It has been a decade since multicellularity was proposed as a general bacterial trait. Intercellular communication and multicellular coordination are now known to be widespread among prokaryotes and to affect multiple phenotypes. Many different classes of signaling molecules have been identified in both Gram-positive and Gram-negative species. Bacteria have sophisticated signal transduction networks for integrating intercellular signals with other information to make decisions about gene expression and cellular differentiation. Coordinated multicellular behavior can be observed in a variety of situations, including development of E. coli and B. subtilis colonies, swarming by Proteus and Serratia, and spatially organized interspecific metabolic cooperation in anaerobic bioreactor granules. Bacteria benefit from multicellular cooperation by using cellular division of labor, accessing resources that cannot effectively be utilized by single cells, collectively defending against antagonists, and optimizing population survival by differentiating into distinct cell types.

Proteus mirabilis colonies exhibit striking geometric regularity. Basic microbiological methods and imaging techniques were used to measure periodic macroscopic events in swarm colony morphogenesis. We distinguished three initial phases (lag phase, first swarming phase, and first consolidation phase) followed by repeating cycles of subsequent swarming plus consolidation phases. Each Proteus swarm colony terrace corresponds to one swarming-plus-consolidation cycle. The duration of the lag phase was dependent upon inoculation density in a way that indicated the operation of both cooperative and inhibitory multicellular effects. On our standard medium, the second and subsequent swarm phases displayed structure in the form of internal waves visible with reflected and dark-field illumination. These internal waves resulted from organization of the migrating bacteria into successively thicker cohorts of swarmer cells. Bacterial growth and motility were independently modified by altering the composition of the growth medium. By varying the glucose concentration in the substrate, it was possible to alter biomass production without greatly affecting the kinetics of colony surface area expansion. By varying the agar concentration in the substrate, initial bacterial biomass production was unaffected but colony expansion dynamics were significantly altered. Higher agar concentrations led to slower, shorter swarm phases and longer consolidation phases. Thus, colony growth was restricted by higher agar concentrations but the overall timing of the swarming-plus-consolidation cycles remained constant. None of a variety of factors which had significant effects on colony expansion altered terracing frequencies at 32؇C, but the length of the swarming-plus-consolidation cycle was affected by temperature and medium enrichment. Some clinical isolates displayed significant differences in terracing frequencies at 32؇C. Our results defined a number of readily quantifiable parameters in swarm colony development. The data showed no connection between nutrient (glucose) depletion and the onset of different phases in swarm colony morphogenesis. Several observations point to the operation of density-dependent thresholds in controlling the transitions between distinct phases.Proteus mirabilis colonies have fascinated microbiologists for over a century (13). On typical laboratory media, mature P. mirabilis swarm colonies display striking patterns characterized by circular symmetry and regularly spaced concentric terraces or zones (Fig. 1) (6). These terraces develop as a result of periodic events during colony growth, most notably the cyclic repetition of alternating phases: swarming (active migration) and consolidation (growth without movement of the colony perimeter) (2, 3). Colony expansion is a dynamic process involving movement over the solid substrate by multicellular rafts of specially differentiated swarmer cells (7,18,30,32). The swarmer cells are elongated and hyperflagellated but have the same DNA/length ratio as the shorter oligofla...

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There are clear theoretical reasons and many well-documented examples which show that repetitive, DNA is essential for genome function. Generic repeated signals in the DNA are necessary to format expression of unique coding sequence files and to organise additional functions essential for genome replication and accurate transmission to progeny cells. Repetitive DNA sequence elements are also fundamental to the cooperative molecular interactions forming nucleoprotein complexes. Here, we review the surprising abundance of repetitive DNA in many genomes, describe its structural diversity, and discuss dozens of cases where the functional importance of repetitive elements has been studied in molecular detail. In particular, the fact that repeat elements serve either as initiators or boundaries for heterochromatin domains and provide a significant fraction of scaffolding/matrix attachment regions (S/MARs) suggests that the repetitive component of the genome plays a major architectonic role in higher order physical structuring. Employing an information science model, the 'functionalist' perspective on repetitive DNA leads to new ways of thinking about the systemic organisation of cellular genomes and provides several novel possibilities involving repeat elements in evolutionarily significant genome reorganisation. These ideas may facilitate the interpretation of comparisons between sequenced genomes, where the repetitive DNA component is often greater than the coding sequence component.

A series of molecular events will explain how genetic elements can transpose from one DNA site to another, generate a short oligonucleotide duplication at both ends of the new insertion site, and replicate in the transposition process. These events include the formation of recombinant molecules which have been postulated to be intermediates in the transposition process. The model explains how the replication of bacteriophage Mu is obligatorily associated with movement to new genetic sites. It postulates that all transposable elements replicate in the transposition process so that they remain at their original site while moving to new sites. According to this model, the mechanism of transposition is very different from the insertion and excision of bacteriophage X.Recent research on transposable elements in bacteria has provided important insights into the role of nonhomologous recombination in genetic rearrangements (1-4). These elements include small insertion sequences (IS elements), transposable resistance determinants (Tn elements), and bacteriophage Mu (3). There are detailed differences in the genetic behavior of these various elements (such as differences in specificity of site selection for insertion), but there is a consensus that they all share underlying recombination mechanisms (3, 5). Although this consensus originally included the bacteriophage X (cf. refs. 3 and 4), the considerations elaborated below argue that phage Mu and other transposable elements differ radically from the elegant and simple X model (6, 7).The mechanisms by which transposable elements move from one genetic site to another are still unknown. However, there has been a rapid accumulation of information about the structural consequences of transposition events, the genetic control of transposition, and the replication of bacteriophage Mu from work in many laboratories. While some of this information is not yet based on completely unambiguous data, the outlines appear sufficiently clear to propose a partially detailed molecular model to explain the transposition process. This model also explains how phage Mu replicates. Its justification draws examples from work on various elements (particularly Mu, IS1, Tn3, and Tn5) on the assumption that they are all mechanistically equivalent as far as the details of the present model go. This proposal does not address the question of site selection for transposition events and specifically deals only with events that occur after an initial donor-target complex has been formed. OBSERVATIONS TO BE EXPLAINED