White : Diseases of the Skin - Bacterial Infections

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Gary M. White & Neil H. Cox

Diseases of the Skin

24	Bacterial Infections

INTRODUCTION

The skin has a variety of natural defenses, making infection by bacteria more difficult. These include an arid surface, antimicrobial properties of secreted lipids, and competition from the normal bacterial flora. In addition, the skin has an active immune system that combats invasion. Factors that promote infection include:

•	chronic moisture and occlusion, as may occur in the body folds;
•	breaks in the surface of the skin (e.g. after scratching or from insect bites);
	decreased lipids, as occurs during isotretinoin therapy;
	a compromised immune system; and
	external factors, such as infrequent bathing or poor hygiene.

In addition, there are several points on the integument that are weaknesses in the body's defenses. Specifically, the follicle is a common site for bacterial invasion and proliferation, as well as the potential space between the periungual skin and nail.

Years ago, bacterial infections of the skin had the potential to be life-threatening. Nowadays, however, with recent advances in the culture of these organisms and the widespread availability of antibiotics, these infections of the skin are usually dealt with swiftly and effectively. Antibiotic resistance, however, is becoming more of a problem, most notably with the rise in cases of infection by methicillin-resistant Staphylococcus aureus (MRSA).

Superantigens are a class of highly potent immunostimulatory molecules produced by Staph. aureus and Streptococcus pyogenes . These toxins possess the unique ability to interact simultaneously with major histocompatibility complex (MHC) class II molecules and T-cell receptors, causing profound T-cell activation and proliferation but ‘bypassing' the normal antigen-presenting mechanisms. The resultant massive cytokine release causes epithelial damage and leads to capillary leak and hypotension. This mechanism is important in scarlet fever, toxic shock syndrome, and possibly in Kawasaki disease.

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