Effect of pH and haem compounds on the killing of Pasteurella septica by specific antiserum

The killing of Pasteurella septica by horse antiserum has features not previously associated with serum bactericidal reactions. The present work showed that lowering the pH from 7-4 to 6-8 abolished the action of antiserum. The bactericidal effect and the degradation of RNA seen when antiserum is added to P. septica growing in horse serum, were abolished at pH 6-8 in much the same way as when haem compounds were added to the system. Addition of chloramphenicol, rifampicin or puromycin to P. septica growing apparently normally in antiserum at pH 6-8 or in antiserum containing haem compounds led to rapid killing of the bacteria and to degration of their RNA. Addition of these antibiotics to P. septica growing in normal serum produced only bacteriostasis and did not induce RNA breakdown. In contrast, nalidixic acid, although inhibiting growth, did not induce rapid killing and RNA breakdown under the same conditions. These findings were unexpected and led to a reassessment of ideas concerning the mechanism of action of specific antiserum to P. septica. Although iron compounds clearly abolish the bactericidal based simply on an interference with bacterial iron supply is no longer sufficient. The process is more complex and must involve other factors.