Staphylococcus aureus surgical site infection rates in 5 European countries
Antimicrobial Resistance & Infection Control volume 12, Article number: 104 (2023)
To determine the overall and procedure-specific incidence of surgical site infections (SSI) caused by Staphylococcus aureus (S. aureus) as well as risk factors for such across all surgical disciplines in Europe.
This is a retrospective cohort of patients with surgical procedures performed at 14 European centres in 2016, with a nested case–control analysis. S. aureus SSI were identified by a semi-automated crossmatching bacteriological and electronic health record data. Within each surgical procedure, cases and controls were matched using optimal propensity score matching.
A total of 764 of 178 902 patients had S. aureus SSI (0.4%), with 86.0% of these caused by methicillin susceptible and 14% by resistant pathogens. Mean S. aureus SSI incidence was similar for all surgical specialties, while varying by procedure.
This large procedure-independent study of S. aureus SSI proves a low overall infection rate of 0.4% in this cohort. It provides proof of principle for a semi-automated approach to utilize big data in epidemiological studies of healthcare-associated infections.
Trials registration The study was registered at clinicaltrials.gov under NCT03353532 (11/2017).
Surgical site infections (SSI) are still among the most frequent healthcare associated infections (HAI) and entail significant morbidity and mortality globally . Considering antimicrobial resistance (AMR) a global threat, mutual international efforts must focus on reducing infection rates of key players such as SSI. In Europe, the most common causative pathogen is Staphylococcus aureus being part of the human skin microbiota . S. aureus SSI is associated with prolonged duration of hospitalization, death rates and treatment costs . In contrast to infections caused by antibiotic-resistant organisms like methicillin-resistant S. aureus (MRSA), S. aureus SSI rates are independent on regional epidemiological influences .
Recent efforts to understand and reduce SSI could diminish infection rates [2, 4,5,6]. In the light of individualized medicine also growing in the field of infectious diseases, detailed insights into epidemiology of different patient populations are urgently needed. This may enable the development of more targeted prevention approaches. Current epidemiological studies measure SSI rates in select indicator procedures assumed representative for surgical subspecialties. This approach has been questioned by data from single institutions or provider networks suggesting relevant SSI rate variability within surgical disciplines and limited applicability of risk criteria in multiple procedure types .
We thus established a cohort of all patients undergoing surgery at 14 high-volume surgical care centres in Europe. We included all types of surgery rather than select indicator procedures to generate a comprehensive picture of S. aureus SSI. Thereby, we aimed to assess the overall S. aureus SSI infection rate and, consequently, also rates within alle included procedures.
This is a retrospective multinational, multicentre cohort study with a nested case–control analysis. The study includes all surgical procedures performed in adult patients in 2016, excluding minimal invasive biopsies and eye surgery at 14 surgical centres in Europe (Fig. 1; Additional file 3: Table S1o ensure appropriate representation of each type of surgery, only centres with more than 10 000 annual procedures were considered. Sites were identified by their publication activity on SSI, prior SSI study participation and membership in respective European surgical, microbiological, or infectious diseases societies. Sites were contacted and selected using a feasibility questionnaire (Additional file 1).
The study was submitted to the Research Ethics Commission of the University of Cologne (No. 17–078) for advice; the requirement for informed consent was waived due to the retrospective nature as well as the anonymous data capture strategy of this study. The study was registered at clinicaltrials.gov under NCT03353532.
Patient selection criteria
Inclusion criteria consisted of surgery in 2016 and age ≥ 18 years at the time of surgery. Exclusion criteria comprised minimal invasive biopsies and eye surgery, SSI at the time of surgery as well as cases with missing data defined as “missing completely at random” (MCAR).
Surgery was defined in analogy to established epidemiological approaches  as any procedure taking place in an operating room and including at least one incision. Minimal invasive needle biopsies without incision, as well as all types of eye surgery were excluded.
Data were assessed at two levels: The cohort and the case–control population. All anonymously documented data and cases were reviewed by infectious disease specialists.
Data for the entire cohort of all included patients were exported from electronic patient records. This included demographics, surgical procedure code, procedure duration, comorbidity by international classification of diseases (ICD), and wound class of all patients undergoing surgery if available.
Cases of S. aureus SSI were identified by crossmatching bacteriology laboratory data of all S. aureus isolates with data of all patients undergoing surgery, thus generating a comprehensive list of all possible S. aureus SSI cases (Fig. 2). Presence of S. aureus SSI among these possible cases was verified by single-case evaluation which was performed by infectious diseases specialists and surgeons in each hospital to ensure inclusion of only those SA found in a relevant clinical culture. True cases were ascertained as having S. aureus presence and either a documented diagnosis of SSI or as exhibiting both, a clinical picture suspicious of SSI and having undergone an intervention. Patients with non-SSI S. aureus (e.g., contamination, colonization, etc.) were excluded.
SSI caused by pathogens other than S. aureus and culture-negative SSI were excluded from the study. Codes that did not comprise surgical procedures, e.g., haemodialysis, were defined by a committee of infectious disease specialists and surgeons of the respective specialties at the coordinating centre. Minimal invasive procedures and eye surgery excluded from the study are listed in the Additional file 2. Country-specific procedure codes were harmonized as described previously : We included the existing surgical procedure coding systems of the five participating European countries (France, Germany, Italy, Spain, and the UK). In an iterative process, country specific codes were grouped in ever more categories until each group represented a coherent unit based on mode, interventions performed, extent and site of the surgical procedure. Next, two ID specialist (arbitrated by a third in case of disagreement) independently assigned country-specific codes to the resulting categories. Finally, specialist from each surgical discipline reviewed these assignments for their respective field. A total number of 15,432 surgical procedures were assigned to 153 codes from 10 specialties .
Matching of case-controls
To allow data assessment for outcomes, S. aureus SSI cases were matched to controls who underwent the same procedure using optimal propensity score matching based on cohort data, in particular age, diabetes, duration of procedure as percentile for this procedure. Six of the participating centres were not equipped to export comorbidities electronically (n = 77 494). Out of all comorbidities, only the item diabetes was provided by all centres and therefore included. Before inclusion, controls were verified to be free of SSI by infectious disease experts and surgeons of the respective centres. Controls determined to have had SSI were excluded from the cohort and associated cases were rematched.
The following additional data were manually documented from S. aureus SSI patients (cases) and 1:1 matched patients (controls) from the same centre: American Society of Anaesthesiologists (ASA) score, body mass index (BMI), length of hospitalization, length of intensive care unit (ICU) stay, reason and attribution to SSI in case of ICU admission, survival at 30 and at 90 days, antibiotic treatment including treatment duration in days, functional status at admission and at last discharge, necessity for surgical revision, and death attributed to SSI. If readmission was necessary, reason and relatedness to SSI, length of hospitalization and ICU stay as well as all antibiotic treatments and their duration were recorded. For cases, the causative pathogens, antibiotic resistance patterns, and type of SSI according to ECDC criteria  were also captured.
S. aureus SSI incidence was defined as the number of S. aureus SSI per 100 surgical procedures. Based on the available literature, the S. aureus SSI rate was assumed to be 1.0%, independent of the surgical specialty involved and the type of procedure performed [1, 2, 4]. Thus, by observing 1,500 surgical procedures, cumulative incidences were expected to be determined with a 95% confidence interval of ± 0.5%. We therefore aimed to include 90 000 to 150 000 patients allowing to calculate incidence with a meaningful precision for all surgical procedures performed on at least 1.0% to 1.5% percent of the overall surgical population at participating centres.
The primary study objective was to determine the overall and procedure-specific incidence of S. aureus surgical site infections in Europe. The cohort of all patients who underwent surgery defined the denominator, while patients developing S. aureus SSI defined the numerator.
For the primary analysis, the cumulative incidence was calculated using 95% confidence intervals. Secondary analyses focussed on the overall and procedure-specific outcomes of S. aureus SSI. The dependent variable was S. aureus SSI. Within the nested case–control design, continuous variables are presented as mean (standard deviation) and median (interquartile range [IQR]) and compared using Student’s t-test or Wilcoxon rank-sum test after performing a normality test. We present categorical variables as proportions and compared those using Fisher’s Exact test.
We performed descriptive statistics of all parameters observed. Country-based and institution-based incidence was determined for each procedure (e.g., ventral hernia repair) and each category (e.g., vascular surgery). For each incidence, the 95% confidence intervals for a binomial proportion were calculated.
Based on case–control matching, the composition of the surgical patient population was characterized.
Further statistical analysis included comparison of SSI, in particular S. aureus SSI incidence, in the different participating countries. We used logistic regression to calculate odds ratios (OR).
Statistical analysis and generation of all tables, listings and figures were performed using SPSS® (IBM Corporation, Chicago, IL, USA).
Role of the funding source
This study was an investigator-initiated trial with the University of Cologne as sponsor. The study was funded by a restricted research grant from Pfizer. The company provided advisory input into the trial design and was provided the primary raw data. Pfizer did not participate in site section, trial conduct, data analysis or manuscript writing.
From 259 459 initially exported data sets of patients who had undergone surgery, 178 902 were included in the analysis; characteristics are depicted in Table 1. In total, 80 557 were excluded due to age < 18 years [7 855], year of surgery not 2016 (10 943), MCAR (29 845), eye or minimal invasive surgery (17 846) or duplicate entries (13 904) (Fig. 3). The procedure most frequently performed within the entire cohort was DER01 (Incision and excision of skin and subcutaneous tissue; n = 9483), followed by GYN08 (Caesarean section; n = 8146).
Data from 178 902 patients who had undergone surgery in five European countries (Fig. 1) in 2016 were analysed. Of those, 764 had S. aureus SSI constituting an overall incidence of 0.4% (Table 2). The overall relative proportions of superficial, deep, and organ/space SSI were 46.3%, 27.0%, and 26.7%, respectively (Additional file 3: Table S2). A total of 660 (86.0%) S. aureus SSIs were caused by methicillin susceptible S. aureus (MSSA) and 104 cases (14.0%) by MRSA.
Cohort data and incidence of S. aureus SSI per country are depicted in Fig. 4. For the nested case–control part 20 (2.6%) of 764 cases were excluded due to missing documentation of the matched cases.
Procedure-specific S. aureus SSI distribution
S. aureus SSI incidence did not differ across surgical specialties. Median discipline-specific incidence was 0.5 (IQR 0.265) as depicted in Fig. 4. Highest procedure-specific incidence (Table 2) was seen in visceral surgery (minimally invasive surgery of the bile duct, 4.6% [95% CI 1.2 -17.6%], and laparoscopic excision of the small intestine, 4.2% [95% CI 0.6–28.4%]) as well as cardiothoracic surgery (operation on the diaphragm, 3.6% [95% CI 0.5–24.5%]). With regard to distribution of S. aureus SSI, it most frequently occurred in caesarean Sects. (8.9%), operations on scull, brain, and meninges (5.8%), and open repair of fractured long tubular bones (4.32%). Surgery performed in cardiothoracic surgery (0.8%) and vascular surgery (0.6%) had a higher specialty-specific S. aureus SSI incidence than anticipated for the respective group along a prediction model (adjusted residual values 5.7 and 3.2, respectively). Further in depth-analyses revealed that patients undergoing surgery in one of those mentioned groups had significantly more comorbidities than patients from other surgical fields (data not shown) and were the oldest of the entire cohort (mean age in the cardiothoracic surgery group 64.3 years and in the vascular surgery group 63.3 years). A list of all procedure specific incidences is included in the Additional file 2.
Structural equality of cases and controls
Between cases and controls, no statistically significant differences in sex, age, or BMI were observed hence indicating successful matching; distribution of comorbidities was similar and significant differences were only seen in patients with peripheral vascular diseases, hemiplegia, and solid tumours (Table 1). Cases and controls were matched along the SALT (Staphylococcus aureus Surgical Site Infection Multinational Epidemiology in Europe [SALT] study) code  and 760 of 764 controls had undergone the same type of procedure as the respective case. For optimal matching, four patients were allocated to similar types of procedures – three within the respective specialty (two neurosurgical procedures) and one with a surgical procedure from another specialty (dermatological procedure with one visceral surgical procedure, Additional file 3: Table S3).
Sex did not differ across the entire cohort (51.7% female and 48.3% male), but there were more men than women among SSI cases (51.9% and 48.1%, respectively; p = 0.043; Table 1). Mean age was 56.7 years for controls and 58.1 years for cases. Distribution of age groups was comparable, and in both populations the majority of patients was older than 60 years. Within the group of SSI cases, most patients (65.4%) were overweight (BMI ≥ 25). However, there was no statistically significant difference between cases and controls (p = 0.77).
The most frequent comorbidity in the overall cohort was diabetes (11.4%), followed by solid malignancies (7.3%), chronic cardiovascular disease (CVD, 4.4%), peripheral vascular disease (3.4%), and chronic kidney disease (CKD, 3.2%) (Table 1). Within the population of S. aureus SSI cases, CVD (23.1%), solid malignancies (22.3%), diabetes (21.0%), and CKD (7.8%) were also the most frequent comorbidities. Comorbidities of the control group were distributed in a comparable manner (significantly differing prevalence only reported for hemiplegia, solid tumours, and peripheral vascular disease; Table 1).
Comparison of cases and controls
Univariate analyses showed that in comparison with matched uninfected controls, patients with S. aureus SSI were more likely to be re-admitted to the hospital (p < 0.005), more likely in need of revision surgery (p < 0.005) and hospitalized for a longer period (mean hospitalization 17 versus 12 days; p < 0.005) (Table 2). ICU stay after a surgical procedure was as frequent in cases as in controls (144 versus 130, p = 0.349). Out of 144 ICU stays in the case cohort, 42 (29.2%) were associated to SSI. An in depth-analysis of the different surgical categories showed, that case patients undergoing cardiothoracic, gynaecological, neuro- or vascular surgery were associated with significantly higher rates of readmission to hospital, revision surgery, and longer hospitalization periods (Table 3 and 4).
In a multivariate analysis risk factors (Table 3) for S. aureus SSI were male sex, chronic cardiovascular disease (OR 1.9, confidence interval [CI] 1.3–2.6), peripheral vascular disease (OR 2.7, CI 2.0–3.7), lymphoma [OR 3.9, CI 1.6–9.4], solid tumour [OR 2.1, CI 1.6–2.8], diabetes (OR 1.7, CI 1.4–2.1) and chronic kidney disease (OR 1.7, CI 1.3–2.5). Smoking status was similar in both, cases, and controls (p = 0.774) (Table 5 and 6).
We report results from the first multinational study of surgical site infection sufficiently powered to determine SSI rates irrespective of procedure type. Our approach allowed to detect and analyse 764 cases of culture-proven S. aureus surgical site infections among 178 902 included patients – an investigation of a scale comparable to the most recent ECDC SSI report (1 016 S. aureus SSI cases) – while providing data of much higher granularity than in single-country surveillance efforts .
The percentage of comorbidities in our cohort mirrors the distribution among the European population, in particular the diabetes rate of 11.4% compared to 9.5% in the general population . Our study population was older than the European average (37.1% above 65 years in our cohort vs 18.4% to 22.8% in the respective countries ) reflecting that older patients are more likely to undergo surgery [14, 15].
The overall S. aureus SSI incidence was 0.43%, which is about 50% lower than assumed in our sample size calculation based on European averages. While lower than the European average, our findings are in line with prior publications by centres with similar expertise (ranging between 0.2% and 0.9%) , [17,18,19,20] Except for a higher incidence in the UK (1.9%), we saw a uniform S. aureus SSI distribution among all countries – most likely reflecting similar standards among leading surgical centres. The higher incidence in the UK might be a direct result of the lower-case volume at the sole participating UK centre and a resulting true higher SSI rate. Alternatively, it might be a statistical effect as a function of the lower number of contributed cases (9 168 vs mean of 42 424). As our trial was restricted to culture proven S. aureus SSI differences in incidence might also be the result in differences in clinical approaches to SSI (e.g. the frequency of obtaining cultures or use of antibiotic prophylaxis).
In line with our hypothesis that indicator procedures are not representative of their respective categories, S. aureus SSI incidence displayed a high degree of intra-disciplinary variability. Currently monitored indicator procedures were representative of their respective category in some disciplines (e.g., total prosthetic replacement of knee joint 0.4% [0.2–0.7%] for orthopaedic surgery 0.5% [0.4–0.6%]), but not others (e.g., revascularization of the heart 1.6% [1.1–2.2%] for cardiothoracic surgery 0.8 [0.6–1.0%]). While statistical interference testing for each procedure was beyond the scope of this work, non-overlapping confidence intervals strongly suggest real differences rather than random effects. These differences highlight the need to expand surveillance efforts beyond indicator procedures.
Beyond our expectation of relevant S. aureus SSI rates in all surgical domains our data show similar average S. aureus SSI rates across all subspecialties. This finding is explained by S. aureus pertaining to the skin microbiome common to all surgical sites in contrast to SSI caused by site-specific organisms (e.g., Enterobacterales in GI surgery). These characteristics further strengthen our assumption that S. aureus is a prototypical causative organism in SSI. This supports the notion that S. aureus can be used as a marker or sentinel pathogen in trials focusing on overall periprocedural care and host defence in a discipline-independent fashion, i.e., in contrast to current discipline-specific approaches . This finding also highlights a need for further exploration of risk factors across all procedures and subspecialties.
The identified risk factors for S. aureus SSI were male sex, chronic cardiovascular disease, peripheral vascular disease, lymphoma, solid tumour, diabetes and chronic kidney disease reflecting a population with severe intern diseases. As recommended in general for these patients, high awareness for infections including adequate prophylactic treatment should guide medical decisions.
The choice of our retrospective study design can be regarded as either a strength or a weakness. We believe that, in the context of HAI by a known pathogen, a retrospective study prevents the Hawthorne effect of confounding by observation while not affecting detection rates [21, 22]. This study design cannot be easily applied to the study of HAI without a microbiologically proven pathogen. Furthermore, while the choice of S. aureus as a marker organism is indeed compelling, our study results cannot be extrapolated to culture-negative SSI or SSI caused by other organisms. However, the current work may be regarded as a proof of concept for a novel epidemiological approach to HAI.
We limited our investigation to adult patients; our findings are thus not applicable to children. No available risk stratification approach has been established in a procedure-independent fashion and research shows wide SSI rate variability within risk categories depending on the specific surgery type . Consequently, we matched cases and controls by a propensity score rather than by more conventional approaches like the NHSN SSI risk index. The paucity of variables used in matching cases and controls resulted directly from limitations of available electronic health record data and hospital information technology capabilities. However, our subsequent analysis demonstrated successful matching.
As reported elsewhere  our study was initially designed to establish S. aureus SSI patterns representative of the overall European surgical population. However, during the centre selection process it became apparent that centres with the necessary technical expertise to generate exports and perform local matching would constitute a highly select sample and thus not be representative of the overall European S. aureus SSI epidemiology. We thus decided to focus on analysing high performance centres rather than trying to extrapolate results from a highly select sample to the overall surgical population. We believe this approach generates more valid and robust data. Post-hoc changes to study design risk can introduce bias and thus compromise validity of results. This risk should, however, not apply to our analysis as we merely abstained from over-interpreting our data by trying to extrapolate it to the overall European surgical population, while not deviating in any other aspect from the trial protocol or the statistical analysis plan.
Future endeavours in applying real-world data to epidemiological research in HAI will be aided by progressive digitalization of medicine, and the compatibility of database interfaces. Technical aspects regarding the harmonization of different procedure coding systems have been discussed elsewhere .
Our methodology and results diverge in important aspects from ongoing SSI surveillance and highlight the expanded possibilities provided by electronic health records and big data. Discrepancies between results from prospective trials and real-world evidence are a well-known issue. Contrary to prior hierarchical views of evidence levels, current approaches integrate real-world evidence with the results of more formal study types [24, 25]. We believe that such an integrative approach of supplementing prospective surveillance results with real-life data will provide a comprehensive grasp of the current state of HAI.
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All data may be available upon request to the corresponding author.
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Open Access funding enabled and organized by Projekt DEAL. This study was an investigator-initiated trial funded by restricted research grant by Pfizer (WI212601). The company provided advisory input into the trial design and was provided the primary raw data. Pfizer did not participate in site selection, trial conduct, data analysis or drafting of this manuscript.
Ethics approval and consent to participate
The study is a retrospective study and was submitted to the Research Ethics Commission of the University of Cologne (No. 17–078) for advice; the requirement for informed consent was waived due to the retrospective nature as well as the anonymous data capture strategy of this study. The study was registered at clinicaltrials.gov under NCT03353532.
All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare the following: EG reports grants from German Federal Ministry of Education and Research, the EU, and the German Statutory Health Insurances, personal fees from the Bavarian association of statutory health insurance registered doctors, not related to this work. TG reports report personnal fees from Pfizer, Gilead and Viiv healthcare. FK reports grants from Else Kröner-Fresenius-Stiftung, grants from German Research Foundation under Germany's Excellence Strategy—EXC 2030: CECAD—Excellent in Aging Research—Project number 390661388, grants from Koeln Fortune program / Faculty of Medicine, University of Cologne, outside the submitted work. SCM reports grants from University of Cologne (KoelnFortune), grants from Dr Manfred Plempel Stipend, grants from DZIF Clinical Leave, personal fees from Octapharma, outside the submitted work. MWP reports grants for the German Research Foundation, the German Ministry for Education and Research, grants and personal fees from Pfizer, personal fees from MSD, personal fees from Infectopharm, personal fees from Thermofisher, personal fees from Shionogi, personal fees from Angelini, personal fees from Chiemsi, personal fees from Novartis, outside the submitted work. HS reports grants from German Centre for Infection Research (DZIF), personal fees from Basilea Pharmaceuticals, personal fees from Gilead, personal fees from MSD, personal fees from Entasis, grants from Accelerate, personal fees from Shionogi, personal fees from ThermoFisher, personal fees from bioMérieux, personal fees from Becton Dickinson, personal fees from Shionogi, personal fees from Eumedica, outside the submitted work. AS reports grants from PFIZER, grants from MERCK SHARP DHOME, grants from GILEAD, personal fees from MENARINI, personal fees from ANGELINI, personal fees from SHIONOGY, outside the submitted work. JS reports grants from German Federal Ministry of Education and Research, grants from Basilea Pharmaceuticals Inc., grants from German Society for Infectious Diseases, grants from Meta-Alexander Foundation, outside the submitted work. JJV has personal fees from Merck / MSD, Gilead, Pfizer, Astellas Pharma, Basilea, German Centre for Infection Research (DZIF), University Hospital Freiburg/ Congress and Communication, Academy for Infectious Medicine, University Manchester, German Society for Infectious Diseases (DGI), Ärztekammer Nordrhein, University Hospital Aachen, Back Bay Strategies, German Society for Internal Medicine (DGIM), Shionogi, Molecular Health, Netzwerk Universitätsmedizin, Janssen, NordForsk, and grants from Merck / MSD, Gilead, Pfizer, Astellas Pharma, Basilea, German Centre for Infection Research (DZIF), German Federal Ministry of Education and Research (BMBF), (PJ-T: DLR), University of Bristol, Rigshospitalet Copenhagen. OAC reports grants or contracts from Amplyx, Basilea, BMBF, Cidara, DZIF, EU-DG RTD (101037867), F2G, Gilead, Matinas, MedPace, MSD, Mundipharma, Octapharma, Pfizer, Scynexis; Consulting fees from Abbvie, Amplyx, Biocon, Biosys, Cidara, Da Volterra, Gilead, IQVIA, Janssen, Matinas, MedPace, Menarini, Molecular Partners, MSG-ERC, Noxxon, Octapharma, Pardes, Pfizer, PSI, Scynexis, Seres; Honoraria for lectures from Abbott, Abbvie, Al-Jazeera Pharmaceuticals, Astellas, Gilead, Grupo Biotoscana/United Medical/Knight, Hikma, MedScape, MedUpdate, Merck/MSD, Mylan, Noscendo, Pfizer, Shionogi; Payment for expert testimony from Cidara; Participation on a Data Safety Monitoring Board or Advisory Board from Actelion, Allecra, Cidara, Entasis, IQVIA, Janssen, MedPace, Paratek, PSI, Pulmocide, Shionogi, The Prime Meridian Group; A patent at the German Patent and Trade Mark Office (DE 10 2021 113 007.7); Stocks from CoRe Consulting; Other interests from DGHO, DGI, ECMM, ISHAM, MSG-ERC, Wiley., outside the submitted work. BJL reports grants from Pfizer, during the conduct of the study. The remaining authors have nothing to disclose.
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Mellinghoff, S.C., Bruns, C., Albertsmeier, M. et al. Staphylococcus aureus surgical site infection rates in 5 European countries. Antimicrob Resist Infect Control 12, 104 (2023). https://doi.org/10.1186/s13756-023-01309-w
- Surgical site infection
- Staphylococcus aureus
- Hospital acquired infection