Nr. Luke et Aa. Campagnari, Construction and characterization of Moraxella catarrhalis mutants defective in expression of transferrin receptors, INFEC IMMUN, 67(11), 1999, pp. 5815-5819
We have previously reported the construction of an isogenic mutant defectiv
e in expression of OmpB1, the TbpB homologue, in Moraxella catarrhalis 7169
, In this report, we have extended these studies by constructing and charac
terizing two new isogenic mutants in this clinical isolate. One mutant is d
efective in expression of TbpA, and the other mutant is defective in expres
sion of both TbpA and TbpB. These isogenic mutants were confirmed by using
PCR analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, an
d sequencing. In vitro growth studies, comparing all three mutants, demonst
rated that the tbpA mutant and the tbpAB mutant were severely limited in th
eir ability to grow with human holotransferrin as the sole source of iron.
In contrast, the ompB1 (tbpB) mutant was capable of utilizing iron from hum
an transferrin, although not to the extent of the parental strain. While af
finity chromatography with human holotransferrin showed that each Tbp was c
apable of binding independently to transferrin, solid-phase transferrin bin
ding studies using whole cells demonstrated that the tbpA mutant exhibited
binding characteristics similar to those seen with the wild-type bacteria.
However, the ompB1 (tbpB) mutant exhibited a diminished capacity for bindin
g transferrin, and no binding was detected with the double mutant. These da
ta suggest that the M. catarrhalis TbpA is necessary for the acquisition of
iron from transferrin. In contrast, TbpB is not essential but may serve as
a facilitory protein that functions to optimize this process. Together the
se mutants are essential to provide a more thorough understanding of iron a
cquisition mechanisms utilized by M. catarrhalis.