<i>P. pneumotropica</i> is resistant to the complement system action.

<p>(<b>A</b>) <i>P. pneumotropica</i> suspensions (10³ bacteria/tube) were incubated with 40% of normal human serum (NHS) or heat inactivated (HI-NHS) diluted in PBS pH 7.4, at 37°C for 30 min and then sera was removed and bacteria suspended in PBS, diluted (2 tenfold dilutions) and cultivated in LB agar plates (triplicate) at 37°C for 28 h. Data are expressed as percentage of colony forming units, considering HI-NHS as 100%. The data obtained from ten independent experiments is shown. (<b>B</b>) C3b binding to <i>P. pneumotropica.</i> Bacteria suspensions (10⁸ bacteria/tube) were incubated for 1 h at 37°C with 20% NHS or HI-NHS. After washing, C3b deposition was quantified by ELISA with anti-C3 polyclonal antibodies. Data were obtained from three independent experiments and analyzed statistically using Student’s <i>t</i>-test where *<i>p</i><0.05; ***<i>p</i><0.0001.