Australia has the highest reported incidence of salmonellosis in the developed world, with three times higher reported cases compared to the UK or US. Recently, there has been a rapid emergence of a monophasic variant of Salmonella enterica serovar Typhimurium, 4,[5],12:i:-. These monophasic strains are characterised by increased multi-drug resistances and deletions in the FljAB operon, preventing expression of the phase two flagellar antigen FljB. In the absence of this key antigenic factor, we hypothesised that monophasic S.Typhimurium variants would induce altered host intracellular responses to infection and replication. We infected human macrophages and colonic epithelial cells with a panel of monophasic S. Typhimurium isolates and compared their intracellular growth and effect on cell viability to control biphasic isolates (expressing both flagellar antigens). We observed limited infection of monophasic strains in epithelial cells with the exception of two strains which were highly attenuated. However, in macrophages we observed 5-10-fold higher intracellular replication of most monophasic strains compared to biphasic isolates, despite lower initial infection rates. Intriguingly, this did not adversely affect macrophage viability, which remained equal to or higher than biphasic-infected cells. We also saw decreased activation of caspases 1, 3 and 8 at early stages of replication, suggesting that the monophasic isolates were able to replicate to high levels within macrophages without activating the intracellular cell death pathways typically observed in S. Typhimurium infections. This was also associated with an early type I interferon and NF-κB activation signature, which we are currently investigating in vitro and in vivo to determine the mechanism by which monophasic S.Typhimurium strains can modulate host detection and immune responses.