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REVIEWS
Candida albicans morphogenesis and host defence: discriminating invasion from colonization
Neil A.R.Gow1, Frank L.van de Veerdonk2, Alistair J.P.Brown1 and Mihai G.Netea2
Abstract | Candida albicans is a common fungal pathogen of humans that colonizes the skin and mucosal surfaces of most healthy individuals. Until recently, little was known about the mechanisms by which mucosal antifungal defences tolerate colonizing C.albicans but react strongly when hyphae of the same microorganism attempt to invade tissue. In this Review, we describe the properties of yeast cells and hyphae that are relevant to their interaction with the host, and the immunological mechanisms that differentially recognize colonizing versus invading C.albicans.
Many fungal pathogens of humans, such as Candida albicans, Histoplasma capsulatum, Paracoccidioides brasiliensis, Penicillium marneffei and Blastomycoides dermatitidis, are capable of growing as unicellular budding yeast cells or as filamentous hyphae or
pseudohyphae a trait termed fungal dimorphism or, more precisely, fungal polymorphism. Those fungi with a life cycle that includes a saprophytic environmental phase, such as H.capsulatum, P.brasiliensis, P.marneffei and B.dermatitidis, normally grow in filamentous forms outside the human body but convert to yeast forms in human tissues1. By contrast, there is no known terrestrial life cycle for C.albicans, the most common fungal pathogen of humans, and this microorganism can grow as both yeast and filamentous forms in the host2 (FIG.1).
The relative attributes of these C.albicans forms during the colonization of skin and mucosae, and later in the invasion of the bloodstream and deep tissues, have long been debated36.
One intriguing aspect of the interaction betweenthe human host and the microbial flora that colonizes the skin and mucosal surfaces is the immunological tolerance of the mucosae to colonizing microorganisms. This contrasts with the induction of potent defence mechanisms following tissue invasion. C.albicans colonizes the skin, genital mucosa and/or intestinal mucosa of 3070% of healthy individuals at any given moment, and it is therefore noteworthy that under normal circumstances the fungus does not cause significant disease7. However, in the absence of proper immune recognition, the inability to control colonizing C.albi-cans on mucosal surfaces can lead to severe infection
(for example, in patients with chronic mucocutaneous
candidiasis (CMC))8. Perturbations in the composition of the competitive commensal bacteria can also predis-pose individuals to C.albicans infections, and distorted immuno logical responses to...