Pathogenic neisseriae can use hemoglobin, transferrin, and lactoferrin independently of the tonB locus

Redundant TonB systems which function in iron transport from TonB-dependent ligands have recently been identified in several gram-negative bacteria. We demonstrate here that in addition to the previously described tonB locus, an alternative system exists for the utilization of iron from hemoglobin, transferrin, or lactoferrin in Neisseria meningitidis and Neisseria gonorrhoeae. Following incubation on media containing hemoglobin, N. meningitidis IR3436 (tonB exbB exbD deletion mutant) and N. gonorrhoeae PD3401 (tonB insertional mutant) give rise to colonies which can grow with hemoglobin. Transfer of Hb⁺ variants (PD3437 or PD3402) to media containing hemoglobin, transferrin, and/or lactoferrin as sole iron sources resulted in growth comparable to that observed for the wild-type strains. Transformation of N. meningitidis IR3436 or N. gonorrhoeae PD3401 with chromosomal DNA from the Hb⁺ variants yielded transformants capable of growth with hemoglobin. When we inactivated the TonB-dependent outer membrane hemoglobin receptors (HmbR or HpuB) in the Neisseria Hb⁺ variants, these strains could not grow with hemoglobin; however, growth was observed with transferrin and/or lactoferrin. These results demonstrate that accumulation of iron from hemoglobin, transferrin, and lactoferrin in the pathogenic neisseriae can occur via a system that is independent of the previously described tonB locus.