Experimental procedures
Bacterial strains. All strains were derivatives ofSalmonella enterica subsp. enterica serovar Typhimurium str. LT2 (hereafter S . Typhimurium) (Table 1). Gene deletions were constructed using the protocol that employs λ Red recombinase to engineer in-frame deletions as described elsewhere (Datsenko and Wanner, 2000).