The lag period had the most distinctive transcriptional profile with few genes affected under other conditions. However, a small number of genes induced during lag phase were also induced in immobilized cells. The majority of genes down-regulated during lag find more and in stationary phase were not affected under any other situation. A large number of up-regulated genes in immobilized cultures were also induced in stationary phase. The transcription of several genes in response to environmental stresses was inversely related

with their expression during exponential growth. Figure 3 shows that the node representing genes induced during exponential growth was connected with few genes repressed under stressing environments while the node SBE-��-CD molecular weight for genes repressed in exponential growth was linked with genes up-regulated in response to stress conditions. Figure 3 Network 2 is an extension of Network 1 that represents genes up(down)-regulated at various growth stages and immobilization condition together with those responding to several environmental stresses and anoxic condition included in Network 1. The genes LY411575 manufacturer degree (k) distribution of the transcriptional response networks decayed as a power law, P(k) ~ k –2.7(Figure 4A), i.e. the network belonged to the family of scale-free

networks characterized by the presence of few highly connected genes or hubs corresponding to the genes that were differentially transcribed in many conditions. A list of 54 genes forming hubs in Network 2 is included in supplementary material (Additional file 2: Oxalosuccinic acid Table S2). Figure 5 shows a sub-network extracted from Network 2 (termed Network 2.1), containing exclusively the 54 genes that formed hubs together with the conditions at which they were differentially transcribed. The transcription of none of these

hubs was regulated during the lag phase. Figure 4 Nodes degree distribution -P( k ) represents the probability that the number of links per node is equal to k – of the genes connected to environmental stresses, growth stage or immobilization condition in the environmental Network 2 (A) and of the genes connected to metabolic pathways and cellular roles in the S . Typhimurium genome scale Network 3 (B). Distributions followed the power law indicating the existence of highly connected genes or hubs in both networks. Figure 5 Network 2.1, which is a sub-network from Network 2 including only genes differentially transcribed in the majority of environmental conditions (hubs). Analysis of the genome scale network for S. Typhimurium shows a scale free topology with hubs formed by genes involved in many metabolic pathways and cellular functions. To explore the presence of hubs in the genome of Salmonella, we looked for genes involved in a large number of cellular functions and metabolic pathways in a genome scale bi-partite network (termed Network 3) constructed for the genome and plasmids of S.