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March 2013 | Volume 12 | Issue 3 | Feature | 369 | Copyright © 2013
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Bacterial Skin and Soft Tissue Infections: A Review for the Dermatologist
The skin is the largest organ in the human body, and when combined with the underlying soft tissues, it comprises the bulk of the body’s solid mass. It is colonized with resident microflora, including various species of staphylococci, corynebacteria, and propionibacteria, that competitively inhibit pathogenic microbes.1 It is important to differentiate between normal colonization, which is not a disease process, and infection with pathogenic strains, which may necessitate treatment.1
In the first century ad, Celsus described the classical symptoms of an inflammatory infection as calor, rubor, tumor, and dolor (ie, heat, redness, swelling, and pain).2,3 In modern medicine, skin and soft tissue infections (SSTIs) are defined as invasion of pathogens into the epidermis, dermis, and subcutaneous tissues, causing subsequent inflammation. One might also augment Celsus’s description with a fifth characteristic: functio laesa, or loss of function. These infections often require treatment, and although it is not within the scope of this review, the selection of suitable antibiotics and their appropriate use is a key issue that physicians face.
SSTIs have developed into a much more complicated clinical problem since Celsus first described them. They are becoming increasingly common in both the outpatient and inpatient settings, and microbial resistance is rising as more virulent strains evolve. Bacterial SSTIs are among the most common diseases seen by dermatologists, who are often the first point of care for these patients.4 As the public health and economic burdens of SSTIs rise, dermatologists continue to play a key role in identifying and managing these diseases.
EpidemiologyBecause bacterial SSTIs can manifest as various clinical entities, accurate assessment of incidence and prevalence has been difficult. 5 Furthermore, because many SSTIs resolve within 7 to 10 days, measurement of prevalence is variable.5 The estimated prevalence among all hospitalized patients is 7% to 10%,6 and from 2000 to 2004, hospital admissions for SSTIs increased by 29%.7 Approximately 70% to 75% of all SSTI cases are managed in an outpatient setting.8,9 A large portion of outpatient cases involve the lower extremity.9,10 In addition, the prevalence of SSTIs is higher in men, accounting for 60% to 70% of all cases, as well as in patients aged 45 to 64 years.5
Though the microbial makeup of SSTIs has been shifting over the past 20 years, recent epidemiological studies yield conflicting results.11 One study found that from 1997 to 2005, the annual incidence of SSTIs rose from 32 to 48 clinical visits per 1,000 persons. 12 Greater than 95% of this increase in incidence was due to the growing number of cases of cellulitis and abscesses.12 However, other studies established that though the overall incidence of SSTIs did not change during this time period, the frequency of cellulitis and abscesses has indeed grown in number.13,14 Although these studies differ in their outcomes with regard to incidence, there is a consensus opinion that cellulitis in particular is increasing in frequency, and that SSTIs are an increasingly common occurrence.11 In addition, a recent study found that the relative proportion of methicillin-resistant Staphylococcus aureus (MRSA) isolates identified in an outpatient dermatology clinic rose by 17.0% during the past 3 years.15 MRSA-related infections have unquestionably increased in number and are likely responsible for the rise in diagnoses.12,13,15
MicrobiologyMost bacterial SSTIs are the result of aerobic Gram-positive cocci, particularly S aureus and Streptococcus species.1,8,11 Streptococcal infections can be further divided into several types, with groups A and B predominantly causing SSTIs.8 In recent years, S aureus has become an even more significant pathogen. The emergence of community-acquired MRSA strains has complicated treatment of bacterial SSTIs.11,16 Though S aureus infections are on the rise and are commonly identified on cultures, good clinical response to β-lactam antibiotics implies that Streptococcus species in Lancefield groups A, C, and G are still predominant microbes in skin infections.11,17,18 In particular, group A β-hemolytic streptococci is often identified.11
In addition, the microbiology of SSTIs varies in communityacquired and hospital-acquired infections. The top 10 bacteria overall in SSTIs are as follows: S aureus (45%), Pseudomonas aeruginosa (11%), Enterococcus species (9.3%), Escherichia coli (7.2%), Enterobacter species (4.8%), Klebsiella species (4.2%), ß-hemolytic Streptococcus(4.1%), Proteus mirabilis (2.8%), coagulase-negative Staphylococcus (2.8%), and Serratia species (2.1%).19,20 However, hospital-acquired SSTIs demonstrate a greater percentage of resistant species such as MRSA (up to 40% of cases), P aeruginosa, and Enterococcus species.19 In contrast, the majority of community-acquired SSTIs are caused by β-hemolytic streptococci.5 According to the SENTRY Antimicrobial Surveillance program, this pathogen appears as only the seventh most common cause of SSTIs in North America20; however, it is likely underrepresented, since mild or superficial infections may not necessitate hospital admission and positive cultures are difficult to obtain in streptococcal infections.19 It is important to note that given the variable presentation of SSTIs and the disparity in severity, assessing incidence and prevalence is difficult; the statistics often vary between studies.5 It is