Serum sensitivity of strains isolated and antibodies against O antigen in gram-negative bacteraemia

The sensitivity of blood culture isolates to the bactericidal activity of normal human serum (NHS) has been studied in 101 patients with gram-negative sepsis. These results were compared with clinical status and outcome, and to the presence of specific IgG or IgM antibodies to O antigens of bacteraemic strains in autologous serum. 23% of the strains were markedly resistant, 27% markedly sensitive and 50% intermediately sensitive to the bactericidal activity of NHS. Shock or death occurred more frequently in immunocompromised patients and those infected with serum-resistant strains. IgG antibody titres to O antigens were significantly lower in patients with serum-resistant organisms regardless of their immune status. Resistance to natural bactericidal antibodies and low immunogenicity of the infecting organism, plus immunodeficiency in the host, may account for apparent increased virulence of some gram-negative bacilli.