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.