The surface of the skin teems with micro-organisms, which are most numerous in moist hairy areas, and areas rich in sebaceous glands. The move away from culture-based approaches to genomic characterization has revealed that the skin microbiome is greatly more diverse than we had previously thought and generally exists in a delicate balance with their human host. Organisms are found, in clusters, in irregularities in the stratum corneum and within the hair follicles. The resident flora is a mixture of harmless and poorly classified staphylococci, micrococci and diphtheroids. Staphylococcus epidermidis and aerobic diphtheroids predominate on the surface, and anaerobic diphtheroids (Propionibacteria sp.) deep in the hair follicles. As a general rule, Gram-positive species such as Staphylococcus epidermidis, Corynebacteria, Staphylococcus aureus and Streptococcus pyogenes colonize the skin above the waist, while Gram-negative bacteria such as Enterobacteriaceae and Enterococci are additionally found below the waist. Several species of lipophilic yeasts also exist on the skin. The proportion of the different organisms varies from person to person but, once established, an individual’s skin flora tends to remain stable and helps to defend the skin against outside pathogens by bacterial interference or antibiotic production. Nevertheless, overgrowth of skin diphtheroids can itself lead to clinical problems. The role of Propionibacteria in the pathogenesis of acne is discussed on p. 157. Overgrowth of aerobic diphtheroids causes the following conditions.

The axillary hairs become beaded with concretions, usually yellow, made up of colonies of commensal diphtheroids. Clothing becomes stained in the armpits. Topical antibiotic ointments, or shaving, will clear the condition, and frequent washing with antibacterial soaps will keep it clear.

Some diphtheroid members of the skin flora produce porphyrins when grown in a suitable medium. Overgrowth of these strains is sometimes the cause of symptom-free macular wrinkled, slightly scaly, pink, brown or macerated white areas, most often found in the armpits or groins, or between the toes. In diabetics, larger areas of the trunk may be involved. Diagnosis is helped by the fact that the porphyrins produced by these diptheroids fluoresce coral pink with Wood’s light. Topical fusidic acid or azoles will clear the condition.

Staphylococcus aureus is not part of the resident flora of the skin other than in a minority who carry it in their nostrils, perineum or armpits. Carriage rates vary with age. Nasal carriage is almost invariable in babies born in hospital, becomes less frequent during infancy, and rises again during the school years to the adult level of roughly 30%. Rather fewer carry the organism in the armpits or groin. Staphylococci can also multiply on areas of diseased skin such as eczema, often without causing obvious sepsis. A minor breach in the skin’s defences is probably necessary for a frank staphylococcal infection to establish itself; some strains are particularly likely to cause skin sepsis. Of concern is the rise in incidence of methicillin-resistant Staphylococcus aureus (MRSA) in the community and in hospitalized patients. This can result in abscesses and furunculosis as well as other infections. Prevention of emerging resistant strains is with a combination of scrupulous hygiene, patient isolation, antiseptic agents and restriction of antibiotics.

This term describes ulcers forming under a crusted surface infection. The site may have been that of an insect bite or of neglected minor trauma. The bacterial pathogens and their treatment are similar to those of impetigo. Whereas in impetigo the erosion is at the stratum corneum, in ecthyma the ulcer is full thickness, and thus heals with scarring.

A group of adjacent hair follicles becomes deeply infected with Staphylococcus aureus, leading to a swollen painful suppurating area discharging pus from several points. The pain and systemic upset are greater than those of a boil. Diabetes must be excluded. Treatment needs both topical and systemic antibiotics. Incision and drainage has been shown to speed up healing, although it is not always easy when there are multiple deep pus-filled pockets. Consider the possibility of a fungal kerion (p. 241) in unresponsive carbuncles.

Bacterial toxins are also responsible for this condition, in which fever, a rash – usually a widespread erythema – and sometimes circulatory collapse are followed a week or two later by characteristic desquamation, most marked on the fingers and hands. Many cases have followed staphylococcal overgrowth in the vagina of women using tampons, although group A streptococci can also be responsible. The bacterial toxins act as superantigens, directly stimulating T cells and causing massive cytokine release. Systemic antibiotics and irrigation of the infected site are needed. Intravenous immunoglobulin may neutralize superantigen and reduce tissue damage.

This inflammation of the skin occurs at a deeper level than erysipelas. The subcutaneous tissues are involved; and the area is more raised and swollen, and the erythema less marginated than in erysipelas. Cellulitis often follows an injury and favours areas of hypostatic oedema. Streptococci, staphylococci, or other organisms may be the cause. Treatment is elevation, rest – sometimes in hospital – and systemic antibiotics, sometimes given intravenously and active against both staphylococci and streptococci. A combination of a macrolide with a streptogramin may be more effective than penicillins.

Toe web intertrigo and lymphoedema are risk factors for the development of both erysipelas and cellulitis, which in turn predispose patients to persistent lymphoedema. It is important to treat these underlying factors as well as the bacterial infection to reduce the risks of recurrence.

A mixture of pathogens, usually including streptococci and anaerobes, is responsible for this rare condition, which is a surgical emergency. Diabetics and post-surgical patients are predisposed. At first the infection resembles a dusky, often painful, cellulitis, but it quickly turns into an extending necrosis of the skin and subcutaneous tissues. Classically, the central area of skin involvement becomes anaesthetic due to cutaneous nerve damage. Hyponatraemia is often present and in a septic patient with clinical signs of soft tissue infection should arouse the doctor’s suspicions. A deep ‘stab’ incision biopsy through the skin into the fascia may be necessary to establish the diagnosis and to obtain material for bacteriological culture. Computed tomography, or a magnetic resonance imaging (MRI) scan may help to establish how far the infection has spread. The prognosis is often poor despite early wide surgical debridement and prompt intravenous antibiotic administration, even when given before the bacteriological results are available.

It is convenient to mention this here, but the causative organism is Erysipelothrix rhusiopathiae. and not a streptococcus. This Gram-positive rod infects a wide range of animals, birds and fish. In humans, infections are most common in butchers, fishmongers and cooks, the organisms penetrating the skin after a prick from an infected bone. The disease has become less common with improvements in the animal handling industry. Infections are usually mild, and localized to the area around the inoculation site. The swollen purple area spreads slowly with a clear-cut advancing edge. With penicillin the condition clears quickly; without it, resolution takes several weeks.

The infective agent is the bacillus Bartonella (Rochalimea) henselae. A few days after a cat bite or scratch, a reddish granulomatous papule appears at the site of inoculation. Tender regional lymphadenopathy follows some weeks later, and lasts for several weeks, often being accompanied by a mild fever. The glands may discharge before settling spontaneously. There is no specific treatment. In immunosuppressed patients, most commonly those with HIV, Bartonella henselae causes bacillary angiomatosis (p. 237).

Yaws is distributed widely across the poorer parts of the tropics. The spirochaete, Treponema pallidum ssp. pertenue, gains its entry through skin abrasions. After an incubation period of up to 6 months, the primary lesion, a crusting and ulcerated papule known as the ‘mother yaw’, develops at the site of inoculation; later it may enlarge to an exuberant raspberry-like swelling which lasts for several months before healing to leave an atrophic pale scar. In the secondary stage, other lesions may develop in any area but do so especially around the orifices. They are not unlike the primary lesion but are smaller and more numerous (‘daughter yaws’). Hyperkeratotic plaques may appear on the palms and soles. The tertiary stage is characterized by ulcerated gummatous skin lesions, hyperkeratosis of the palms and soles, and a painful periostitis that distorts the long bones. Serological tests for syphilis are positive. Treatment is with penicillin.

This condition is usually acquired through contact with infected livestock or animal products such as wool or bristles. Previously rare in industrialized countries, its importance increased after the infectious agent was used in the United States for a bioterrorism attack.

Anthrax has two main clinical variants: the often fatal inhalational anthrax, which is outside the scope of this book; and cutaneous anthrax. The incubation period of the latter is usually 2–5 days. A skin lesion then appears on an exposed part, often in association with a variable degree of cutaneous oedema, which can sometimes be massive, especially on the face. Within a day or two, the original small painless papule shows vesicles that quickly coalesce into a larger single blister. This ruptures to form an ulcer with a central dark eschar, which falls off after 1–2 weeks leaving a scar. The skin lesions are often accompanied by fever, headache, myalgia and regional lymphadenopathy. The mortality rate for untreated cutaneous anthrax is up to 20%; with appropriate antibiotic treatment, this falls to less than 1%.

Cultures of material taken from the vesicle may be positive in 12–48 hours; a Gram stain will show Gram-positive bacilli, occurring singly or in short chains. Quicker results may be obtained by a direct fluorescence antibody test, or by an enzyme-linked immunoabsorbant assay (ELISA) – both of which are currently available only at reference laboratories. Before the results are available, it is wise to assume that the organism is penicillin- and tetracycline-resistant, and to start treatment with ciprofloxacin at 400 mg intravenously every 12 hours or, for milder cases, ciprofloxacin 500 mg orally every 12 hours. The latter dose is suitable for prophylactic use in those who are known to have been exposed to spores. A switch to an alternative regimen can be made once the antibiotic sensitivity of the organism has been established. At present, anthrax vaccine is in short supply; it requires six injections over 18 months, with subsequent boosters, to prevent anthrax. The spores of Bacillus anthracis, the causative organism, are highly resistant to physical and chemical agents.

Skin lesions are important clues to the diagnosis of this condition, in which the symptoms and signs of classic gonorrhoea are usually absent. The patient, usually a menstruating woman with recurring fever and joint pains, develops sparse crops of skin lesions, usually around the hands and feet. The grey, often haemorrhagic, vesicopustules are characteristic. Rather similar lesions are seen in chronic meningococcal septicaemia.

Most infections in the United Kingdom are caused by Mycobacterium tuberculosis. Mycobacterium bovis infection, endemic in cattle, can be spread to humans by milk, but human infection with this organism is now rare in countries where cattle have been vaccinated against tuberculosis and the milk is pasteurized. The steady decline of tuberculosis in developed countries has been reversed in some areas where AIDS is especially prevalent. Dormant tuberculosis of the skin can also be reactivated by systemic corticosteroids, immunosuppresants and anti-tumour necrosis factor biological agents.