Studies on the mechanism of bacterial resistance to complement-mediated killing. I. Terminal complement components are deposited and released from Salmonella minnesota S218 without causing bacterial death

The mechanism of resistance of gram-negative bacteria to killing by complement was investigated. Complement consumption and uptake of purified, radiolabeled complement components on bacteria was studied using a serum-sensitive and a serum-resistant strain of Salmonella minnesota. Twice as many molecules of ¹²⁵I C3 were bound per colony-forming unit (CFU) of the smooth, serum-resistant S. minnesota S218 as were bound per CFU of the rough, serum-sensitive S. minnesota Re595 in 10% pooled normal human serum (PNHS), although 75-80% of C3 was consumed by both organisms. Hemolytic titrations documented total consumption of C9 by 5 min and >95% consumption of C5 and C7 by 15 min in the reaction with S218 with 10% PNHS. In contrast, negligible C5 depletion, 10% C7 consumption, and only a 26% decrease in C9 titer occurred with the serum-sensitive Re595. Binding of ¹²⁵I C5, ¹²⁵I C7, and ¹²⁵I C9 to S218 and Re595 was measured in 10% PNHS. A total of 6,600 molecules C5/CFU, 5,200 molecules C7/CFU, and 3,100 molecules C9/CFU bound to S218 after 5-10 min of incubation at 37°C, but 50-70% of the C5, C7, and C9 bound to S218 was released from the organism during incubation at 37°C for 60 min. Binding of 2,000 molecules C5/CFU, 1,900 molecules C7/CFU, and 9,000 molecules C9/CFU to Re595 was achieved by 20 min and was stable. The ratio of bound C9 molecules to bound C7 molecules, measured using ¹³¹I C9 and ¹²⁵I C7, was constant for both organisms after 15 min and was 4.3:1 on Re595 and 0.65:1 on S218 in 10% PNHS. With addition of increasing amounts of purified, unlabeled C9 to 10% PNHS, there was no change in the C9:C7 ratio on Re595. However, with S218 there was a linear increase of the C9:C7 ratio, which approached the ratio on Re595. There was no ¹⁴C release from S218 incubated in PNHS, nor was there evidence by electron microscopy of outer membrane damage to S218. Therefore, S. minnesota S218 is resistant to killing by PNHS, despite the fact that the organism consumes terminal complement components efficiently and that terminal components are deposited on the surface in significant amounts. The C5b-9 complex is released from the surface of S218 without causing lethal outer membrane damage.