- We previously demonstrated that incubation of E. coli in normal human serum (NHS) resulted in the release of a finite fraction (approximately 30%) of LPS from the bacterial outer membrane. In experiments reported here, we examined factors which may enhance or diminish the capacity of NHS to mediate this limited LPS release. Both the susceptibility to serum killing and LPS release were dependent on growth phase. Optimal killing and release coincided with the midlogarithmic growth phase. The composition of LPS subunits in the outer membrane appeared to influence serum-mediated LPS release. Serum treated E. coli enriched for Rc-chemotype LPS released less LPS from their outer membrane than the wildtype 'smooth' bacteria during exponential growth. LPS fractions released by NHS or EDTA appeared to a large degree to overlap, suggesting that NHS-mediated LPS release may involve the action of a serum chelator. A serum-resistant mutant failed to release LPS in either NHS or EDTA. This latter observation suggests that LPS release may be a relevant event in serum killing. We did not detect any modulation of LPS release when E. coli were pre-incubated with a series of antibiotics prior to treatment with NHS.