In vivo expression of the developmentally regulated Candida albicans hyphal wall protein 1 (HWP1) gene was analysed in human subjects who were culture positive for C. albicans and had oral symptoms (n=40) or were asymptomatic (n=29), or had vaginal symptoms (n=40) or were asymptomatic (n=29). HWP1 mRNA was present regardless of symptoms, implicating hyphal and possibly pseudohyphal forms in mucosal carriage as well as disease. As expected, in control subjects without oral symptoms (n=10) and without vaginal symptoms (n=10) who were culture negative in oral and vaginal samples, HWP1 mRNA was not detected. However, exposure to Hwp1 in healthy culture-negative controls, as well as in oral candidiasis and asymptomatic mucosal infections, was shown by the existence of local salivary and systemic adaptive antibody responses to Hwp1. The results are consistent with a role for Hwp1 in gastrointestinal colonization as well as in mucosal symptomatic and asymptomatic infections. Overall, Hwp1 and hyphal growth forms appear to be important factors in benign and invasive interactions of C. albicans with human hosts. INTRODUCTIONCandida albicans is a member of the normal flora of the gastrointestinal tract that frequently causes serious oral and vaginal mucosal invasion and systemic disease in hosts with impaired immune defences. For pathogens that persist in the host permanently or for extended periods, understanding the mechanisms that lead to progression from commensalism to virulence is an emerging area of medical research. It has recently become recognized that for pathogens whose ecological niche is the host, molecular factors believed to be important for virulence may also be considered adaptive factors that play an essential role in allowing the pathogen to persist in the host (Falkow, 2006). In these organisms, mechanisms of persistence and virulence may overlap via common determinants that function in both states. For C. albicans, the factors contributing to low-level asymptomatic persistence in the gastrointestinal tract, i.e. colonization, among inhibitory microbial flora, to frequent occurrence of asymptomatic oral and vaginal carriage, and to virulence in immunocompetent and immunodeficient hosts, are poorly understood. In previous work, we have found that expression of specific secreted aspartyl proteinase (SAP) genes is correlated with active disease and anatomical location, whereas expression of other SAP genes reflects the presence of the organism but not disease (Naglik et al., 2003).Hyphal wall protein (Hwp1), a transglutaminase substrate which functions as an adhesin, is important for the pathogenesis of candidiasis (Staab et al., 1996(Staab et al., , 1999Sundstrom et al., 2002). HWP1 mRNA and protein are abundant in hyphae in vitro, and greatly reduced or undetectable in yeast forms (Staab et al., 1996(Staab et al., , 1999 Sharkey et al., 1999;Nantel et al., 2002). Hypha-specific expression of Hwp1 extends to fungal growth in the host, as shown by the presence and absence of Hwp1 on hyphae and yeast, re...