Evidence that the Serum Resistance Genetic Locus sac-3 of Neisseria gonorrhoeae Is Involved in Lipopolysaccharide Structure

SUMMARY: The gonococcal chromosome contains a sequence of closely linked genes (for example, sac-1, sac-3, nmp) known or presumed to affect cell envelope structure and which appear to influence susceptibility of gonococci to killing by normal human sera (NHS). Previous work has shown that the serum-resistant isolate FA19, and FA899, a serum-sensitive transformant of FA19, differ in outer membrane protein I (PI) and at the sac-3 genetic locus. However, the sac-3 locus is separable from changes determined by nmp-3, the gene determining PI species. We found that FA 19 and FA899 differ in lipopolysaccharide (LPS) molecular size and in reactivity with a monoclonal antibody which recognizes an LPS (L8) epitope. To address the question of whether the changes in LPS were due to the sac-3 locus, we constructed new transformants of FA 19 using donor DNA prepared from FA899. The new transformants could be divided into three groups: (1) those identical to FA19 in serum resistance (>90% survival at 120 min), in LPS molecular size and in expression of the L8 epitope; (2) those identical to FA899 in serum sensitivity (100% killed at 30 min), in LPS molecular size and in lack of expression of the L8 epitope; (3) those significantly killed by 50% NHS at 120 min, whose LPS molecular size was greater than that of FA 19 but less than that of FA899 and which did not express the L8 epitope. Except for PI there were no differences in other outer-membrane proteins (e.g. PII, PIII, H.8) among these transformants. Studies of PI using monoclonal antibodies which distinguish between PI serovars confirmed that changes in PI were separate from changes in serum resistance or alteration in LPS chemotype. These data indicate that a genetic locus involved in LPS structure appears to be identical with the sac-3 locus, which influences serum susceptibility in Neisseria gonorrhoeae.