Hypertonic sucrose inhibited the bactericidal activity of lysozyme-free serum against a rough strain of Escherichia coli. The duration of the inhibition correlated with the duration of plasmolysis caused by the sucrose. Although the lethal action of the serum was delayed, the prompt release of alkaline phosphatase by the cells suggested that nonlethal damage to the cell wall had taken place under these conditions. In contrast, the crypticity of the cells for ,B-galactosidase did not deteriorate until the viability of the bacteria began to decrease. It is concluded that the primary site of action of serum is at the bacterial cell wall; however, in the absence of lysozyme, the lethal event was subsequent damage to the bacterial cell membrane.

The mechanism of the lethal action of human serum on a rough strain of Escherichia coli was investigated by use of serum with and without lysozyme, in medium of low and high osmotic pressure, with cells radioactively labeled in the peptidoglycan polymer, and by electron microscopy. The results suggested that there are two separate components in the bacterial cell wall that afford structural support for the cell. Lysozyme attacked one of these, the peptidoglycan polymer. Serum damaged the other, which is probably the peripherally located lipopolysaccharide-phospholipid complex. The cell wall damage caused by lysozyme-free serum promptly resulted in cell death under usual conditions. In plasmolyzed cells, however, the wall damage was not lethal, presumably because the membrane of the plasmolyzed cell was protected from secondary lethal changes which otherwise occur.

No abstract