Most infections are the result of a surface-attached community of bacteria that displays many unique characteristics. Our understanding is still limited, however, with respect to how pathogens are colonizing surfaces to begin infection. It is known that the arrival of bacteria to host tissues is often aided by self-generated motility of the organism. Pathogens such as Salmonella enterica, Vibrio cholerae, Proteus mirabilis, and Pseudomonas aeruginosa are able to spread rapidly over surfaces by the process of swarming. Many bacteria generate flat, spreading colonies, called swarms because they resemble swarms of insects. In the beginning of the talk, swarms of the myxobacterium, Myxococcus xanthus will be described in detail. Individual M. xanthus cells are elongated; they always move in the direction of their long axis; and they are in constant motion, repeatedly touching each other. Remarkably, they regularly reverse their gliding directions. We have constructed a detailed cell- and behavior-based computational model of M. xanthus swarming that allows the organization of cells to be simulated [1]. By using the model, it will be shown that reversals of gliding direction are essential for swarming and that specific reversal frequencies result in optimal swarming rate of the whole population [2]. This suggests that the circuit regulating reversals evolved to its current sensitivity under selection for growth achieved by swarming. Also, an orientation correlation function will be used to show that microscopic social interactions help to form the ordered collective motion observed in swarms. In the second part of the talk we will discuss a model of the swarming of the Pseudomonas aeruginosa. 1. Wu, Y., Jiang, Y., Kaiser, D., and M. Alber [2007], Social Interactions in Myxobacterial Swarming, PLoS Computational Biology 3 12, e253. 2. Wu, Y., Jiang, Y., Kaiser, D., and M. Alber [2009], Periodic reversal of direction allows Myxobacteria to swarm, Proc. Natl. Acad. Sci. USA 106 4 1222-1227 (featured in the Nature News, January 20th, 2009, doi:10.1038/news.2009.43).