Fig. 1.

Initial cell densities were critical for the outcomes of interaction between SP and NTHi. Co-cultures were set up with mixtures of different amounts of TIGR4 cells and 86028NP cells. Viable cells were counted at 16 and 24 hour time points after inoculation. (H: high concentration/~10⁶ cfu/ml; L: medium concentration/~10⁴ cfu/ml). High densities of TIGR4 completely inhibited 86028NP survival no matter how many 86028NP cells there were initially. However, higher cell densities of 86028NP could overcome the inhibition of TIGR4 in lower densities and enhance TIGR4 survival. The bacterial cultures and viable cell counts were performed four times independently. *: P<0.05; **: P<0.01.