, 1986;

Parkhill et al., 2003; Diavatopoulos et al., 2005). Despite evolving independently, these pathogens share a number of virulence factors including filamentous hemagglutinin, pertactin, adenylate cyclase toxin and tracheal cytotoxin (Mattoo & Cherry, 2005). However, B. pertussis is unique among the Bordetellae in that it produces the virulence factor pertussis toxin (PT), an AB5 toxin 105 kDa in size. The enzymatically active A subunit, also referred to as S1, is an ADP ribosyltransferase that modifies heterotrimeric Gi proteins of mammalian cells, leading to inhibitory effects on G protein-coupled receptor signaling pathways (Katada et al., 1983; Moss et al., 1983). The B-oligomer is organized into a pentameric ring structure made up of subunits S2, S3, two S4 and S5, which bind to unknown glycoconjugate receptors on the surface of the host cell, allowing MLN2238 internalization by endocytosis (Witvliet et al., 1989). Bordetella parapertussis also carries the genes encoding PT, but does not express them due to multiple mutations in the promoter region (Arico & Rappuoli, 1987). Bordetella parapertussis, unlike B. pertussis, does not express BrkA, which is responsible for

conferring serum resistance (Goebel et al., 2008). Instead, B. parapertussis expresses an O-antigen on its lipopolysaccharide, which provides serum resistance and promotes bacterial colonization of the respiratory tract

(Goebel et al., 2008). Thus, the two pathogens, find more although closely related, have evolved distinct pathogenic mechanisms through expression of different virulence factors. We previously found that PT contributes to B. pertussis respiratory infection in mouse models by the suppression and modulation of innate and adaptive immune responses (Carbonetti et al., 2003, 2004, 2005, 2007; Andreasen & Carbonetti, 2008). We hypothesize that this immunomodulatory activity of PT may sensitize B. pertussis-infected hosts to secondary respiratory infections with other pathogens. Because little is known about the dynamics of coinfection with B. pertussis and B. parapertussis, in this study, we investigated mixed infection of the two pathogens in the mouse Meloxicam respiratory tract and hypothesized that the presence of B. pertussis would enhance the ability of B. parapertussis to infect the host. Bordetella parapertussis strain 12822, the type strain whose genome has been sequenced (Heininger et al., 2002; Parkhill et al., 2003), was used in this study. The B. pertussis strains used for this study were streptomycin- and nalidixic acid-resistant derivatives of Tohama I and were produced as described previously (Carbonetti et al., 2003). Bordetella pertussis and B. parapertussis strains were grown on Bordet–Gengou (BG) agar plates containing 10% defibrinated sheep blood.