Actinobacillus pleuropneumoniae Iron Transport and Urease Activity: Effects on Bacterial Virulence and Host Immune Response

Abstract

ABSTRACT Actinobacillus pleuropneumoniae , a porcine respiratory tract pathogen, has been shown to express transferrin-binding proteins and urease during infection. Both activities have been associated with virulence; however, their functional role for infection has not yet been elucidated. We used two isogenic A. pleuropneumoniae single mutants (Δ exbB and Δ ureC ) and a newly constructed A. pleuropneumoniae double (Δ ureC Δ exbB ) mutant in aerosol infection experiments. Neither the A. pleuropneumoniae Δ exbB mutant nor the double Δ ureC Δ exbB mutant was able to colonize sufficiently long to initiate a detectable humoral immune response. These results imply that the ability to utilize transferrin-bound iron is required for multiplication and persistence of A. pleuropneumoniae in the porcine respiratory tract. The A. pleuropneumoniae Δ ureC mutant and the parent strain both caused infections that were indistinguishable from one another in the acute phase of disease; however, 3 weeks postinfection the A. pleuropneumoniae Δ ureC mutant, in contrast to the parent strain, could not be isolated from healthy lung tissue. In addition, the local immune response—as assessed by fluorescence-activated cell sorter and enzyme-linked immunosorbent spot analyses—revealed a significantly higher number of A. pleuropneumoniae -specific B cells in the bronchoalveolar lavage fluid (BALF) of pigs infected with the A. pleuropneumoniae Δ ureC mutant than in the BALF of those infected with the parent strain. These results imply that A. pleuropneumoniae urease activity may cause sufficient impairment of the local immune response to slightly improve the persistence of the urease-positive A. pleuropneumoniae parent strain.