Infections of Skin and Soft Tissues

Infections of the skin, soft tissues, joints, and bones

• Infections of the skin and soft tissue encompass a wide variety of clinical situations and microbes.
• Skin breaches often precede these infections, emphasizing the importance of the skin's protective function.
• Microbes can enter deeper soft tissues once the skin barrier is compromised.
• Surgical incisions disrupt the barrier, making surgical site infection (SSI) a significant concern.
• Staphylococcus aureus is frequently implicated, including methicillin-resistant strains (MRSAs).
• Animal bites (dogs, cats) introduce oral microbes, including Pasteurella multocida and Capnocytophaga canimorsus, potentially producing localized or systemic infections.
• Eikenella corrodens, a human oral microbe, plays a role in soft tissue and joint injuries, especially those involving a "closed fist" injury.

Microorganisms

• Some key microbes associated with infections like the ones discussed, are common colonizers or colonizers present in the normal human microbiome.
• Spirochetes, such as Treponema pallidum (syphilis), and Borrelia burgdorferi (Lyme disease), can cause skin rashes.
• Lyme disease has an animal reservoir (deer), and transmission via ticks. The rash can migrate from the tick bite site. Joint inflammation is another manifestation of Lyme disease.
• Osteomyelitis can follow a head injury from a compound fracture.
• Sternum osteomyelitis can arise after cardiac surgery.
• Sickle cell disease patients may developed acute osteomyelitis from Salmonella species.
• Infected prosthetic joint replacements can induce adjacent osteomyelitis, leading to implant failure.
• Staphylococcus coagulase-negative species are frequently identified in these cases.
• Staphylococcus aureus is a common cause of osteomyelitis.

Pathogenesis

• Intact skin, relative dryness, cell shedding, a pH of 5.0-6.0, and normal skin microbiota are primary defenses against infections.
• Sebum, converted into free fatty acids by normal skin microbiota, inhibits harmful pathogens such as Streptococcus pyogenes.
• Bacteria can breach the skin barrier through abrasions, cuts, or hair follicles.
• Skin lesions, like impetigo, bullous impetigo, folliculitis, ecthyma, cellulitis, can result from infections.
• Pseudomonas aeruginosa can cause hot tub folliculitis.

Fasciitis and Myositis

• Streptococcus pyogenes commonly causes necrotizing fasciitis of the extremities.
• Abdominal surgery can result in polymicrobial infections involving streptococci, coliforms, and anaerobes.
• Staphylococcus aureus can also be implicated.
• Gas gangrene, caused by Clostridium perfringens, is a serious complication of soft tissue injuries in poorly perfused areas. Crepitus (crackling sound) is an indication of gas production in infected tissues.
• Clostridium perfringens produces powerful toxins that worsen the condition and can lead to tissue death within affected tissues.
• Polymicrobial infections, common in surgical sites, involve multiple microbes working together. The presence of intestinal bacteria in the surgical site creates a favorable breeding environment for anaerobic species. Oxygen demand limits by anaerobic microbes are satisfied by facultative anaerobes that consume it in the wound , allowing the establishment of this kind of infection.