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Salmonella typhimurium and Escherichia coli K12 mutants of R chemotype, with varying contents of major proteins, were studied with respect to serum-mediated killing. The mutants demonstrated a different susceptibility to serum lytic action. These results were related to phospholipid and fatty acid content, as well as different physico-chemical surface properties, such as outer membrane fluidity. Tests were carried out on all parameters considered in the literature to demonstrate the resistance to complement. Our results showed that in sensitive strains such as Salmonella strains SH6261, SH6378, SH5551, SH6017 and E. coli PC0479 tests taken alone were not sufficient to explain the resistance to complement. Therefore, complement susceptibility is probably determined by many factors influencing the microheterogeneity of the membrane system.

Studies have been carried out on protein and fatty acid patterns present in the outer layers of E. coli strains which are considered pathogenic in relation to their serogroup. The patterns were related to surface properties of hydrophobicity of the same cells and to their resistance to phagocytosis. The results obtained demonstrate the presence in the pathogenic serogroups of a proteic band of high molecular weight as well as differences in the percentages of fatty acids with respect to the non-pathogenic serogroups. Furthermore, these serogroups are more resistant to phagocytosis; this resistance seems to be correlated to a greater surface hydrophilia of the pathogenic serogroups, leading to different relations of the lipidic and protein fraction of the outer layers.

In this study we investigated the cytotoxic activity mediated by antibody dependent cells (K cell) in C57BL/6 mice experimentally infected with Brucella abortus. The involvement of the K cells in experimental infection from Brucella was studied by evaluating the bacteriolysis in the presence of specific antibodies. After 3 h of incubation, approximately 30 +/- 3% of the labeled Brucellae undergo lysis, whereas the presence of lymphocytic populations alone, or of a specific decomplemented serum, determines a lysis of 5 +/- 3%. Cytotoxic antibody dependent activity of purified lymphomonocytic populations (ADCC) evaluated against cells labeled with 51Cr sensitized with specific antibodies obtained from mice is considerably increased in mice experimentally infected with Brucellae with respect to that present in non-infected mice. The increased activity begins to manifest itself 5 to 7 days after inoculation of the Brucellae reaching a maximum on the 10th day. The same results were obtained in the lymphocytic populations prepared from normal animals and those infected by Brucellae searching for K cells with the ability to form rosettes (EA-RFC).

Escherichia coli K12 suspended in different media showed a loss of phospholipids. Mg2+ and Ca2+ 0.01 M prevented phospholipid loss and stabilized Escherichia coli K12 for bactericidal and bacteriolytic assays.