Myxococcus xanthus is a gliding bacterium that possesses two motility systems, the adventurous (A-motility) and social (S-motility) systems. A-motility is used for individual cell gliding, while S-motility is used for gliding in multicellular groups. Video microscopy studies showed that nla24 cells are non-motile on agar surfaces, suggesting that the nla24 gene product is absolutely required for both A-motility and S-motility under these assay conditions. S-motility requires functional type IV pili, wild-type LPS O-antigen, and an extracellular matrix of exopolysaccharide (EPS) and protein called fibrils. The results of expression studies and tethering assays indicate that the nla24 mutant has functional type IV pili. The nla24 mutant also produces wild-type LPS. However, several lines of evidence suggest that the nla24 mutant is defective for production of the EPS portion of the fibril matrix. The nla24 mutant is also defective for transcription of two genes (aglU and cglB) known to be required for A-motility, which is consistent with the idea that nla24 cells are defective for A-motility. Based on these findings, it is proposed that the putative transcriptional activator Nla24 regulates a subset of genes that are important for A-motility and S-motility in M. xanthus.

Coordinated group movement (swarming) is a key aspect of Myxococcus xanthus' social behavior. Here we report observation of domain structures formed by multiple cells within large three-dimensional swarming groups grown on amorphous glass substrates, using the atomic force microscope (AFM). Novel analyses revealed that 90% of the wild type swarms displayed some form of preferential cell alignment. In contrast, cells with mutations in the social and adventurous motility systems displayed a distinct lack of cell alignment. Video microscopy observations of domain features of in vivo swarming M. xanthus cells were also consistent with the AFM data. The results presented here reveal that unique domain formation within swarms of wild type cells is a biologically driven process requiring the social and adventurous motility systems and is not a statistical phenomenon or thermodynamic process arising from liquid crystal behavior.

The dif operon is essential for fruiting body formation, fibril (exopolysaccharide) production and social motility of Myxococcus xanthus. The dif locus contains a gene cluster homologous to chemotaxis genes such as mcp (difA), cheW (difC), cheY (difD), cheA (difE) and cheC (difF), as well as an unknown ORF called difB. This study used yeast two-hybrid analysis to investigate possible interactions between Dif proteins, and determined that DifA, C, D and E interact in a similar fashion to chemotaxis proteins of Escherichia coli and Bacillus subtilis. It also showed that DifF interacted with DifD, and that the novel protein DifB did not interact with Dif proteins. Furthermore, DifA-F proteins were used to determine other possible protein-protein interactions in the M. xanthus genomic library. The authors not only confirmed the specific interactions among known Dif proteins, but also discovered two novel interactions between DifE and Nla19, and DifB and YidC, providing some new information about the Dif signalling pathway. Based on these findings, a model for the Dif signalling pathway is proposed.