Exposure of many strains of gram-negative bacteria to suitable concentrations of human or animal serum results in loss of viability, and sometimes dissolution, of the bacterial cells. Since the recognition, towards the end of the last century, that the bactericidal and bacteriolytic properties of serum are destroyed by heating at 56 C, an extensive literature has accumulated indicating that the killing process is effected by deposition on or insertion into the bacterial envelope of thbe assembled terminal proteins of the complement cascade, the membrane attack complex (MAC). It is now clear that activation of complement by gram-negative bacteria can occur via the classical or the alternative path-way; the former usually requires for its activation recognition of bacterial surface antigens by certain antibody classes, whereas activation of the latter can be initiated and amplified, in the absence of antigen-antibody interactions, by poorly understood structural or conformational characteristics of the cell surface. Killing by serum is often, but not invariably, accompanied by bacteriolysis, an event dependent on adequate amounts of lysozyme (mucopeptide N-acetylmuramoylhydrolase; EC 3.2. 1.17); this basic enzyme degradespeptidoglycan to form monomers or multiples of the disaccharide-tetrapeptide unit (265).

The majority of investigators of bactericidal and bacteriolytic phenomena have utilized serum from humans (26, 222, 328) or from domestic (194, 2%) and laboratory animals (7, 47, 144) as sources of complement; the capacity to kill gram-negative bacteria is associated, however, with serum from a wide variety of warm-blooded and cold-blooded animals, including fish (218, 322), and probably reflects the distribution of immunological responsiveness and the comple-ment system in the animal kingdom. Susceptibil-ity to the serum bactericidal system is a widespread characteristic of gram-negative bacteria; in addition to the many well-documented in-stances ofenterobacterial susceptibility to com-plement, serum is known to possess bactericidal and bacteriolytic activity against susceptible representatives of practically every gram-negative genus so far examined. In fact, any procaryote that presents a lipid bilayer membrane to the external environment would appear to bepoten-tially susceptible to complement killing. Although outside the scope of thisreview, comple-