Demodex mites--commensals, parasites or mutualistic organisms?

ated, he proposed that the life cycle of D. folliculorum mites was about 14.5 days. He also demonstrated that all life stages of these mites were negatively phototaxic, that is they were more mobile in a dark environment and relatively inert when bright light was shone on them. However, until optimal in vitro culture techniques and conditions allow Demodex proliferation in the laboratory, the true life cycle of Demodex remains uncertain. Mites are mobile and can travel at a speed of up to 16 mm/h [6] . Mites found from time to time on the skin surface suggest that they emerge from follicles (probably at night to avoid light exposure) and migrate across the surface of the facial skin. Mites are known to contain lipase enzymes [7] , to carry bacteria on the surface [6] and they may have endobacteria [8] . Any potential role of these complex organisms in the biobalance of the skin has been largely ignored. They are regarded by most investigators as simple commensals benefiting from the human sebum in its sheltered ecological follicular niche and without adversely affecting its host, but in animals the pathogenic potential of Demodex species is well documented. Demodectic mange in dogs is a potentially lethal condition, and goats can be similarly affected. Both disorders are caused by a massive proliferation of the normal mite population [9] . In humans there is mounting evidence that Demodex mites, like othThe German dermatologist Gustav Simon is credited with the first description of Demodex mites [1] . He was studying the microanatomical structure of acne vulgaris lesions by examining material expressed from sebaceous follicles under the microscope. He noted structures within this material and was able to identify a worm-like object with a head, legs, and anterior and posterior body parts that made him think this was ‘an animal’ of some sort. Suspicion became certainty when he pressed the object gently between two slides and observed that ‘it moved’! The term Demodex was coined by Richard Owen in 1843 for this genus [2] , borrowing from the Greek the words ‘demo’ (lard) and ‘dex’ (boring worm) to describe the form and location of preference of this organism. The anatomical details were subsequently described by Desch and Nutting [3, 4] . These included (for Demodex folliculorum ) 4 pairs of articulated legs, complex mouth parts, genital organs (either penis or vagina), a rudimentary gastrointestinal tract but surprisingly no anus! We now know that 2 mite species (Demodex brevis and D. folliculorum) inhabit normal adult human facial sebaceous follicles. Mites are not found in the skin of newborn infants. Sebaceous follicles are thought to become colonised during later childhood and early adult life by transfer from adult family members. The mites’ life cycle was studied by Spickett [5] by histological and rudimentary in vitro experiments. From a synthesis of this data generPublished online: January 11, 2011