SPI1 and infection: However, induction of SPI2 in the intestines prior to invasion [50], [51] and the expression of SPI1 in response to diverse stimuli in vitro[41] indicate that additional un-identified signals affect expression of virulence genes in Salmonella. To our surprise, we found that the exogenous presence of putrescine and spermidine, in the culture media prior to infection significantly enhanced intracellular survival of the polyamine mutant indicating that Salmonella sense and respond to exogenous polyamines by transcriptional priming of the bacteria for intracellular survival.