Surface properties of Treponema pallidum in relation to phagocytosis by human polymorphonuclear leucocytes in vitro

Surface charge and hydrophobicity of Treponema pallidum have been investigated in relation to phagocytosis by human polymorphonuclear leucocytes (PMNs) in vitro. The treponemal surface was relatively hydrophobic and negatively charged but despite these properties, phagocytosis, as assessed by luminol-enhanced chemiluminescence, was minimal in the absence of serum. Preopsonization of bacteria with serum reduced surface hydrophobicity but promoted phagocytosis, suggesting that specific immune mechanisms may be more important in controlling phagocytosis of T. pallidum in vitro than non-specific surface properties. T. pallidum evoked a much weaker chemiluminescence response from PMNs than the non-pathogenic treponeme Treponema phagedenis biotype Reiterii even though similar numbers of bacteria were phagocytosed, suggesting differences in the reactivity of the surface components of the two organisms toward PMNs. The reactivity of T. pallidum towards PMNs could be increased by removal of the bacterial outer membrane by Triton X-100 treatment. These observations reinforce the suggestion that the outer surface of T. pallidum is inherently inert.