Objective—To determine the importance of iron for
in vitro growth of Rhodococcus equi, define potential
iron sources in the environment and mechanisms by
which R equi may obtain iron from the environment,
and assess expression and immunogenicity of
Sample Population—10 virulent and 11 avirulent
strains of R equi.
Procedure—In vitro growth rates and protein patterns
of R equi propagated in media with normal,
excess, or limited amounts of available iron were
compared. Immunoblot analyses that used serum
from foals naturally infected with R equi and monoclonal
antibody against virulence-associated protein
(Vap)A were conducted to determine immunogenicity
and identity of expressed proteins.
Results—Excess iron did not alter growth of any
R equi strains, whereas growth of all strains was significantly
decreased in response to limited amounts
of available iron. Virulent R equi were able to use iron
from ferrated deferoxamine, bovine transferrin, and
bovine lactoferrin. Only virulent R equi expressed an
iron-regulated, immunogenic, surface-associated protein
identified as VapA.
Conclusions and Clinical Relevance—Iron is
required for the growth and survival of R equi.
Sources of iron for R equi, and mechanisms by which
R equi acquire iron in vivo, may represent important
virulence factors and novel targets for the development
of therapeutic and immunoprophylactic strategies
to control R equi infection in foals. Expression of
VapA is substantially upregulated when there is a limited
amount of available iron. (Am J Vet Res 2003;64:1337–1346)