Fungal infections and critically ill adults

Abstract

Fungi are eukaryotes (i.e. having membranes that cover the nucleus and other intracellular organelles); this makes them structurally similar to animals and plants, but different from prokaryotes such as bacteria. Fungi have rigid cell walls containing chitin, chitosan, mannan, and glucan. Fungi also have cell membranes structurally different from that of animals as they contain ergosterol rather than cholesterol. The simplest subclassification of fungi responsible for human infections is as either moulds (e.g. Aspergillus species) or yeasts (e.g. Candida species). Under the microscope, yeasts are small rounded cells that can bud, while moulds demonstrate a stranded, filamentous appearance caused by hyphae. Some fungi can exist in both forms (these are said to be dimorphic, e.g. Blastomyces), and some yeasts can develop pseudo-hyphae (e.g. Candida species). When the hyphae of filamentous fungi develop a matted, intermeshed network, this is referred to as a mycelium. Fungi are slow-growing, with cell-doubling times often as long as days, which can affect the ability to identify clinically relevant infections. Reproduction may be sexual, asexual, or both; and may result in the production of ‘daughter cells’ or spores. Many fungi and spores are environmentally ubiquitous, for example, Aspergillus species are commonly found in soil, and their spores are prevalent in the atmosphere. Several fungi are common human flora (for example, Candida occur within the human gut) or are able to colonize structures such as the gut, oropharynx, or upper and lower airways. It can sometimes be difficult to tell whether or not a positive fungal culture is indicative of invasive disease or simply the result of the capture of normal flora.