Coupling cell movement to multicellular development in myxobacteria

The myxobacteria are Gram-negative organisms that are capable of multicellular, social behaviour. In the presence of nutrients, swarms of myxobacteria feed cooperatively by sharing extracellular digestive enzymes, and can prey on other bacteria. When the food supply runs low, they initiate a complex developmental programme that culminates in the production of a fruiting body. Myxobacteria move by gliding and have two, polarly positioned engines to control their motility. The two engines undergo coordinated reversals, and changes in the reversal frequency and speed are responsible for the different patterns of movement that are seen during development. The myxobacteria communicate with each other and coordinate their movements through a cell-contact-dependent signal. Here, the cell movements that culminate in the development of the multicellular fruiting body are reviewed.     

Myxobacteria, Polarity, and Multicellular Morphogenesis

Myxobacteria are renowned for the ability to sporulate within fruiting bodies whose shapes are species-specific. The capacity to build those multicellular structures arises from the ability of M. xanthus to organize high cell-density swarms, in which the cells tend to be aligned with each other while constantly in motion. The intrinsic polarity of rod-shaped cells lays the foundation, and each cell uses two polar engines for gliding on surfaces. It sprouts retractile type IV pili from the leading cell pole and secretes capsular polysaccharide through nozzles from the trailing pole. Regularly periodic reversal of the gliding direction was found to be required for swarming. Those reversals are generated by a G-protein switch which is driven by a sharply tuned oscillator. Starvation induces fruiting body development, and systematic reductions in the reversal frequency are necessary for the cells to aggregate rather than continue to swarm. Developmental gene expression is regulated by a network that is connected to the suppression of reversals.     

Making waves: pattern formation by a cell-surface-associated signal

Starving Myxococcus xanthus cells organize into two strikingly different spatio-temporal patterns, either rippling or aggregation of cells into fruiting bodies. Formation of both patterns depends on a cell-surface-associated, non-diffusible signal, the C-signal. A key motility parameter modulated by the C-signal during pattern formation is the frequency at which cells reverse their gliding direction, with low and high levels of C-signalling causing an increase and a decrease in the reversal frequency, respectively. Recently, a simple yet elegant mathematical model was proposed to explain the mechanism underlying the non-linear dependence of the reversal frequency on C-signalling levels. The mathematical solution hinges on the introduction of a negative feedback loop into the biochemical circuit that regulates the reversal frequency. This system displays an oscillatory behaviour in which the oscillation frequency depends in a non-monotonic manner on the level of C-signalling. Thus, the biochemical oscillator recapitulates the effect of the C-signal on the reversal frequency. The challenge for biologists now is to test the mathematical model experimentally.     

Improved methods of isolation and purification of myxobacteria and development of fruiting body formation of two strains

By using baiting techniques and different purification methods, a high number of myxobacterial strains have been isolated as pure cultures from soil of different regions of China. Because myxobacterial cells do not disperse easily in liquid media, a medium containing an enzymatic hydrolysate of casein (CEH) medium have been used for purification and purity tests combined in a single step. The key method, in which isolates are reintroduced to sterile rabbit dung to induce fruiting bodies formation, facilitates purification of myxobacteria. Sterile rabbit dung pellets are used to mimic the natural growth substance of these organisms which has the advantage that characteristic fruiting bodies emerge, which is a key characteristics in the taxonomy of myxobacteria. In this study, the optimum program of isolation and purification of some myxobacteria strains has been established which will facilitate screening programs. Moreover, the development of fruiting body formation of strain BD20 (Chondromyces) and strain BD54 (Cystobacter) have been recorded in this study.     

Fruiting and non-fruiting myxobacteria: a phylogenetic perspective of cultured and uncultured members of this group

The diversity of myxobacteria present in campus garden soil was surveyed by both cultivation-based and cultivation-independent methods. Detailed phylogenetic analysis of cultured and uncultured myxobacteria 16S rRNA gene sequences revealed that many undescribed relatives of the myxobacteria exist in nature. Molecular systematic analyses also revealed that myxobacterial genera described to date on the basis of the morphology of multi-cellular fruiting bodies were mostly monophyletic. However, these known taxa comprised only in a small part of the sequences recovered directly from soil in a cultivation-independent approach, indicating that the group is much more diverse than previously thought. We propose that the myxobacteria exist in two forms: the fruiting and the non-fruiting types. Most of the uncultured myxobacteria may represent taxa which rarely form fruiting bodies, or may lack some or all of the developmental genes needed for fruiting body formation. In order to identify non-fruiting myxobacteria, new morphology-independent cultivation and isolation techniques need to be developed.     

Adaptation of Salt-tolerant <Emphasis Type="Italic">Myxococcus</Emphasis> Strains and their Motility Systems to the Ocean Conditions

More and more studies have indicated that myxobacteria are able to live in seawater conditions, which, however, can decrease the fruiting body formation ability and also the adventurous (A) and social (S) motility systems of the myxobacteria. To learn the adaptation mechanism of the salt-tolerant myxobacteria to marine conditions, we analyzed 10 salt-tolerant Myxococcus strains of their fruiting body formation and motility. The isolates were from marine samples and possessed different levels of salt tolerance. They had the dual motility system and formed fruiting bodies in the presence of suitable seawater concentrations. Some high salt-tolerant strains even lost their fruiting abilities in the absence of seawater. In response to the presence of seawater, the S-motility was found to be increased in the high salt-tolerants but decreased in the low salt-tolerants. The A-motility, on the other hand, was observed in all the salt-tolerant Myxococcus strains, but increased or decreased in response to the presence of seawater. Perceived shifts of fruiting body formation abilities and motilities discovered in the salt-tolerant Myxococcus strains suggested an ecological adaptation of myxobacterial social behaviors to the marine environments.     

不同分离方法对子实体形成和粘细菌分离的影响

[目的]基于模拟原位环境策略、可培养粘细菌的营养策略及细菌互作网络,改良分离培养基,以提高分离粘细菌的多样性.[方法]通过添加土壤浸提液、使用不同种类的诱导菌和改变诱导菌的接种方式设置分离方法,同时以传统的分离方法作对照.[结果]改良的分离方法比对照组诱导出了更多粘细菌子实体种类,采自4个地区的9份样品共分离纯化出40...     

Complete genome sequence of the fruiting myxobacterium Corallococcus coralloides DSM 2259

Corallococcus coralloides, like most other myxobacteria, undergoes a developmental program culminating in the formation of fruiting bodies. C. coralloides fruiting bodies are morphologically distinct from those of other fruiting myxobacteria for which full-length genome sequences are available. The genome sequence of the 10.0-Mb C. coralloides genome is presented herein.     

Role of phase variation in the resistance of Myxococcus xanthus fruiting bodies to Caenorhabditis elegans predation

The phenomenon of phase variation between yellow and tan forms of Myxococcus xanthus has been recognized for several decades, but it is not known what role this variation may play in the ecology of myxobacteria. We confirm an earlier report that tan variants are disproportionately more numerous in the resulting spore population of a M. xanthus fruiting body than the tan vegetative cells that contributed to fruiting body formation. However, we found that tan cells may not require yellow cells for fruiting body formation or starvation-induced sporulation of tan cells. Here we report three differences between the yellow and tan variants that may play important roles in the soil ecology of M. xanthus. Specifically, the yellow variant is more capable of forming biofilms, is more sensitive to lysozyme, and is more resistant to ingestion by bacteriophagous nematodes. We also show that the myxobacterial fruiting body is more resistant to predation by worms than are dispersed M. xanthus cells.     

粘细菌生态多样性的初步研究

通过对河北,云南,青藏高原不同地区的粘细菌进行生态多样的研究,从42个样品中分离得到了10个属（Archangium,Myxococcus,Cystobacter,Corallococcus,Melittangium,Sorangium,Polyangium,Chondromyces,Angiococccus,Stigmatella)的150余株粘细菌,其中包括一些尚未有描述的菌株,有待鉴定,根据这些菌株的子实体结构,菌落形态,营养细胞,粘孢子形态等特征将它们初步鉴定到属,对这些特有自然生态环境的粘细菌按不同地点,植被,营养基质进行统计比较,结果表明,粘细菌生态分布极为广泛,具有丰富的生态多样性,以上结果为粘细菌生物资源的有效开发利用奠定了基础。     

Interconnected Cavernous Structure of Bacterial Fruiting Bodies

The formation of spore-filled fruiting bodies by myxobacteria is a fascinating case of multicellular self-organization by bacteria. The organization of Myxococcus xanthus into fruiting bodies has long been studied not only as an important example of collective motion of bacteria, but also as a simplified model for developmental morphogenesis. Sporulation within the nascent fruiting body requires signaling between moving cells in order that the rod-shaped self-propelled cells differentiate into spores at the appropriate time. Probing the three-dimensional structure of myxobacteria fruiting bodies has previously presented a challenge due to limitations of different imaging methods. A new technique using Infrared Optical Coherence Tomography (OCT) revealed previously unknown details of the internal structure of M. xanthus fruiting bodies consisting of interconnected pockets of relative high and low spore density regions. To make sense of the experimentally observed structure, modeling and computer simulations were used to test a hypothesized mechanism that could produce high-density pockets of spores. The mechanism consists of self-propelled cells aligning with each other and signaling by end-to-end contact to coordinate the process of differentiation resulting in a pattern of clusters observed in the experiment. The integration of novel OCT experimental techniques with computational simulations can provide new insight into the mechanisms that can give rise to the pattern formation seen in other biological systems such as dictyostelids, social amoeba known to form multicellular aggregates observed as slugs under starvation conditions.     

Territorial interactions between two Myxococcus Species.

It is unusual to find fruiting bodies of different myxobacteria occupying the same territory on natural samples. We were thus interested in determining whether myxobacteria establish territorial dominance and, if so, what the mechanism of that interaction is. We had previously observed that vegetative swarms of Myxococcus xanthus and Stigmatella aurantiaca placed close to each other on an agar surface initially merged but eventually separated. Further studies indicated that these two species also formed separate fruiting bodies when mixed together on developmental agar (unpublished observation). We examined the interactions between two more closely related myxobacteria, M. xanthus and M. virescens, in greater detail. When mixtures of a kanamycin-resistant strain of M. xanthus and a kanamycin-sensitive strain of M. virescens were placed together under developmental conditions, the cells sorted themselves out and established separate fruiting body territories. In addition, differential viable counts of a mixture of the two species during development indicated that each strain was producing an extracellular component that inhibited the growth and development of the other. Nevertheless, finally, M. virescens invariably outcompeted M. xanthus at all input ratios of M. xanthus/M. virescens tested. This is consistent with the observation that M. virescens is by far the more commonly encountered of the two species. The properties of the inhibitory substance from M. virescens are consistent with the possibility that it is a bacteriocin. Our working hypothesis is that the bacteriocin plays a role in the establishment of myxobacterial territoriality. If so, this is an example of an ecological function of bacteriocins.     

Aggregation during fruiting body formation in Myxococcus xanthus is driven by reducing cell movement

When starved, Myxococcus xanthus cells assemble themselves into aggregates of about 10(5) cells that grow into complex structures called fruiting bodies, where they later sporulate. Here we present new observations on the velocities of the cells, their orientations, and reversal rates during the early stages of fruiting body formation. Most strikingly, we find that during aggregation, cell velocities slow dramatically and cells orient themselves in parallel inside the aggregates, while later cell orientations are circumferential to the periphery. The slowing of cell velocity, rather than changes in reversal frequency, can account for the accumulation of cells into aggregates. These observations are mimicked by a continuous agent-based computational model that reproduces the early stages of fruiting body formation. We also show, both experimentally and computationally, how changes in reversal frequency controlled by the Frz system mutants affect the shape of these early fruiting bodies.     

Dynamics of fruiting body morphogenesis

Myxobacteria build their species-specific fruiting bodies by cell movement and then differentiate spores in specific places within that multicellular structure. New steps in the developmental aggregation of Myxococcus xanthus were discovered through a frame-by-frame analysis of a motion picture. The formation and fate of 18 aggregates were captured in the time-lapse movie. Still photographs of 600 other aggregates were also analyzed. M. xanthus has two engines that propel the gliding of its rod-shaped cells: slime-secreting jets at the rear and retractile pili at the front. The earliest aggregates are stationary masses of cells that look like three-dimensional traffic jams. We propose a model in which both engines stall as the cells' forward progress is blocked by other cells in the traffic jam. We also propose that these blockades are eventually circumvented by the cell's capacity to turn, which is facilitated by the push of slime secretion at the rear of each cell and by the flexibility of the myxobacterial cell wall. Turning by many cells would transform a traffic jam into an elliptical mound, in which the cells are streaming in closed orbits. Pairs of adjacent mounds are observed to coalesce into single larger mounds, probably reflecting the fusion of orbits in the adjacent mounds. Although fruiting bodies are relatively large structures that contain 10(5) cells, no long-range interactions between cells were evident. For aggregation, M. xanthus appears to use local interactions between its cells.     

Cell polarity, intercellular signalling and morphogenetic cell movements in Myxococcus xanthus

In Myxococcus xanthus morphogenetic cell movements constitute the basis for the formation of spreading vegetative colonies and fruiting bodies in starving cells. M. xanthus cells move by gliding and gliding motility depends on two polarly localized engines, type IV pili pull cells forward, and slime extruding nozzle-like structures appear to push cells forward. The motility behaviour of cells provides evidence that the two engines are localized to opposite poles and that they undergo polarity switching. Several proteins involved in regulating polarity switching have been identified. The cell surface-associated C-signal induces the directed movement of cells into nascent fruiting bodies. Recently, the molecular nature of the C-signal molecule was elucidated and the motility parameters regulated by the C-signal were identified. From the effect of the C-signal on cell behaviour it appears that the C-signal inhibits polarity switching of the two motility engines. This establishes a connection between cell polarity, signalling by an intercellular signal and morphogenetic cell movements during fruiting body formation.     

Cell flexibility affects the alignment of model myxobacteria

Myxobacteria are social bacteria that exhibit a complex life cycle culminating in the development of multicellular fruiting bodies. The alignment of rod-shaped myxobacteria cells within populations is crucial for development to proceed. It has been suggested that myxobacteria align due to mechanical interactions between gliding cells and that cell flexibility facilitates reorientation of cells upon mechanical contact. However, these suggestions have not been based on experimental or theoretical evidence. Here we created a computational mass-spring model of a flexible rod-shaped cell that glides on a substratum periodically reversing direction. The model was formulated in terms of experimentally measurable mechanical parameters, such as engine force, bending stiffness, and drag coefficient. We investigated how cell flexibility and motility engine type affected the pattern of cell gliding and the alignment of a population of 500 mechanically interacting cells. It was found that a flexible cell powered by engine force at the rear of the cell, as suggested by the slime extrusion hypothesis for myxobacteria motility engine, would not be able to glide in the direction of its long axis. A population of rigid reversing cells could indeed align due to mechanical interactions between cells, but cell flexibility impaired the alignment.     

A myxobacterial S-motility protein dances with poles

Coordinated movement of packs of S-motile Myxococcus xanthus cells relies on extrusion and retraction of pili that are located at one cell pole. At regular intervals the pili switch their polar location and cells reverse direction. Recently, the FrzS S-motility protein was observed to localize predominantly to the piliated pole. In time, FrzS was redeployed to the opposite pole and its sequestration at the new site coincided with cell reversal. The C-terminal region of FrzS, a response regulator homolog, is rich in coiled-coil motifs and is required for dynamic localization and proper motility. These results raise the possibility that proper spatial control of FrzS has an important role in the regulation of cell reversal and S-motility.     

粘细菌基因组学研究进展

粘细菌(Myxobacteria)隶属于δ变形菌纲(Deltaproteobacteria)的粘球菌目(Myxococcales),是一类革兰氏阴性杆状细菌。它是继放线菌和真菌之后又一重要的活性次级代谢产物产生菌,尽管如此,由于分离纯化困难,粘细菌的研究进展一直较为缓慢。随着测序技术的进步和生物信息学的应用,大量粘细菌基因组被完成测序和报道。本文对粘细菌研究意义及该类资源开发价值、分离培养存在的困难进行了阐述,对粘细菌基因组注释及目前已测菌株的全基因组进行了归纳总结,同时介绍了基因组学在粘细菌生态、捕食机制、子实体形成以及次级代谢产物合成方面的研究进展。本文有助于了解基因组学在粘细菌研究中的重要价值,为联合应用多组学技术深入研究粘细菌代谢机制和社会性行为提供了参考,对粘细菌基础研究、资源发掘和开发利用具有重要意义。     

Characteristics and living patterns of marine myxobacterial isolates

The growth, morphology, and life cycle of two marine myxobacterial isolates, halotolerant Myxococcus fulvus strain HW-1 and halophilic Haliangium ochraceum strain SMP-2, were studied as models to determine the living patterns of myxobacteria in the ocean. The growth, morphology, and development of halotolerant strain HW-1 shifted in response to salinity. The optimal seawater concentration for growth of HW-1 was 0 to 80% (salinity, 0.1 to 2.9%), and the strain grew poorly in media with a salinity of more than 4%. The cells became shorter as the seawater concentration increased. The fruiting body structure was complete only on agar prepared with low concentrations of seawater or salts (less than 60% seawater; salinity, 2.1%), and rudimentary structures or even simple cell mounds appeared as the seawater concentration increased. In contrast, the halophilic strain SMP-2 was unable to grow without NaCl. The cell length and the morphology of the fruiting body-like structure did not change in response to salts. In seawater liquid medium, the cells of both strains were confirmed to be able to form myxospores directly from vegetative cells, but they could not do so in medium containing a low seawater concentration (10% or less). HW-1 cells from medium containing a high concentration of seawater grew independent of cell density, while cells from medium containing a low concentration of seawater (10% or less) showed density-dependent growth. SMP-2 cells showed density-dependent growth under all salinity conditions. The results suggest that the halotolerant myxobacteria are the result of degenerative adaptation of soil myxobacteria to the marine environment, while the halophilic myxobacteria form a different evolutionary group that is indigenous to the ocean.