SPI1 and infection: The complex expression patterns of SPI1 and SPI2 during infection led us and others to develop the concept of the Salmonella extracellular (STEX) and intracellular (STIN) virulence gene expression programmes, [4,5] Successful host invasion and colonisation requires expression of the STEX virulence gene programme followed by expression of the STIN programme (characterised by SPI1 and SPI2 expression respectively) [6].