Invasion of HeLa cells by recombinant E. coli. The invasion of HeLa cells by E. coli expressing Mce4A and Mce1A proteins was monitored by transmission electron microscopy. The stages of invasion by recombinant E. coli (pET28a/mce4A) (fig. 3A), E. coli (pET28a/mce1A) (fig. 3B) and E. coli (pET28a) (fig. 3C: j) are shown. A: a and B: f infection with uninduced recombinant E. coli. A: b, c, d, e and B: g, h, i infection with induced recombinant E. coli expressing Mce4A and Mce1A respectively. Unfilled arrow (bacteria attached to cell membrane), thick arrow (membrane ruffling), thin arrow (internalized bacteria). Bars = 1 μm in all the cases, except in A: c, d and B: g, h, where it is 0.1 μm.