sion of this capsule alone does not correlate with Neutrophil Killing of Opsonized B. Pseudomallei their ability to evade neutrophil clearance, as acapsular B. thailandensis displayed a similar ability to resist neutrophil killing as B. pseudomallei. Thus, both bacteria appear able to inherently evade neutrophil clearance and additional immune mechanisms must be involved if neutrophils are able to control these infections in vivo. Antibodies and other serum opsonins are known to be crucial for neutrophils to recognize and kill certain bacteria, and particularly those 21990348 that possess a capsule. We chose to focus on complement as a critical opsonin to promote efficient killing of these Burkholderia species, particularly since these innate components would be present early during infection and before the development of Burkholderia-specific antibodies. Quantification of C3 deposition on the bacterial surface indicated that B. thailandensis acquired significantly more C3 on its surface compared to B. pseudomallei. Parallel studies using the DCPS B. pseudomallei mutant suggested that this capsular material is largely responsible for the reduced C3 levels deposited on B. pseudomallei, and titration studies indicated that this protection was most apparent in low levels of serum/complement, which likely reflects the levels encountered in most host tissues. When serum levels were relatively low, components of the classical or lectin pathways were necessary for complement activation on both bacterial species; however when the serum concentration was increased to 20% NHS, activation through the alternative pathway contributed to the majority of C3 on B. thailandensis, but not for B. pseudomallei. Although, B. thailandensis acquired more surface C3 than B. pseudomallei, both Burkholderia species were equally resistant to complement-mediated direct killing. The mechanism that Burkholderia species use to resist direct killing by complement is not known. While B. pseudomallei LPS is known to be involved in serum resistance, and our findings indicate it is absolutely required for the complete complement resistance observed by B. pseudomallei, it has not been determined how it mediates this effect and what host or other bacterial factors are involved in this process. The length of the LPS O-antigen has been associated with serum resistance in some Gram-negative bacteria, and the structure of B. pseudomallei and B. thailandensis O-antigen are similar, suggesting this could represent a common serum-resistance mechanism between these closely-related bacterial species. Since the B. pseudomallei DLPS mutant had less C3 deposition than wild-type B. pseudomallei but still resulted in direct bacterial killing, 10854736 this indicated that C3 is directly deposited onto LPS and may thus prevent the assembly of the MAC on the bacterial outer membrane. Multiple bacterial species including Haemophilus influenzae, Neisseria meningitidis and N. SB366791 price gonorrhoeae, Borrelia burgdorferi, Streptococcus pyogenes, and Moraxella catarrhalis bind negative regulators of the complement system as a means to avoid direct killing by complement, particularly via the alternative pathway. However, there have been no reports that B. 
pseudomallei or B. thailandensis can similarly bind complement regulatory proteins to avoid direct killing. Our data also indicate that B. pseudomallei are resistant to activation of the complement system by the alternative pathway. This finding goes against previous studie
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