Activation of human complement by Yersinia enterocolitica: ultrastructural alterations and C3b-deposition

Rapid killing of Yersinia enterocolitica strain 75 in smooth form (Ye 75 S) was observed in the presence of serum or of lysozyme-free serum whereas the killing activity of EGTA-serum was slow, and absent in heated (30 min 56 degree C) serum. Similarly, complement (C) activation by Ye 75 S was rapid in serum and lysozyme-free serum but slow via the alternative pathway (EGTA-serum). These data suggest that C is sufficient for killing of the cells and most active via an intact classical pathway. Electronmicroscopic studies were performed on bacterial killed by serum (C + lysozyme) or by lysozyme-free serum (C). In these experiments cell fragmentation and spheroplast formation were seen after exposure of Ye 75 S to serum; in bacteria incubated with lysozyme-free serum "blebs" formation was observed as the most prominent alteration. These blebs most likely originate from the outer membrane as a result of C activation on the cell surface. The deposition of activated C (C3b) on Ye 75 S was analyzed kinetically in the presence of serum or EGTA-serum. With serum (30 vol%) massive C3b deposition was observed within 20--30 min whereas with EGTA-serum (30 vol%) the deposition of C3b was slower and less complete. Experiments with EGTA-serum also revealed that the deposition of C3b started at single sites mainly located in the region of the cell poles; from these sites spreading of C3b occurred until large areas of the cell surface were covered. These data suggest that C activation via the alternative pathway is restricted to certain regions of the bacterial surface.