• bacterial:
  • erythrasma: brownish patches in intertriginous areas (armpits, under breasts, groin); it is due to Wood's-light fluorescing (because the bacteria contain porphyrins) Corynebacterium minutissimum (Wood's exam can be neg if patient bathed vigorously prior to office exam...therefore, could histology be false neg in same situation?. [clinical photo]¹ Histology is of Gram negative rods & filamentous shapes in the cornified layer [S07-7665]. Treat with topical gel of 2% erythromycin.
• mycobacterial:
  • histologically obvious granulomatous reaction: infectious vs. sarcoid (naked...no surrounding lymphocytes...& non caseous granulomata which tend to be separate & negative for polys, eos, or plasma cells).
  • non-granulomatous reaction:
    • when severely immunocompromised patient (AIDS, etc.).
    • some of the atypical AFBs [S-02-6281].
• viral: HSV vesicles (fever blister); dermatomal hypesthesia with & without vesicles...VZV; small pox (variola virus).
• parasitic:
  • Demodex: a mite living in hair follicles...may cause demodecosis of eyelid margins and nose [HERE]...folliculitis & erythema.
  • babesia: a group of tick-borne protozoan parasites possibly visible in a blood smear within RBCs & causing hemolysis.
• fungal: follicles & hairs [S-04-4151], skin, nails:
• Malassezia yeast/fungal ("tinea versicolor"...caused by the various M. & Pityrosporum species): pustular follicultitis, skin-multi-discolored & faintly scaling dermatosis, Gougerot-Carteau disease (confluent and reticulated papillomatosis), a pigmented eruption occurring mainly on the chest, back and neck of adolescent girls, & seborrheic dermatitis...excellent patient photos, etc., HERE. The fungal stains may show the yeasts in all cases (very scant in seb. derm.) and a mix of yeasts & hyphae in tinea versicolor (in a "spaghetti and meatballs" pattern...the skin lesions colored under Wood's light fluorescent light exam). Treatment
• dermatophytic: dermatophytosis (skin infected); onychomycosis (nails infected).
• spirochetal:
• large pox: the syphilis spirochete (Treponema pallidum).
• erythema chronicum migrans (erythema migrans): of Lyme disease & caused by tick-borne Borrelia burgdorferi (any single tick-borne illness has a reasonable likelihood that that tick or another tick from that tick infested environment might have also passed on an additional agent such as babesia.
• leptospirosis: skin jaundice when severe hepatitis.

References:

1. Miller SD, NEJMed 351(16):1666, 14 October 2004.

(posted 2 February 2004; latest addition 5 July 2009)