FIG. 4.
Expression of β-galactosidase by EDL933(pUJ3) after infection of HeLa cells. At different time intervals after infection, enzymatic activity was determined in bacteria present in supernatants (■) or attached to HeLa cells (●) and compared to that produced by EDL933(pUJ3) grown in DMEM (▴). The basal values of β-galactosidase obtained from EDL933 containing the promoterless plasmid under matching conditions were at least 10-fold lower than the basal levels of the tested clones and were subtracted from each sample. β-Galactosidase activities are expressed as relative light units (rlu) per 105 bacteria and are means of three independent experiments; standard deviations were lower than 5%. Open symbols indicate numbers of CFU at each time point.