Interaction of Escherichia coli with catecholamine hormones, transferrin and lactoferrin

The correlation between stress and increased susceptibility to infectious disease has been widely established. A direct consequence of stress, whether physical or mental, is the release of catecholamine stress hormones (adrenaline and noradrenaline), which as well as reducing immune function, have recently been shown to directly enhance the growth of a variety of Gram positive and Gram negative bacteria. The mechanism of catecholamine-induced growth stimulation involved catecholamines facilitating iron removal from the high affinity mammalian iron binding proteins transferrin and lactoferrin. In the case of Escherichia coli, the direct binding of transferrin was also an important part of this process. However, the precise mechanism(s) by which catecholamines enabled bacterial access to transferrin was not clear, and neither was the identity of the E. coli protein(s) responsible for the binding of transferrin. Using the enteropathogenic E. coli strain E2348/69 as model organism, the objectives of this project were therefore to determine how catecholamines facilitated iron removal from transferrin and lactoferrin, identify and characterise the E. coli transferrin-binding protein, and to investigate globally the effects of catecholamine stress hormones on E. coli growth and virulence. In this study, using a combination of electron paramagnetic resonance spectroscopy, chemical and polyacrylamide gel electrophoresis techniques, it was found that catecholamine stress hormones form a complex with the iron(III) associated with transferrin and lactoferrin, causing its reduction into iron(II), to which these proteins have much lower affinity. This enables the dissociation of iron from transferrin or lactoferrin, which then becomes available for bacterial uptake through ferric or ferrous iron uptake systems. It was also shown that although catecholamines enabled E. coli to acquire iron from transferrin, the amounts of iron made available by the stress hormones, while sufficient to enable growth, the levels of iron actually within the catecholamine-treated bacteria was not enough to switch off the expression of iron-regulated genes. The effects of catecholamines on expression of E. coli proteins was also investigated, and it was found that expression of intimin, important in formation of attaching and effacing lesion, was up-regulated in the presence of catecholamines. Investigation of the role of quorum sensing in the mechanism of E. coli catecholamine responsiveness was also undertaken. Analysing catecholamine responsiveness of a series of E. coli E2346/89 luxS mutants showed that luxS is not required for the ability of E. coli to interact with catecholamine stress hormones.