AIEC disrupts MDCK-I monolayers and replicates in the cell cytoplasm. MDCK-I monolayers were either left uninfected (sham control; Panel A) or infected with AIEC for 4 h (Panel B & D) and 48 h (Panel C). While uninfected cells maintained normal intercellular spaces (Panel A), transmission electron photomicrographs demonstrated disruptions in intercellular junctions between epithelial cells (*), as well as adhesion (black arrow) and invasion and replication (arrowheads and white arrow, respectively) of bacteria in 4 h AIEC, strain LF82-infected MDCK-I cells (Panel B). After 48 h of bacterial infection, monolayers were severely disrupted, accompanied by morphological changes within cells (Panel C). Some of the invasive bacteria appeared within membrane-bound vacuoles after 4 h of infection (arrowheads in Panel D). Measurement bar = 1 μ.