Accessibility of enterobacterial common antigen to antibodies in encapsulated and non-capsulated S and R forms of Escherichia coli.

Antiserum specific for the enterobacterial 'common antigen' (ECA) was obtained by absorbing a rabbit ECA antiserum with an ECA-negative mutant. Accessibility of ECA to antibodies in encapsulated and non-capsulated S and R forms derived from Escherichia coli O8:K27 was studied using the indirect immunoferritin technique (whole-mount electron microscopy). The number of ferritin particles on the bacterial surface decreased in the order, non-capsulated R greater than encapsulated R greater than on-capsulated S greater than encapsulated S form, indicating that both the O and K antigens partly cover ECA on the surface of the outer membrane. Whole-mount and thin-section electron microscopy showed that ferritin was evenly distributed on the surface of R mutants, whereas it formed clusters on the S forms.