Micro-organisms are constantly monitoring their surrounding environment and making important lifestyle decisions. This decision process is governed by large genetic networks that process the information leading to a phenotypic response from the cell. Using the food-borne pathogen, Salmonella, as a model organism, we try to investigate how cells encode strategies in networks to optimally control cellular behavior. Salmonella, on ingestion with contaminated food, swims in small intestine using propeller-like structures, flagella, on its surface. On reaching the site of infection, it assembles a hypodermic needle on its surface using which it injects proteins into host cells. These proteins cause a change in the host-cell shape leading to internalization of the bacterium. If the bacterium fails to get internalized, it assembles finger-like projections, fimbriae, on its surface to adhere to and persist in the intestine. How Salmonella dynamically regulates gene expression and assembly of these organelles is the focus of this study. Our results demonstrate that the networks controlling genes necessary for flagella, needle, and fimbriae are designed so as to encode logic gates and limit expression to conditions optimum for infection. In addition, there is cross-talk between these three systems which serves to dynamically control the timing of activation and de-activation of these networks. Collectively, we demonstrate that cells dynamically process information in genetic networks which ensures that the encoded products are produced at the correct locales, at the appropriate levels, and for the appropriate amount of time
