The attachment to and entry into L mouse fibroblast cells of viable ph
ase I and phase II Nine Mile Coxiella burnetii was investigated. The u
se of P-32-labelled rickettsiae showed that phase II C. burnetii attac
hed more readily to L cells than phase I organisms; this probably acco
unts for the more rapid establishment of infection of host cells by th
e phase II agents. Two lines of evidence indicated that C burnetii pla
ys a passive role in both attachment and entry into host cells: (1) in
activation of rickettsiae by either heat or glutaraldehyde did not aff
ect either process, and (2) metabolic inhibitors of L cell phagocytic
function - NaF and cytochalasin B and D - abolished rickettsiae uptake
. These results indicate that it is an endocytotic event. While the pr
esence of purified phase I lipopolysaccharide (LPS) did not interfere
with attachment of rickettsiae to the surface of host cells, it marked
ly impaired entry of C burnetii in both phases. This suggests that LPS
is not an adhesin and that it is toxic to the host cell. Treatment of
L cells with either pronase, subtilisin or subtilopeptidase A signifi
cantly reduced the number of C. burnetii that adhered to the host cell
surface; this result suggests that proteins are either proximate to o
r components of the C. burnetii attachment site.