Serious infections due to gram-negative micro-organisms have been increasing over the last decade. Despite aggressive antimicrobial therapy the mortality of these infections is still high.
Data on pathogenesis of these infections are conflicting. Therefore, the phagocytic and bactericidal functions of polymorphonuclear leucocytes (PMNs) and monocytes (MNs) and the opsonic activity of serum from patients with gram-negative bacillaemia were evaluated. Leucocytes from five of 20 patients (25 ~) showed diminished phagocytic cell function; in three of the patients the chemotactic activity of leucocytes was also decreased. Compared with normal serum, none of 37 patient sera tested had defects in levels of immunoglobulins, CH s o and C 3. Forty-nine percent of 75 patients were infected with bacterial strains that were poorly opsonized in normal serum. Thirty percent of the isolates were also ineffectively opsonized in the corresponding patient's serum. In some patients an increase in opsonic activity due to heat-stable opsonins was found after two weeks.
Impaired opsonization of E. coli strains correlated with the presence of K-capsular polysaccharide in bacteria. It was concluded that impaired leucocyte function and ineffective opsonization play a role in the pathogenesis of gram-negative bacteraemia. Specific heat stable opsonins (presumably specific antibodies), appear to be necessary for effective phagocytosis of gram-negative blood isolates.