Final answer:
Erysipelas and cellulitis are bacterial infections, often caused by S. pyogenes, leading to inflamed, painful skin. Treatment typically involves antibiotics like penicillin, while culture tests can confirm the causative agent. Erythema nodosum involves inflammation in the subcutaneous fat and may not always require antibiotics.
Step-by-step explanation:
Cellulitis and erysipelas are common streptococcal conditions of the skin caused by infection. Cellulitis occurs when bacteria, often S. pyogenes or staphylococci, enter through a cut or abrasion, leading to a warm, painful, and reddened area usually in the dermis or hypodermis. Erysipelas is also often caused by S. pyogenes, presenting as a large, intensely inflamed and raised patch of skin, typically on the legs or face. Another related condition is erythema nodosum, characterized by inflammation in the subcutaneous fat cells leading to red nodules, most frequently on the shins.
Treatment for these infections partly depends on the identification of the specific pathogen, which can be done using immunological tests like agglutination reactions and enzyme-linked immunosorbent assays (ELISAs). The antibiotic penicillin is commonly prescribed for cellulitis and erysipelas, given that streptococci are typically susceptible to it. Erythema nodosum, which is often self-limiting, may not require antimicrobial drugs and instead may be treated with nonsteroidal anti-inflammatory drugs (NSAIDs) and other supportive measures.
In cases where a skin infection is rapidly progressing, leads to blistering or gas pockets under the skin, or has a putrid smell, a more severe infection such as necrotizing fasciitis might be considered, prompting immediate medical treatment and potentially a culture of the fluid to identify the causative agent. Group A streptococci (GAS) are frequently implicated in such infections, but there's an ongoing problem with community-acquired and hospital-acquired staphylococcal infections due to asymptomatic carriers.