Infections of Skin and Soft Tissues

Study Notes

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.

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.

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.

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.