Myxococcus xanthus

Myxococcus xanthus
Scientific classification
Domain: Bacteria
Class: Myxococcota
Class: Myxococcia
Order: Myxococcales
Family: Myxococcaceae
Genus: Myxococcus
Species:
M. xanthus
Binomial name
Myxococcus xanthus
Beebe 1941

Myxococcus xanthus is a gram-negative, rod-shaped species of myxobacteria that exhibits various forms of self-organizing behavior in response to environmental cues. Under normal conditions with abundant food, it exists as a predatory, saprophytic single-species biofilm called a swarm. Under starvation conditions, it undergoes a multicellular development cycle.[1]

Colony growth

A swarm of M. xanthus is a distributed system, containing millions of bacteria that communicate among themselves in a non-centralized fashion. Simple patterns of cooperative behavior among the members of the colony combine to generate complex group behaviors in a process known as "stigmergy". For example, the tendency for one cell to glide only when in direct contact with another results in the colony forming swarms called "wolf-packs" that may measure up to several inches wide. This behavior is advantageous to the members of the swarm, as it increases the concentration of extracellular digestive enzymes secreted by the bacteria, thus facilitating predatory feeding. M. xanthus feeds on dead biomass of a broad range of bacteria and some fungi, discriminating live from dead cells, and causing cell death and lysis when required.[2]

During stressful conditions, the bacteria undergo a process in which about 100,000 individual cells aggregate to form a structure called the fruiting body over the course of several hours. On the interior of the fruiting body, the rod-shaped cells differentiate into spherical, thick-walled spores. They undergo changes in the synthesis of new proteins, as well as alterations in the cell wall, which parallel the morphological changes. During these aggregations, dense ridges of cells move in ripples, which wax and wane over 5 hours.[3]

Motility

Social motility leads to a spatial distribution of cells with many clusters and few isolated single cells.

An important part of M. xanthus behavior is its ability to move on a solid surface by a mechanism called "gliding".[4] Gliding Motility is a method of locomotion that allows for movement, without the help of flagella, on a solid surface. Gliding Motility is also called A-motility (adventurous).[5] In A motility, single cells move, resulting in a distribution with many single cells. M. xanthus have the ability to use a second type of motility. This motility is called Social motility, single cells do not move, but cells that are close to one another move. This leads to a spatial distribution of cells with many clusters and few isolated single cells.[5] This motility depend on the presence of the Type IV pili[6] and divers polysaccharides.[7][8]

More than 37 genes are involved in the A-motility system, which comprises multiple motor elements that are arrayed along the entire cell body. Each motor element appears to be localized to the periplasmic space and is bound to the peptidoglycan layer. The motors are hypothesized to move on helical cytoskeletal filaments. Gliding force generated by these motors is coupled to adhesion sites that move freely in the outer membrane, and which provide a specific contact with the substratum, possibly aided by extracellular polysaccharide slime.[9][10]

S-motility may represent a variation of twitching motility, since it is mediated by the extension and retraction of type IV pili that extend through the leading cell pole. The genes of the S-motility system appear to be homologs of genes involved in the biosynthesis, assembly, and function of twitching motility in other bacteria.[9][10]

Cell differentiation, fruiting and sporulation

In the presence of prey (here E. coli), M. xanthus cells self-organize into periodic bands of traveling waves, termed ripples (left-hand side). In the areas without prey, M. xanthus cells are under nutrient stress and as a result self-organize into haystack-shaped, spore-filled structures termed fruiting bodies (right-hand side, yellow mounds).

In response to starvation, myxobacteria develop species-specific multicellular fruiting bodies. Starting from a uniform swarm of cells, some aggregate into fruiting bodies, while other cells remain in a vegetative state. Those cells that participate in formation of the fruiting body transform from rods into spherical, heat-resistant myxospores, while the peripheral cells remain rod-shaped.[11] Although not as tolerant to environmental extremes as Bacillus endospores, the relative resistance of myxospores to desiccation and freezing enables myxobacteria to survive seasonally harsh environments. When a nutrient source becomes once again available, the myxospores germinate, shedding their spore coats to emerge into rod-shaped vegetative cells. The synchronized germination of thousands of myxospores from a single fruiting body enables the members of the new colony of myxobacteria to immediately engage in cooperative feeding.[12] M. xanthus cells can also differentiate into environmentally-resistant spores in a starvation-independent manner. This process, known as chemically-induced sporulation, is triggered by the presence of glycerol and other chemical compounds at high concentrations.[13] The biological implications of this sporulation process have been controversial for decades due to the unlikeliness to find such high concentrations of chemical inducers in their natural environment.[14][15] However, the finding that the antifungal compound ambruticin acts as a potent natural inducer at concentrations expected to be present in soil, suggests that chemically induced sporulation is the result of competition and communication with the ambruticin-producing myxobacterium Sorangium cellulosum.[16]

Intercellular communication

It is very likely that cells communicate during the process of fruiting and sporulation, because a group of cells that starved together form myxospores inside fruiting bodies.[1] Intercellular signal appears to be necessary to ensure that sporulation happens in the proper place and at the proper time.[17] Research supports the existence of an extracellular signal, A-factor, which is necessary for developmental gene expression and for the development of a complete fruiting body.[18]

Ability to eavesdrop

It has been shown that an M. xanthus swarm is capable of eavesdropping on the extracellular signals that are produced by the bacteria it preys upon, leading to changes in swarm behaviour increasing its efficiency as a predator.[19] This allows for a highly adaptive physiology that will have likely contributed to the near ubiquitous distribution of the myxobacteria.

Importance in research

The complex life cycles of the myxobacteria make them very attractive models for the study of gene regulation as well as cell to cell interactions. The traits of M. xanthus make it very easy to study, and therefore important to research. Laboratory strains of M. xanthus are available that are capable of planktonic growth in shaker culture, so that they are easy to grow in large numbers. The tools of classical and molecular genetics are relatively well-developed in M. xanthus.[20]

Although the fruiting bodies of M. xanthus are relatively primitive compared with, say, the elaborate structures produced by Stigmatella aurantiaca and other myxobacteria, the great majority of genes known to be involved in development are conserved across species.[21] In order to make agar cultures of M. xanthus grow into fruiting bodies, one simply can plate the bacteria on starvation media.[22] Furthermore, it is possible to artificially induce the production of myxospores without the intervening formation of fruiting bodies, by adding compounds such as glycerol or various metabolites to the medium.[23] In this way, different stages in the developmental cycle can be experimentally isolated.

The genome of M. xanthus has been completely sequenced.[24] The size of its genome may reflect the complexity of its life cycle. At 9.14 megabase, it had the largest known prokaryotic genome until the sequencing of Sorangium cellulosum (12.3 Mb), which is also a myxobacterium.

Developmental cheating

Social cheating exists among M. xanthus commonly. As long as mutants are not too common, if they are unable to perform the group beneficial function of producing spores, they will still reap the benefit of the population as a whole. Research has shown that 4 different types of M. xanthus mutants showed forms of cheating during development, by being over-represented among spores relative to their initial frequency in the mixture.[25]

Evolution

In 2003, two scientists, Velicer and Yu, deleted certain parts of the M. xanthus genome, making it unable to swarm effectively on soft agar. Individuals were cloned, and allowed to evolve. After a period of 64 weeks, two of the evolving populations had started to swarm outward almost as effectively as normal wild-type colonies. However, the patterns of the swarm were very different from those of the wild-type bacteria. This suggested that they had developed a new way of moving, and Velicer and Yu confirmed this by showing that the new populations had not regained the ability to make pili, which allows wild-type bacteria to swarm. This study addressed questions about the evolution of cooperation between individuals that had plagued scientists for years.[26]

Very little is known about the evolutionary mechanisms present in M. xanthus. However, it has been discovered that it can establish a generalist predator relationship with different prey, among which is Escherichia coli. In this predator-prey relationship, a parallel evolution of both species is observed through genomic and phenotypic modifications, producing in subsequent generations a better adaptation of one of the species that is counteracted by the evolution of the other, following a co-evolutionary model known Red Queen hypothesis. However, the evolutionary mechanisms present in M. xanthus that produce this parallel evolution are still unknown.[27]

Strains

  • Myxococcus xanthus DK 1622
  • Myxococcus xanthus DZ2
  • Myxococcus xanthus DZF1
  • Myxococcus xanthus NewJersey2
  • Myxococcus xanthus DSM16526T

Whole genome comparisons have indicated that M. virescens is the same species as M. xanthus.[28] M. virescens was first described in 1892, so has precedence.[29]

References

  1. 1 2 Kroos, Lee; Kuspa, Adam; Kaiser, Dale (1986). "A global analysis of developmentally regulated genes in Myxococcus xanthus". Developmental Biology. 117 (1): 252–266. doi:10.1016/0012-1606(86)90368-4. PMID 3017794.
  2. ↑ Thiery S, Kaimer C (2020). "The Predation Strategy of Myxococcus xanthus". Frontiers in Microbiology. 11: 2. doi:10.3389/fmicb.2020.00002. PMC 6971385. PMID 32010119.
  3. ↑ Velicer GJ, Stredwick KL (2002). "Experimental social evolution with Myxococcus xanthus". Antonie van Leeuwenhoek. 81 (1–4): 155–64. doi:10.1023/A:1020546130033. PMID 12448714. S2CID 20018104.
  4. ↑ Shi, Wenyuan. "Myxococcus xanthus". University of California at Los Angeles. Archived from the original on 25 December 2014. Retrieved 26 May 2015.
  5. 1 2 Kaiser, Dale (1979). "Social gliding is correlated with the presence of pili in Myxococcus xanthus". Proc. Natl. Acad. Sci. 76 (11): 5952–5956. Bibcode:1979PNAS...76.5952K. doi:10.1073/pnas.76.11.5952. PMC 411771. PMID 42906.
  6. ↑ Wu, Samuel S.; Kaiser, Dale (1995). "Genetic and functional evidence that Type IV pili are required for social gliding motility in Myxococcus xanthus". Molecular Microbiology. 18 (3): 547–558. doi:10.1111/j.1365-2958.1995.mmi_18030547.x. ISSN 1365-2958. PMID 8748037. S2CID 1257522.
  7. ↑ Islam, Salim T.; Alvarez, Israel Vergara; Saïdi, Fares; Guiseppi, Annick; Vinogradov, Evgeny; Sharma, Gaurav; Espinosa, Leon; Morrone, Castrese; Brasseur, Gael; Guillemot, Jean-François; Benarouche, Anaïs (2020-06-09). "Modulation of bacterial multicellularity via spatio-specific polysaccharide secretion". PLOS Biology. 18 (6): e3000728. doi:10.1371/journal.pbio.3000728. ISSN 1545-7885. PMC 7310880. PMID 32516311.
  8. ↑ Bowden, M. Gabriela; Kaplan, Heidi B. (1998). "The Myxococcus xanthus lipopolysaccharide O-antigen is required for social motility and multicellular development". Molecular Microbiology. 30 (2): 275–284. doi:10.1046/j.1365-2958.1998.01060.x. ISSN 1365-2958. PMID 9791173.
  9. 1 2 Spormann, A. M. (1999). "Gliding motility in bacteria: insights from studies of Myxococcus xanthus". Microbiol. Mol. Biol. Rev. 63 (3): 621–641. doi:10.1128/mmbr.63.3.621-641.1999. PMC 103748. PMID 10477310.
  10. 1 2 Zusman, D. R.; Scott, A. E.; Yang, Z.; Kirby, J. R. (2007). "Chemosensory pathways, motility and development in Myxococcus xanthus". Nature Reviews Microbiology. 5 (11): 862–72. doi:10.1038/nrmicro1770. PMID 17922045. S2CID 2340386.
  11. ↑ Julien, B.; Kaiser, A. D.; Garza, A. (2000). "Spatial control of cell differentiation in Myxococcus xanthus". Proceedings of the National Academy of Sciences. 97 (16): 9098–9103. Bibcode:2000PNAS...97.9098J. doi:10.1073/pnas.97.16.9098. PMC 16828. PMID 10922065.
  12. ↑ Lee, Keesoo; Shimkets, Lawrence J. (1994). "Cloning and Characterization of the socA Locus Which Restores Development to Myxococcus xanthus C-Signaling Mutants". Journal of Bacteriology. 176 (8): 2200–2209. doi:10.1128/jb.176.8.2200-2209.1994. PMC 205340. PMID 8157590.
  13. ↑ Dworkin, Martin; Gibson, Sally M. (1964). "A System for Studying Microbial Morphogenesis: Rapid Formation of Microcysts in Myxococcus xanthus". Science. 146 (3641): 243–244. Bibcode:1964Sci...146..243D. doi:10.1126/science.146.3641.243. PMID 14185314. S2CID 43847755.
  14. ↑ O'Connor, Kathleen A.; Zusman, David R. (2003). "Starvation‐independent sporulation in Myxococcus xanthus involves the pathway for β‐lactamase induction and provides a mechanism for competitive cell survival". Molecular Microbiology. 24 (4): 839–850. doi:10.1046/j.1365-2958.1997.3931757.x. PMID 9194710.
  15. ↑ Müller, Frank-Dietrich; Treuner-Lange, Anke; Heider, Johann; Huntley, Stuart M.; Higgs, Penelope I. (2010). "Global transcriptome analysis of spore formation in Myxococcus xanthus reveals a locus necessary for cell differentiation". BMC Genomics. 11: 264. doi:10.1186/1471-2164-11-264. PMC 2875238. PMID 20420673.
  16. ↑ Marcos-Torres, Francisco Javier; Volz, Carsten; Müller, Rolf (2020). "An ambruticin-sensing complex modulates Myxococcus xanthus development and mediates myxobacterial interspecies communication". Nature Communications. 11 (1): 5563. Bibcode:2020NatCo..11.5563M. doi:10.1038/s41467-020-19384-7. PMC 7643160. PMID 33149152.
  17. ↑ Hagan, David C.; Bretscher, Anthony P.; Kaiser, Dale (1978). "Synergism between morphogenetic mutants of Myxococcus xanthus". Developmental Biology. 64 (2): 284–296. doi:10.1016/0012-1606(78)90079-9. PMID 98366.
  18. ↑ Kuspa, Adam; Kroos, Lee; Kaiser, Dale (1986). "Intercellular signaling is required for developmental gene expression in Myxococcus xanthus". Developmental Biology. 117 (1): 267–276. doi:10.1016/0012-1606(86)90369-6. PMID 3017795.
  19. ↑ Lloyd, D.; Whitworth, D. (2017). "The Myxobacterium Myxococcus xanthus Can Sense and Respond to the Quorum Signals Secreted by Potential Prey Organisms". Frontiers in Microbiology. 439 (8): 439. doi:10.3389/fmicb.2017.00439. PMC 5348527. PMID 28352265.
  20. ↑ Stephens, Karen; Kaiser, Dale (1987). "Genetics of gliding motility in Myxococcus xanthus: Molecular cloning of the mgl locus". Molecular and General Genetics. 207 (2–3): 256–266. doi:10.1007/bf00331587. S2CID 10173079.
  21. ↑ Huntley, S.; Hamann, N.; Wegener-Feldbrugge, S.; Treuner-Lange, A.; Kube, M.; Reinhardt, R.; Klages, S.; Muller, R.; Ronning, C. M.; Nierman, W. C.; Sogaard-Andersen, L. (2010). "Comparative Genomic Analysis of Fruiting Body Formation in Myxococcales". Molecular Biology and Evolution. 28 (2): 1083–1097. doi:10.1093/molbev/msq292. PMID 21037205.
  22. ↑ Inouye, Masayori; Inouye, Sumiko; Zusman, David R. (1979). "Gene expression during development of Myxococcus xanthus: Pattern of protein synthesis". Developmental Biology. 68 (2): 579–591. doi:10.1016/0012-1606(79)90228-8. PMID 108160.
  23. ↑ Bui, N. K.; Gray, J.; Schwarz, H.; Schumann, P.; Blanot, D.; Vollmer, W. (2008). "The Peptidoglycan Sacculus of Myxococcus xanthus Has Unusual Structural Features and is Degraded during Glycerol-Induced Myxospore Development". Journal of Bacteriology. 191 (2): 494–505. doi:10.1128/JB.00608-08. PMC 2620817. PMID 18996994.
  24. ↑ Goldman, B. S.; Nierman, W. C.; Kaiser, D.; Slater, S. C.; Durkin, A. S.; Eisen, J. A.; Ronning, C. M.; Barbazuk, W. B.; Blanchard, M.; Field, C.; Halling, C.; Hinkle, G.; Iartchuk, O.; Kim, H. S.; MacKenzie, C.; Madupu, R.; Miller, N.; Shvartsbeyn, A.; Sullivan, S. A.; Vaudin, M.; Wiegand, R.; Kaplan, H. B. (2006). "Evolution of sensory complexity recorded in a myxobacterial genome". Proceedings of the National Academy of Sciences. 103 (41): 15200–15205. Bibcode:2006PNAS..10315200G. doi:10.1073/pnas.0607335103. PMC 1622800. PMID 17015832.
  25. ↑ Velicer, G. J.; Kroos, L.; Lenski, R. E. (2000). "Developmental cheating in the social bacterium Myxococcus xanthus". Nature. 404 (6778): 598–601. Bibcode:2000Natur.404..598V. doi:10.1038/35007066. PMID 10766241. S2CID 4381783.
  26. ↑ Velicer, Gregory; Yu, Yuen-tsu (2003). "Evolution of novel cooperative swarming in the bacterium Myxococcus xanthus". Nature. 425 (6953): 75–78. Bibcode:2003Natur.425...75V. doi:10.1038/nature01908. PMID 12955143. S2CID 4321295.
  27. ↑ Nair, Ramith R.; Vasse, Marie; Wielgoss, Sébastien; Sun, Lei; Yu, Yuen-Tsu N.; Velicer, Gregory J. "Bacterial predator-prey coevolution accelerates genome evolution and selects on virulence-associated prey defences", Nature Communications, 2019, 10:4301.
  28. ↑ Chambers, James; Sparks, Natalie; Sydney, Natasha; Livingstone, Paul G.; Cookson, Alan R.; Whitworth, David E. (6 October 2020). "Comparative genomics and pan-genomics of the Myxococcaceae, including a description of five novel species: Myxococcus eversor sp. nov., Myxococcus llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogochensis sp. nov., Myxococcus vastator sp. nov., Pyxidicoccus caerfyrddinensis sp. nov. and Pyxidicoccus trucidator sp. nov". Genome Biology and Evolution. 12 (12): 2289–2302. doi:10.1093/gbe/evaa212. PMC 7846144. PMID 33022031.
  29. ↑ Thaxter, R. (1892). "On the Myxobacteriaceae, a new order of Schizomycetes". Botanical Gazette. 17 (12): 389–406. doi:10.1086/326866. S2CID 84654678.

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