Bacterial skin infections (also known as pyodermas) can be caused by a wide array of different bacteria – most commonly by either staphylococci or streptococci, or sometimes by both organisms together. Less common bacteria may also play a role in hospitalized individuals, those living in a nursing home, or during certain activities such as gardening or swimming in a pond or ocean.

In a majority of cases there are distinct morphological characteristics that enable clinicians to recognize potentially treatable and reversible conditions. Accordingly, most cutaneous infections cause edema, erythema or other signs of inflammation, but can also give rise to focal accumulation of pus or fluid (such as furuncles and vesicles, respectively).

Clinical examination and, in some instances, microbiological staining and culturing of pus or exudate are usually adequate for establishing a diagnosis. When the implementation of antibiotic treatment is necessary, drugs active against gram-positive microorganisms (such as second-generation penicillins, cephalosporins and fluoroquinolones) should be considered as a therapy of choice, and susceptibility testing of a putative isolate should be pursued as resistance is on the rise.

Pathophysiology of bacterial skin infections

Even though a myriad of different bacterial species come in contact or may be resident on the skin, it is considered a very efficient barrier against invasion and subsequent bacterial infections. Most common conditions that can predispose an individual to bacterial skin infections involve cases when the integrity of the skin barrier is breached; therefore trauma such as abrasion, excoriation, laceration or ulceration may result in a vulnerable skin.

Furthermore, certain groups of people have heightened risk of developing cutaneous infections. Diabetic patients usually have poor blood flow (especially to the hands and feet), while high levels of glucose in their blood substantially decreases the propensity of their leukocytes to fight infections. In addition elderly patients, those with HIV/AIDS or those on chemotherapy are also at particular risk due to a weakened immune system.

Primary bacterial skin infections

Primary bacterial infections of the skin are usually initiated by a single organism, most often occur in normal skin and manifest with characteristic course and morphology. Most frequent pathogens associated with this type of infections are Streptococcus pyogenes, Staphylococcus aureus and sometimes coryneform bacteria. Some of the common examples are impetigo, folliculitis and erythrasma.

Impetigo is a superficial and intraepidermal bacterial skin infection that is most commonly observed in children during humid summer months. A dominant form of the disease is a non-bullous type which presents with one sore (erosion) or a cluster of them on exposed areas of the skin. Impetigo first appears in form of small vesicles or pustules that rupture and form a thick adherent honey-yellow crust.

Folliculitis represents an inflammation of the hair follicles as a result of infection (but can also arise due to chemical irritation or physical injury). Superficial type of folliculitis caused by Staphylococcus aureus manifests as a painless or tender pustule that usually heals without any scarring. In deep folliculitis infection spreads further into the follicle, while a furuncle (or boil) involves subcutaneous tissue.

Erythrasma is caused by a non-spore-forming gram-positive microorganism Corynebacterium minutissimum which (under conditions of occlusion or moisture) proliferates and causes a superficial infection of the skin and, most notably, intertriginous areas. Lesions are in the form of patches that are initially pink and dry, and later turn brown.

Secondary bacterial skin infections

The moist and damaged skin provides an opportunistic environment for different bacterial species to thrive and cause secondary cutaneous infections (often as a superimposed condition). Some of the common examples are intertrigo and toe web infections. In addition, various systemic infections can also have skin manifestations.

Intertrigo is a term for a chronic skin disorder with insidious onset of stinging and burning sensations in skin folds, which is accompanied by pruritus (or itching). This secondary bacterial skin infection, most commonly observed in patients with obesity and those who are bedridden, is caused by an overgrowth of either transient or resident bacterial flora.

Toe web infections (also known as athlete's foot) used to be regarded as strictly fungal infections; nevertheless, it is now known that the fungi can create an environment suitable for overgrowth of Brevibacterium species and certain corynebacteria. Proteolytic enzymes that are secreted by some of these bacteria can aggravate this problem even more.