Current Microbiology of Surgical Site Infections in Patients with Cancer: A Retrospective Review

SSIs are the most common post-operative complications associated with surgical procedures. They are defined as infections that occur within 30 days of a surgical procedure (or within 1 year, if an implant has been left in place) affecting either the incision or deep tissue at the operative site [14, 15]. The development of SSIs can lead to prolonged hospital stay with increased costs, medical comorbidity, psychological and emotional trauma, poor cosmetic results, and occasionally, a delay in post-operative adjuvant therapies [16]. Both patient-related and procedure-related factors influence the risk of SSIs, and a recent analysis concluded that patient-related factors (e.g., advanced age, diabetes mellitus, and low serum albumin indicative of poor nutritional status, smoking, pre-existing colonization/infection with S. aureus) were predominant [17]. Many of these factors are present in patients with cancer. Additionally, surgical incisions in previously irradiated areas are more likely to have poor or delayed healing, and develop secondary infections. Consequently, infection prevention is important and antimicrobial prophylaxis is recommended for most of these surgical procedures [7‐9, 18, 19]. The agents commonly used for prophylaxis (e.g., first- or second-generation cephalosporins; amoxicillin/clavulanate) were selected several decades ago, based on microbiologic data available then [20]. In the past, methicillin-susceptible S. aureus and, to a lesser extent, beta-hemolytic streptococci were the predominant organisms causing SSIs [21]. Data from our study provide confirmation that Gram-positive bacteria remain the most frequently isolated pathogens in SSIs in cancer patients, regardless of the specific site of infection. Of the 157 monomicrobial infections, 129 (82%) were caused by Gram-positive pathogens. However, streptococci accounted for only 8% (12 of 157) of these infections, possibly because current prophylactic regimens are still effective against these organisms. In contrast, staphylococci accounted for 109 of 157 episodes of monomicrobial infections (70%), suggesting that prophylaxis was not particularly effective. Of concern is the fact that 40% of S. aureus isolates were methicillin resistant, and that 67% of staphylococcal isolates had a vancomycin MIC ≥1.0 µg/ml. Although there is some variability with susceptibility test results using different methods (E-test, which we used, usually gives higher MIC values than other methods), such organisms have been shown to cause infections that respond slowly, or not at all, to therapy with vancomycin [22‐24]. Also of concern is the fact that such isolates are now almost as common among MSSA as they are among MRSA strains. These data have therapeutic implications and suggest that vancomycin might no longer be the agent of choice for the treatment of staphylococcal SSIs at our institution.

We were surprised to find that Gram-negative bacilli caused a substantial proportion (23 of 157 episodes, 15%) of monomicrobial infections, with P. aeruginosa (16 of 23 episodes, 70%) being the predominant pathogen. Although not frequent, the isolation of multidrug-resistant P. aeruginosa and other Gram-negative organisms is of some concern. While this epidemiologic shift does not justify the administration of prophylaxis with broad-spectrum and/or anti-pseudomonal agents on a routine basis to all patients undergoing surgical procedures (particularly since institutional/geographic differences do occur), it should not be ignored. Other investigators have also reported an increase in the frequency of SSIs caused by Gram-negative organisms (49% in one study), and have suggested that alternatives to cefazolin or similar agents, be considered for prophylaxis [25, 26]. We believe that individual institutions performing substantial numbers of these surgical procedures should generate their own data, and assess their own clinical needs. Since many of these procedures are elective, determining the presence of colonization with problem organisms such as MRSA or P. aeruginosa prior to surgery, and administering targeted prophylaxis to patients who are colonized, might be one approach in this setting. Based on our data, and on some clinical experience, we have instituted a change in our clinical practice in our breast cancer cohort by providing empiric coverage against Gram-negative organisms including P. aeruginosa until microbiologic data become available. We strongly recommend discontinuation of anti-pseudomonal coverage if subsequent cultures do not confirm it as a pathogen.

We were even more surprised to find a high proportion of polymicrobial infections at all surgical sites studied. The combined frequency of polymicrobial infections was 43.3%, ranging from a low of 37.7% for craniotomy site infections, to a high of 48% for PEG-tube insertion site infections. We realize that when multiple pathogens are isolated, it is often not possible to determine the exact role played by individual organisms in the pathogenesis of polymicrobial infections. Nevertheless, it is striking that the organisms isolated most frequently from such infections (Staphylococcus species including MRSA and P. aeruginosa) are the same as those isolated most frequently from monomicrobial infections, probably reflecting the inadequacy of the agents used for prophylaxis as well. We have documented an increase in the overall frequency of polymicrobial infections in cancer patients, including patients with and without neutropenia [27]. Based on our current microbiologic data and guidelines provided by our antimicrobial stewardship initiative and on some recent clinical experience, we have instituted changes in our clinical practice by providing broad-spectrum empiric therapy, followed by de-escalation if possible, once microbiologic data become available [28]. We recommend this approach in other institutions that are experiencing epidemiologic changes similar to ours.