Abstract
Serological investigation of the lipopolysaccharide (LPS) of Acinetobacter calcoaceticus revealed a new antigen to which antibody in high titre is present in the serum of many mammalian species. The passive haemolysis test showed that antibody, in titres ranging from 32-4096, was invariably present in the serum of mice, rats, guinea-pigs, and horses. Rabbits and human beings had lower and more variable titres (less than 2-512). The antigen persisted after prolonged hydrolysis of the LPS in 1% acetic acid at 100 degrees C. Acinetobacter lipid A, which resembled antigenically the lipid A of many gram-negative bacteria, could be distinguished from the new antigen by inhibition and absorption experiments. Antibody to the new antigen could be completely absorbed with acinetobacter lipid A but not with enterobacterial lipid A; moreover, the latter failed to react with the antibody in the passive haemolysis test. Immunisation of rabbits with lipid A-immunogenic acinetobacter cells gave rise to antibodies against the new antigen and to lipid-A antibodies. Absorption of the immune serum with acinetobacter lipid A removed antibody to both antigens, but absorption with enterobacterial lipid A removed only the lipid -A antibodies.