FIG. 1.

The degree of confluence of mammalian cell layers affects the attachment of different Leptospira strains differently. ³⁵S-labeled bacteria were added to wells containing cells that had reached confluence 2 days before the experiment (2 d postconfluent), wells containing cells that were just reaching confluence (with 95 to 100% well area coverage), and wells containing cells expected to reach confluence the day after the experiment (with 45 to 50% well area coverage). Shown are the means of cell-specific attachment levels ± standard deviations, expressed as the percent inoculum bound, from a single experiment representative of multiple experiments. For the HEp-2 cells, the attachment of both L. interrogans strains to postconfluent layers was significantly less efficient than attachment to confluent layers (P ≤ 0.0008); a similar difference (P < 0.0001) in the efficiencies of attachment to confluent versus subconfluent layers was observed, although this result is likely to be affected by the approximately twofold differences in the numbers of cells in the wells. For the other cell lines, confluence did not significantly affect the efficiency of bacterial attachment (P ≥ 0.05). L. interrogans serovar Canicola bound to confluent MDCK cell layers more efficiently than to HEp-2 cell layers (P < 0.016). L. interrogans serovar Copenhageni, in contrast, bound more efficiently to HEp-2 than to MDCK confluent cell layers (P < 0.0001). All comparisons were analyzed by Student's two-tailed t test.