Invasion and intracellular survival of S . Typhimurium strains in cultured cells. S. Typhimurium ATCC 14028 (open bars) and 14028 ΔyqiC::CAT mutant (filled bars) recovered from lysates of J774 murine macrophages (A) or human epithelial HeLa cells (B). The number of viable bacteria from cell lysates was determined 1, 6 and 24 hours post infection as described in Materials and methods. The reported value is the media of duplicates of a representative experiment +/- standard deviation.