Currently there's no evidence to suggest that salads are a major source of food poisoning in the UK and very few incidents of contamination in pre-packed salads have been reported to the Food Standards Agency in the past three years.
Bacteria have tiny channels in their walls which operate like the valve on a pressure cooker - they open to release material when the pressure in a bacterial cell gets too great. If the channel didn't open to relieve pressure the bacteria would explode and die, so this is a target for drug development.
S. Shehab, N. Keller, A. Barkay, L. Leitner, A. Leventhal, and C. Block. European Journal of Clinical Microbiology & Infectious Diseases: Official Publication of the European Society of Clinical Microbiology, 17 (11):
749--53(November 1998)PMID: 9923513.
F. Fusco, and V. Puro. Euro Surveillance: Bulletin Européen Sur Les Maladies Transmissibles = European Communicable Disease Bulletin, (March 2008)PMID: 18445438.