Staphylococcal Infections
Staphylococcal infections are caused by the common gram-positive cocci Staphylococcus (staphylococci).
Staphylococci normally grow in the nose and on the skin of 20 to 30% of healthy adults (and less commonly in the mouth; mammary glands; and urinary, intestinal, and upper respiratory tracts). These bacteria do no harm most of the time; however, a break in the skin, burn, or other injury may allow the bacteria to penetrate the body's defenses and cause infection. Commonly, staphylococcal infections produce collections of pus (abscesses), which can appear not only on the skin but also in internal organs. Staphylococcal infections range from mild to life threatening.
People prone to staphylococcal infections include newborns; injecting drug users; breastfeeding women; and people with skin disorders, surgical incisions, a weakened immune system, or chronic diseases (especially diabetes, lung disease, disease of the veins and arteries, and cancer). Intravenous catheters, particularly those that remain in the body for more than 1 or 2 days, often become contaminated with staphylococci, allowing the bacteria to enter the bloodstream (bacteremia). Postoperative staphylococcal infections usually appear a few days to several weeks after surgery but may develop more slowly if the person received antibiotics at the time of surgery.
Staphylococci tend to infect the skin (see Section 18, Chapter 211), but they can travel through the bloodstream and involve almost any site in the body, particularly the heart (endocarditis (see Section 3, Chapter 29)) and the bones (osteomyelitis (see Section 5, Chapter 65)). Staphylococcal endocarditis is common in injecting drug users. Staphylococcal osteomyelitis predominantly affects children, although it also develops in older people--especially those with deep skin ulcers (bedsores or pressure sores).
Staphylococcal pneumonia is a severe infection (see Section 4, Chapter 42) that develops mainly in people with chronic lung diseases (such as chronic bronchitis and emphysema) or influenza.
Some strains of staphylococci produce toxins. These toxins cause staphylococcal food poisoning (see Section 9, Chapter 122), toxic shock syndrome, and scalded skin syndrome (see Section 18, Chapter 211).
Symptoms
There are many kinds of staphylococcal skin infections. The least serious is folliculitis, an infection of a hair root (follicle) that produces a slightly painful, tiny white pimple at the base of a hair. Impetigo consists of shallow, fluid-filled blisters surrounded by yellow crusts. Impetigo may itch or hurt. Staphylococcal skin abscesses (boils, furuncles) are warm, painful, collections of pus below the skin surface. Staphylococcal cellulitis is a spreading infection that develops under the skin, producing pain and redness. Two particularly serious staphylococcal skin infections are toxic epidermal necrolysis and, in newborns, scalded skin syndrome, both of which lead to large-scale peeling of skin. All staphylococcal skin infections are very contagious.
Staphylococcal breast infections (mastitis) and abscesses typically develop 1 to 4 weeks after delivery. The infected area is red and painful. Breast abscesses often release large numbers of bacteria into the mother's milk, and these milk-borne bacteria may infect the nursing infant.
Staphylococcal pneumonia often causes a high fever, shortness of breath, rapid breathing, and a cough that produces sputum that may be tinged with blood. In both newborns and adults, staphylococcal pneumonia can cause lung abscesses. These abscesses may extend to involve the membranes surrounding the lungs (empyema), which adds to the breathing difficulties caused by the pneumonia.
Staphylococcal bacteremia is a common cause of death in people with severe burns. Typically, the bacteremia causes a persistent, high fever, and sometimes shock.
Staphylococcal endocarditis can quickly damage the heart valves, leading to heart failure (with weakness and difficulty breathing) and death.
Staphylococcal osteomyelitis causes chills, fever, and bone pain. Redness and swelling appear over the infected bone, and fluid may build up in joints near the areas invaded by the bacteria.
Diagnosis and Treatment
Staphylococcal skin infections are usually diagnosed by their appearance without laboratory testing. Other more serious staphylococcal infections require samples of blood or infected fluids for culture. The laboratory establishes the diagnosis and determines which antibiotics can kill the staphylococci. Sometimes x-rays and other imaging scans can identify an area of infection, but they generally do not help the doctor make an early diagnosis.
Minor skin infections--such as folliculitis and tiny patches of impetigo--are usually treated with an ointment such as nonprescription triple-antibiotic mixture (bacitracin, neomycin, polymyxin B) or prescription mupirocin. For most other skin infections, oral antibiotics (such as cloxacillin, dicloxacillin, and cephalexin) are adequate. More severe infections, especially blood infections, require intravenous antibiotic therapy, often for up to 6 weeks.
The choice of antibiotic depends on the site of infection, the severity of the illness, and the susceptibility of the particular staphylococcal strain. Some strains are resistant to many antibiotics. Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to nearly all antibiotics and is increasingly common in big city and university hospitals. Among the few antibiotics that are still effective against MRSA are vancomycin and trimethoprim-sulfamethoxazole.
Antibiotics alone do not cure abscesses; they must also be drained. Abscesses deeper in the body may require surgery.
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