The unidirectional flagellar motor of Rhodobacter sphaeroides WS8 can rotate either clockwise or counterclockwise: characterization of the flagellum under both conditions by antibody decoration

Swimming Using a Unidirectionally Rotating, Single Stopping Flagellum in the Alpha Proteobacterium Rhodobacter sphaeroides

Rhodobacter sphaeroides has 2 flagellar operons, one, Fla2, encoding a polar tuft that is not expressed under laboratory conditions and a second, Fla1, encoding a single randomly positioned flagellum. This single flagellum, unlike the flagella of other species studied, only rotates in a counterclockwise direction. Long periods of smooth swimming are punctuated by short stops, caused by the binding of one of 3 competing CheY homologs to the motor. During a stop, the motor is locked, not freely rotating, and the flagellar filament changes conformation to a short wavelength, large amplitude structure, reforming into a driving helix when the motor restarts. The cell has been reoriented during the brief stop and the next period of smooth swimming is a new direction.

Sex and flow: the consequences of fluid shear for sperm–egg interactions

Fertilization is a complex interaction among biological traits of gametes and physical properties of the fluid environment. At the scale of fertilization (0.01–1 mm), sperm encounter eggs while being transported within a laminar (or viscous) shear flow. Varying laminar-shear in a Taylor-Couette flow tank, our experiments simulated important aspects of small-scale turbulence within the natural habitats of red abalone(Haliotis rufescens), a large marine mollusk and external fertilizer. Behavioral interactions between individual cells, sperm–egg encounter rates, and fertilization success were quantified, simultaneously, using a custom-built infrared laser and computer-assisted video imaging system. Relative to still water, sperm swam faster and moved towards an egg surface,but only in comparatively slow flows. Encounter rate, swim speed and orientation, and fertilization success each peaked at the lowest shear tested(0.1 s–1), and then decayed as shear increased beyond 1.0 s–1. The decay did not result, however, from damage to either sperm or eggs. Analytical and numerical models were used to estimate the propulsive force generated by sperm swimming (Fswim) and the shear force produced by fluid motion within the vicinity of a rotating egg(Fshear). To first order, male gametes were modeled as prolate spheroids. The ratio Fswim/Fshear was useful in explaining sperm–egg interactions. At low shears where Fswim/Fshear>1, sperm swam towards eggs, encounter rates were pronounced, and fertilization success was very high; behavior overpowered fluid motion. In contrast, sperm swimming,encounter rate and fertilization success all decayed rapidly when Fswim/Fshear<1; fluid motion dominated behavior. The shears maximizing fertilization success in the lab typically characterized natural flow microenvironments of spawning red abalone. Gamete behavior thus emerges as a critical determinant of sexual reproduction in the turbulent sea.

Chemotaxis-guided movements in bacteria

Motile bacteria often use sophisticated chemotaxis signaling systems to direct their movements. In general, bacterial chemotactic signal transduction pathways have three basic elements: (1) signal reception by bacterial chemoreceptors located on the membrane; (2) signal transduction to relay the signals from membrane receptors to the motor; and (3) signal adaptation to desensitize the initial signal input. The chemotaxis proteins involved in these signal transduction pathways have been identified and extensively studied, especially in the enterobacteria Escherichia coli and Salmonella enterica serovar typhimurium. Chemotaxis-guided bacterial movements enable bacteria to adapt better to their natural habitats via moving toward favorable conditions and away from hostile surroundings. A variety of oral microbes exhibits motility and chemotaxis, behaviors that may play important roles in bacterial survival and pathogenesis in the oral cavity.

Bacterial chemotaxis: Rhodobacter sphaeroides and Sinorhizobium meliloti--variations on a theme?

We are only beginning to understand the mechanisms involved in tactic sensing in the alpha-subgroup of bacteria. It is clear, however, from recent developments that although the central chemosensory pathways are related to those identified in enteric species, the primary signals and the effect on flagellar behaviour are very different. The expression of chemoreceptors is under environmental control, and the strength of a response depends on the metabolic state of the cell. This is very different from enteric species which always respond to MCP-dependent chemoeffectors, and in which the expression of the receptors is constitutive. Chemotaxis in R. sphaeroides and S. meliloti is therefore more directly linked to the environment in which a cell finds itself. The integration of chemosensory pathways dependent on growth state may be much more suited to the fluctuating environment of these soil and water bacteria. There is still a great deal that needs to be understood about the mechanisms involved in motor control. The presence of at least two CheY homologues and the finding that the swimming speed of these bacteria can vary, and, in the case of S. meliloti, vary with chemosensory stimulation, suggests a different control mechanism at the flagellar motor where speed can be altered, or the motor stopped, with a full delta p still present. Why R. sphaeroides should have at least two functional sets of genes encoding homologues of the enteric chemosensory pathway remains to be determined. The major differences in sensory behaviour between the two alpha-subgroup species so far studied in detail and the differences from the enteric species suggests that many more variations of the chemosensory pathways will be found as more species are studied.

Isolation and ultrastructural study of the flagellar basal body complex from Rhodobacter sphaeroides WS8 (wild type) and a polyhook mutant PG

Filament-Hook-Basal Body (FHBB) complexes were isolated from the purple non-sulphur facultative anaerobic bacterium Rhodobacter sphaeroides (WS8) by lysozyme digestion of the cells followed by an alkaline treatment and ultracentrifugation, and they were analysed by electron microscopy. The structure is composed of a filament linked through an enlarged junction to the hook and a basal body composed of L and P rings, a rod, and a less well-defined cytoplasmic ring that has evidence of additional attached structures. Hook-basal body complexes isolated from a mutant (PG) which produces an extended hook but no filament shows basal body structures identical to those of wild-type FHBBs.

Structural and phylogenetic analysis of the MotA and MotB families of bacterial flagellar motor proteins

MotA and MotB are two well-characterized proteins in Escherichia coli which are believed to function as the proton channel and the anchor, respectively, of the motor component of the bacterial flagellum. We have identified and analysed all currently sequenced members of the MotA and MotB families. Members of these families include (1) these E. coli proteins, (2) their pmf-interacting motor homologues in other bacteria, (3) two ORFs which map downstream of the gene encoding the catabolite repression-mediating CepA protein in Bacillus species and (4) unidentified open reading frames. With one exception (the MotB protein of Rhodobactec sphaeroides), members of the MotB family exhibit a C-terminal domain that is homologous to peptidoglycan-interaction domains of numerous sequenced lipoproteins and outer membrane proteins. Multiple alignments, average hydropathy and similarity plots, and phylogenetic trees have allowed (1) identification of regions of relative conservation, (2) definition of signature sequences for these protein families and (3) determination of relative phylogenetic distances relating all members of each family. The phylogenies of these proteins do not follow those of the organisms from which they were isolated, suggesting the presence of divergent isoforms in many bacteria. Phylogenetic analyses of the peptidoglycan-interaction domains of MotB proteins indicated that, except for MotB of R. sphaeroides, these domains became associated with the MotB proteins early during evolutionary history, before members of the MotB family or members of the outer membrane protein family diverged from each other.

Photobiology of bacteria

The field of photobiology is concerned with the interactions between light and living matter. For Bacteria this interaction serves three recognisable physiological functions: provision of energy, protection against excess radiation and signalling (for motility and gene expression). The chemical structure of the primary light-absorbing components in biology (the chromophores of photoactive proteins) is surprisingly simple: tetrapyrroles, polyenes and derivatised aromats are the most abundant ones. The same is true for the photochemistry that is catalysed by these chromophores: this is limited to light-induced exciton- or electron-transfer and photoisomerization. The apoproteins surrounding the chromophores provide them with the required specificity to function in various aspects of photosynthesis, photorepair, photoprotection and photosignalling. Particularly in photosynthesis several of these processes have been resolved in great detail, for others at best only a physiological description can be given. In this contribution we discuss selected examples from various parts of the field of photobiology of Bacteria. Most examples have been taken from the purple bacteria and the cyanobacteria, with special emphasis on recently characterised signalling photoreceptors in Ectothiorhodospira halophila and in Fremyella diplosiphon.