Phenomenon of growth inhibition of Borrelia burgdorferi sensu lato (BBSL), the agent of Lyme disease, by antiborrelial antibodies was observed and investigated. Some of the antiborrelial antibodies were bactericidal in the absence of complement and phagocytes. Based on growth inhibition phenomenon we developed and evaluated the function-oriented in vitro growth inhibition assay (GIA). GIA proved to be more strain-specific and a better predictor of protection against infectious challenge than matrix-based assays, such as ELISA and Western blot. Growth inhibition phenomenon was also applied as a tool for selection of antibody-resistant BBSL mutants. All BBSL isolates characterized to date have one or a combination of several major outer surface proteins (Osps). Mutants of BBSL with altered or absent Osps were selected with polyclonal or monoclonal antibodies (mAbs) at a frequency of 10(-2) to 10(-6). Based on PAGE and Western blot analysis all examined mutants were catalogued into 4 classes. Some of these mutants were later employed in studies of functional importance of Osps in immunopathogenesis of BBSL. Several studies revealed some possible functions of Osp proteins in borrelias. In one study, Osp-lacking mutant was distinguished from its Osp-bearing parent by autoaggregation and slower growth rate, diminished capacity to adhere to human umbilical vein endothelial cells, decreased plating efficiency on solid medium as well as serum and complement sensitivity. Mutant also was unable to evoke a detectable immune response after intradermal live cell immunization even though mutant cells survived in mouse skin for the same duration as wild type cells.(ABSTRACT TRUNCATED AT 250 WORDS)