The human intestinal pathogen, enteropathogenic Escherichia coli (EPEC), causes a diarrhoeal disease in infants through a mechanism that is dependent on the injection of effector proteins into epithelial cells of the small intestine. Studies with the HeLa (adenocarcinoma human cervix derived epithelial) cell line have allowed the identification of several EPEC encoded bacterial proteins (Tir, Map, EspF-H) that are delivered into host cells (1). Further work with such cells has revealed the multifunctional nature of both Tir (Translocated intimin receptor) and Map (Mitochondria-associated protein) proteins as well as the ability of these effector molecules to induce signalling events within host cells by independent, synergistic and/or antagonistic mechanisms (2,3). The genes encoding the delivery system and the listed effector molecules are encoded on a chromosomally located ‘pathogenicity island’ called LEE (Locus of Enterocyte Effacement; 1). An outer membrane protein, Intimin, is also encoded on LEE and serves as a receptor for Tir following Tir’s delivery and insertion into the host plasma membrane (1). However, the use of such cell lines, although useful, is somewhat limited since they do not possess many of the characteristics of gut epithelial cells, such as a polarised nature and the presence of absorptive microvilli. This deficiency is illustrated by the finding that the EspF effector molecule is required for EPEC-mediated disruption of tight junction integrity of polarised cells (4) – a process likely to contribute to diarrhoea. Recent studies in our laboratory, using polarised Caco-2 cells (colorectal adenocarcinoma) has revealed both the cryptic nature of EPEC effector molecules and the complex nature of EPEC signalling mediated into host cells, illustrating the need to carry out studies using the most appropriate cells. These conclusions arose from our finding that EspF is not the only EPEC-encoded factor required to disrupt intestinal barrier function, but that two additional LEE-encoded factors are also involved. Thus, we found that the Map effector molecule i) is as essential as EspF for disrupting intestinal barrier function ii) can function independently of EspF, iii) functions to alter tight junction structure and iv) mediates these effects in the absence of its mitochondrial targeting sequence (5). Additionally, the outer membrane protein intimin is shown to be crucial for both EspF and Map to exhibit their intestinal barrier disrupting activities, with this Intimin-mediated activity shown to be independent of interaction with its known receptor, Tir. Indeed, Map retains its ability to induce Cdc42-dependent filopodia formation following delivered into host cells by the intimin mutant but can not exhibit its tight junction disrupting activity (5). Thus, this work not only reveals the cryptic nature of EPEC effector molecules and the complex involvement of multiple molecules in disrupting barrier function, but also shows that EPEC can control effector activity within host cells from an extracellular location. Additionally, this study indicates that Intimin interacts with non-Tir receptor(s) to mediate its controlling influence on Map and EspF’s tight junction disrupting activities.