelagic ocean represented by phytoplankton and organic particles are likely important niches that drive the cryptic speciation of the Roseobacter population, though microhabitats contributed by other less abundant pelagic hosts cannot be ruled out.

Uncoupled Quorum Sensing Modulates the Interplay of Virulence and Resistance in a Multidrug-Resistant Clinical Pseudomonas aeruginosa Isolate Belonging to the MLST550 Clonal Complex.

Pseudomonas aeruginosa is a prevalent and pernicious pathogen equipped with extraordinary capabilities both to infect the host and to develop antimicrobial resistance (AMR). Monitoring the emergence of AMR high-risk clones and understanding the interplay of their pathogenicity and antibiotic resistance is of paramount importance to avoid resistance dissemination and to control P. aeruginosa infections. In this study, we report the identification of a multidrug-resistant (MDR) P. aeruginosa strain PA154197 isolated from a blood stream infection in Hong Kong. PA154197 belongs to a distinctive MLST550 clonal complex shared by two other international P. aeruginosa isolates VW0289 and AUS544. Comparative genome and transcriptome analysis of PA154197 with the reference strain PAO1 led to the identification of a variety of genetic variations in antibiotic resistance genes and the hyperexpression of three multidrug efflux pumps MexAB-OprM, MexEF-OprN, and MexGHI-OpmD in PA154197. Unexpectedly, the strain does not display a metabolic cost and a compromised virulence compared to PAO1. Characterizing its various physiological and virulence traits demonstrated that PA154197 produces a substantially higher level of the P. aeruginosa major virulence factor pyocyanin (PYO) than PAO1, but it produces a decreased level of pyoverdine and displays decreased biofilm formation compared with PAO1. Further analysis revealed that the secondary quorum-sensing (QS) system Pqs that primarily controls the PYO production is hyperactive in PA154197 independent of the master QS systems Las and Rhl. Together, these investigations disclose a unique, uncoupled QS mediated pathoadaptation mechanism in clinical P. aeruginosa which may account for the high pathogenic potentials and antibiotic resistance in the MDR isolate PA154197.

Proteomic Delineation of the ArcA Regulon in Salmonella Typhimurium During Anaerobiosis.

Salmonella enterica serovar Typhimurium (S. Typhimurium) is one of the most used models for bacterial pathogenesis and successful infection requires its adaptation to the low oxygen environment in host gastrointestinal tracts. Central to this process is the Arc (aerobic respiratory control) two-component regulatory system that contains a sensor kinase ArcB and a response regulator ArcA. Nevertheless, a comprehensive profile of the ArcA regulon on the proteome level is still lacking in S. Typhimurium. Here we quantitatively profiled Salmonella proteome during anaerobiosis in an arcA-deleting mutant compared with its parental strain. In addition to known processes under its control, notably we found that ArcA represses ethanolamine utilization by directly binding to the promoter region of the eut operon. Furthermore, we found opposing changes of several bacterial genes on the protein and transcript levels in the arcA-deleting mutant including the virulence genes of Salmonella pathogenicity island 1 (SPI-1), thereby indicating potentially prevalent post-transcriptional regulatory mechanisms. Altogether, our study provides important new insights into ArcA-dependent bacterial physiology and virulence during Salmonella anaerobiosis.

Signaling by the heavy-metal sensor CusS involves rearranged helical interactions in specific transmembrane regions.

Two-component systems (TCSs) play important roles in the adaptation of bacteria to stress. Despite their increasingly well understood mechanistic features, it remains poorly understood how TCSs transduce signals across membranes. Here, we use the E. coli Cu/Ag-responsive CusSR TCS as a model to investigate the roles of CusS transmembrane (TM) residues. Proline scanning of TM1 domain led to identification of the T17P, F18P, and S21P variants, which display higher kinase activities relative to wild type. A single point mutation, V202G, in the adjacent TM2 domain specifically suppresses the hyperactivities of these mutants. Disulfide crosslinking analysis demonstrated that T17 and V202 are situated in close proximity, and Cys residues substituted at those two positions form exclusive intramolecular crosslinks when CusS is in the signaling-inactive state. In the signaling-active variant of CusS, however, only intermolecular crosslinking between the two Cys residues could be observed, suggesting that destabilization of an intramolecular constraint and a subsequent rearrangement of helical interactions in this TM region is involved in the activation of CusS. An analogous TM helical interface in the P. aeruginosa heavy metal sensor kinase CzcS is also observed. Together, these results suggested a conserved transmembrane signal transduction mechanism in the heavy metal sensing TCSs.