Inducible serum resistance in Salmonella typhimurium is dependent on wzzfepE-regulated very long O antigen chains

Salmonella typhimurium possesses two wzz genes conferring long (wzzST) and very long (wzzfepE) lipopolysaccharide O antigen modal chain lengths. While the long O antigen modal length was essential for complement resistance, the very long modal length was found to have a minor role. However, when grown in the presence of serum, S. typhimurium demonstrated a wzzfepE-dependent increase in the density of very long O antigen chains, resulting in a significant increase in serum resistance. Similar phenotypic changes were observed after growth under iron-limiting conditions, though iron limitation is unlikely to be the sole signalling mechanism behind the changes induced in serum. A wzzfepE::lacZ promoter fusion was used to determine that regulation of wzzfepE transcription is unlikely to be the mechanism behind the variation in O antigen length. Since systemic infection occurs in a small but significant percentage of human non-typhoid salmonellosis, the phenomenon identified in this study may be significant during the bacteraemic phase of infection.