[Etiology, resistance and diagnostic techniques in skin and skin structure infections]

Skin and soft tissue infections (SSTI) are common and, generally, uncomplicated at the time of initial presentation. However, these infections can worsen quickly when there are delays in diagnosis and treatment. The clinical presentation of most SSTI is the culmination of a microbial three-step process as follow: i) bacterial adherence to host cells; ii) invasion of tissue with evasion of host defences, and iii) elaboration of toxins. Even if the microbiology of wounds has been actively investigated in recent years, there is still much to be learned about the microbial mechanisms that induce infection and prevent wound healing. There are also several means by which bacteria penetrate the skin barrier. The most common route is through a break in the barrier (lacerations, bite wounds, scratches, instrumentations, pre-existing skin conditions, wounds or ulcers, burns and surgery); other routes of penetration include contiguous spread from adjacent infections (e.g. osteomyelitis), entry of water into the skin pores, and, rarely, haematogenous seeding (i.e septic emboli). From what it was said, many microorganisms, above all from the normal skin microbiota, can be involved in these often polymicrobial infections, with Gram-positives such as Staphylococcus aureus, Streptococcus pyogenes, Staphylococcus epidermidis, Corynebacterium spp being predominant. Many other aerobic and anaerobic species, including Gram-negative bacilli, can also be involved. Even if the diagnosis of most SSTI is based on clinical examination, laboratory investigations, guided by clinical information, can help to confirm the diagnosis and elucidate the characteristics of specific pathogens. These microbiological investigations may include blood cultures, tissue swabs with culture, and needle aspiration. In rapidly progressing infections, empirical therapy is essential, although microbiological data are important in confirming subsequently that the chosen regimen is appropriate. Furthermore, the number of microorganisms becoming resistant to many usual drugs and the changing microbial epidemiology of these infections, such as the emergence of CA-MRSA, required a constant cooperation between the microbiology lab and the clinician in order to address microbiological aspects that can be critical to the successful management of SSTI.