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 spp. and related bacteria are large, intestinalsymbionts of surgeonfish. The biggest of these bacteria are cigar-shaped, reaching lengths of 600 μmor more. The architecture of these enormous cells is unusual and it appears that basic bacterial cell components,such as the flagella, cytoplasmic membrane and nucleoids, are present in excess and may serve novel functions.Comparative studies of members of this morphologically diverse group of intestinal symbionts are providinginsight into the role of cytoarchitecture in supporting large cytoplasmic volumes as well as the developmentof unusual reproductive strategies.

In the recent decade, our view of the sub-cellular organization of bacterial cells has been revolutionizedby the application of modern cell biology methods. Cytoskeletal proteins that are precisely targeted tospecific cellular locations and that assemble in dynamic filamentous structures have been discovered. Moreover,most bacterial species contain homologous proteins of eukaryotic tubulin and actin, which, like in eukaryotes,have functions in cell shape maintenance, cell division, DNA movement or alignment of organelles. Additionalcytoskeletal elements are present in bacterial species with more complex cell shapes, e.g., in the curved. Cell wall-free Mollicutes contain cytoskeletalproteins for maintaining their cell shape and for motility.

Cryo-electron tomography (cryo-ET) is an emerging imaging technique that combines the power of three-dimensional(3-D) imaging of large pleomorphic structures such as cells or organelles with a close-to-life preservationof the sample. At the present resolution of approximately 4 nm, supramolecular structures can be studiedin unperturbed cellular environments. Application of this method to prokaryotic cells has provided newinsights into the structural organization of the bacterial cytoskeleton. In , it is built of three parallel ribbons of thicker and thinner filaments that spanthe cell from one end to the other just underneath the cell membrane, thereby giving the cell its helicalshape and enabling it to swim by processive change in body helicity.

synthesizes a complex polar structure,the attachment organelle (AO), which is required for productive adherence to host cells. Because in nature cannot survive outside the host, this structure is essentialto the cell. While it is understood that the AO is thesite at which adhesin proteins are concentrated, how that localization is mediated is unknown. However,the presence at the AO of a set of novel, detergent-insoluble proteins and structures which are indirectlyrequired for normal function of this structure has provided insight into its assembly and architecture.Biochemical, genetic, and immunocytochemical studies of these cytadherence-accessory proteins and theirinterrelationships have revealed remarkable complexity in this supposedly minimal bacterial cell.

Many diverse bacteria move in the absence of any visible locomotory organelle by a mechanismtermed gliding. Gliding motility requires contact with a solid substrate and occurs in the directionparallel to the long axis of the bacterial cell. Recent research indicates that bacteria use at least twodifferent mechanisms to glide. One mechanism, termed social motility, is based on the extension and retractionof pili. The other mechanism, termed adventurous motility, involves the secretion of slime from a specializedorganelle called the junctional pore complex. This work discusses the possible role of the junctional porecomplex in gliding motility of cyanobacteria, myxobacteria, and the alphaproteobacterium .

Many pathogenic and symbiotic bacteria interact with their eukaryotic host using a complex proteinsecretion apparatus termed the type III secretion system. Type III secretion systems are multiprotein machineriesthat form a complex syringe-like organelle spanning the entire Gram-negative bacterial envelope. Thebacteria use these machineries to translocate effector proteins into the host cells that either enable themto evade the host's immune system or help establish a close relationship that is necessary to engagein a mutualistic symbiosis. This work discusses the occurrence, structure, and regulation of thesenovel and important protein translocation systems.

Gas vesicles are gas-filled prokaryotic organelles that provide buoyancy in many planktonic (cyano)bacteriaand halophilic archaea. Remarkably, more and more genomes of soil bacteria, especially those of actinomycetes,show gas vesicle gene () clusters often encoding homologues of atleast the eight genes essential for gas vesicle formation in archaea.Here, I discuss characteristics specific to actinomycete Gvp proteins, their expression under stress conditions,and the apparent absence of a buoyancy phenotype in mutant strains.Alternative functions for gas vesicles in actinomycetes are discussed in relation to their unusually complexdevelopmental life cycle.

We have summarized herein the available data concerning either parasitic or mutualistic bacteria havingas host cell another prokaryote, with particular emphasis on those cases in which endosymbiosis emerges.These types of associations are central to the endosymbiotic theory for the origin of the eukaryotic cell.Endosymbionts of prokaryotes (today restricted to Gram-negative bacteria) may be clustered into two categories:the intraperiplasmic, long time known as bdellovibrios, and the more invading intracytoplasmic, describedmore recently.