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Chapter 175. General Considerations of Bacterial Diseases

Noah Craft

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Bacterial Skin Diseases at a Glance
  • Bacteria cause disease by direct invasion of tissues, by secreting toxins, and by causing immunologic consequences that result in disease.
  • The innate immune system is critical in the initial defense against bacterial entry into the skin.
  • The virulence and pathogenicity of bacteria is related to their ability to avoid activating the innate immune system or resisting killing within immune effector cells.
  • Immunosuppression, especially neutropenia, puts the host at high risk for bacterial infections; some infections are rare except in the immunocompromised host.
  • Consideration of host factors and pathogen virulence factors are critical to choose a safe and effective therapy against bacterial infections.
  • There are many noninfectious conditions that can mimic the clinical presentations of bacterial infections. Consider the noninfectious differential diagnoses carefully.
  • The use of up-to-date computer based resources for antibiotic resistance trends and best-practice guidelines is important when choosing empiric antimicrobial therapy.

The microbes that live on or in the human body are collectively referred to as the human microbiome. Some microbes of the human microbiome cause disease and others do not (commensals). The skin microbiome is a complex and diverse population of organisms that includes many bacteria, both commensals and pathogenic. The Human Microbiome Project includes recent work using metagenomic sequencing that describes the skin microbiome in previously inconceivable detail. The effects of multiple factors, including sebum secretion, body location, lipid content, pH, and sweat production significantly influence the growth of bacteria on the skin.1 The effects of skin diseases such as psoriasis also dictate the composition of the skin microbiome.2 It is clear there are many more bacteria normally living on our skin than previously imagined. Understanding the factors that contribute to healthy skin and skin disease will lead to improved treatment and prevention of skin infections and perhaps many noninfectious skin diseases.

The relationship of bacteria and the skin may be considered in four major categories1: (1) primary skin infections,2 (2) secondary infection of a primary skin disease (e.g., infected atopic dermatitis),3 (3) the skin lesions as manifestations of primary infection in some other organ system, usually the blood, and4 (4) reactive skin conditions resulting from bacterial infection elsewhere (e.g., erythema nodosum due to streptococcal pharyngitis). Thus, the balance of host immunity and the growth of the skin bacteria determine the disease state of the skin. Controlling a disease state, like atopic dermatitis, clearly reduces the number of skin infections that arise in the broken skin barrier. Conversely, controlling skin infections with dilute bleach baths and mupirocin ointment can lead to decreased atopic dermatitis flare as well.3

When considering the patient, it is important to remember that not all skin infections are suppurative but may present as reactive responses (e.g., erythema nodosum). Equally important, not all suppurative skin problems are primary skin infections (e.g., hidradenitis suppurativa). Notably, many erythematous skin lesions are not infectious at all (e.g., stasis dermatitis). ...

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