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The human microbiome or microbiota represents diverse viral, bacterial, fungal, and other species that live on and within us. They are part of us and we are part of this complex ecosystem. The human body contains >10 times more microbial cells than human cells. Skin supports a range of microbial communities that live in distinct niches. Microbial colonization of skin is more dense in humid intertriginous and occluded sites such as axillae, anogenital regions, and webspaces of feet. An intact stratum corneum is the most important defense against invasion of pathogenic bacteria.

Coagulase-negative staphylococci normally colonize skin shortly after birth and are not considered to be pathogens when cultured from skin.

Overgrowth of flora in occluded areas often results in clinical syndromes of erythrasma, pitted keratolysis, and trichomycosis.

Pyoderma is an archaic term, literally "pus in the skin." Skin and soft-tissue infections, commonly caused by Staphylococcus aureus and group A streptococcus (GAS), have been referred to as "pyoderma." Pyoderma gangrenosum is a noninfectious inflammatory process, often associated with a systemic disorder such as inflammatory bowel disease.

S. aureus colonizes the nares and intertriginous skin intermittently, can penetrate the stratum corneum, and cause skin infections, e.g., impetigo, folliculitis. Deeper infection results in soft-tissue infections. Methicillin-resistant S. aureus (MRSA) is an important pathogen for community-acquired (CA-MRSA) and health-care-acquired (HA-MRSA) infections. MRSA strain USA300 is the major cause of skin and soft tissue as well as more invasive infections in community and health-care settings.

GAS usually colonizes the skin first and then the nasopharynx. Group B streptococcus (GBS; Streptococcus agalactiae) and group G β-hemolytic streptococci (GGS) colonize the perineum of some individuals and may cause superficial and invasive infections.

Cutaneous production of toxins by bacteria (S. aureus and GAS) causes systemic intoxications such as toxic shock syndrome (TSS) and scarlet fever.


  • Etiology. Corynebacterium minutissimum, gram-positive (diphtheroid) bacillus; normally in human microbiome. Growth favored by humid cutaneous microclimate.


Asymptomatic except for subtle discoloration.

Patches, sharply marginated (Fig. 25-1). Tan or pinkish; postinflammatory hyperpigmentation in more heavily pigmented individuals.

Figure 25-1

Erythrasma: groins Sharply marginated, tan patches in the genitocrural fold. Wood lamps demonstrates bright coral-red fluorescence differentiating erythrasma from intertriginous psoriasis. KOH preparation was negative for hyphae.

In webspaces of the feet, it may be macerated (Fig. 25-2). Distribution: Intertriginous skin, i.e., toe webs (Fig. 25-2), inguinal folds, axillae, and other occluded sites.

Figure 25-2

Erythrasma: webspace This macerated interdigital webspace appeared bright coral-red when examined with Wood's lamp; KOH preparation ...

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