3). Heat-inactivation (to remove complement activity) abolished serum bactericidal activity, consistent with bacterial killing being complement-dependent as previously shown for D23580 ( MacLennan et al., 2008). S. Paratyphi A CVD1901 was highly sensitive to serum killing with all dilutions of human sera tested killing the bacteria. 1/2, 1/4, 1/8 Adriamycin manufacturer dilutions effected a 3 log10 kill and 1/16 dilution a 1 log10 kill by 180 min. The bactericidal activities of the human sera against S. Typhimurium isolates were more affected by serum dilutions, particularly

D23580 — the highest dilution of the human sera that could still kill LT2 was 1/8 for donor 1 and donor 2 sera, and 1/4 for the pooled Malawian serum, while 1/4, but not E7080 in vivo 1/8 dilution of all sera killed D23580. These findings indicate the limitation of using diluted human serum in serum bactericidal assays against S. Typhimurium. Since both antibody and complement are co-diluted, the individual contributions of anti-Salmonella antibody and complement to killing of Salmonella cannot be determined. Hence, it is necessary to provide an exogenous source of complement in S. Typhimurium serum bactericidal assay when serial dilutions of human

serum are used as the source of antibody. BRS is commonly used as an exogenous source of complement in serum bactericidal assays and was used as the exogenous source of complement in this study. We first measured the ability of BRS alone to kill Salmonella by determining the viable bacterial numbers following exposure to different percentages (20%, 50%, 75%, next 100%) of BRS over a 3 h time course. All percentages of BRS tested (both AbD Serotec and Pel-Freez BRSs) did not kill S. Typhimurium D23580 and LT2 ( Fig. 4). The viable

bacterial count of S. Typhimurium D23580 increased by approximately 1 log10 in all percentages of BRS tested, while S. Typhimurium LT2 was bacteriostatic. With S. Paratyphi A CVD1901, higher percentages of both AbD Serotec and Pel-Freez BRS (100% and 75%) could kill the bacteria by 1–2 log10 over 180 min ( Fig. 4). This antibody-independent killing was removed when BRS was heat-inactivated. The difference in susceptibility of the three Salmonella isolates to killing by neat and diluted human serum suggested that there will be differences in the amount of BRS required for bactericidal activity in the presence of antibody. Using AbD Serotec BRS as the exogenous complement source and heat-inactivated diluted pooled Malawian serum for antibody, we investigated the amount of BRS required to kill the three bacterial isolates. With S. Typhimurium D23580 as the target isolate and 1/40 or 1/400 diluted human sera as antibody source, bacterial growth occurred with 20% BRS, and bacteriostasis with 50% BRS ( Fig. 5). Killing of D23580 occurred with 75% BRS. All three percentages of BRS killed S. Typhimurium LT2 at 1/40, 1/400 and 1/4000 diluted human serum, although with limited killing at 1/4000. With S.