Salmonella enterica is an intracellular bacterial pathogen that replicates within membrane-bound
vacuoles by delivering virulence (effector) proteins across the vacuolar membrane by
the SPI-2 type III secretion system (T3SS). In this thesis I show that through the action
of this T3SS, Salmonella selectively interferes with a subset of endosome-to-trans-Golgi
network (TGN) traffic that is involved in mannose-6-phosphate receptor (MPR) recycling.
Two pathways of endosome-to-TGN traffic have been described: one involves the Q-SNARE
syntaxin 6; the other is defined by the Q-SNARE syntaxin 10 and involves retrograde transport
of the MPRs. Salmonella specifically disrupted syntaxin 10-dependent endosome-to-
TGN traffic and this was accompanied by redistribution of MPR from the TGN, misrouting
of newly synthesized lysosomal enzymes and attenuation of lysosome function. The SPI-2
T3SS effector SifA is an important virulence determinant that is known to interact with
the endolysosomal system. SifA was shown to be required and sufficient to mediate the
effects on lysosome biogenesis during infection or following transfection of host cells. By
contrast, a variant of SifA with a single amino acid substitution that prevents interaction
with its host protein target, SKIP, failed to redistribute MPR or to reduce lysosomal function.
SKIP was found to contribute to lysosomal biogenesis in uninfected cells. Inhibition
of lysosomal enzyme function with a lysosomal protease inhibitor or by depleting cells of
syntaxin 10 enhanced intracellular bacterial replication. I conclude that SifA attenuates
lysosomal activity through its interaction with SKIP and that this facilitates intracellular
bacterial replication
