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Emergence of methicillin-resistant Staphylococcus aureus USA300 genotype as a major cause of late-onset nosocomial pneumonia in intensive care patients in the USA

Open AccessPublished:February 04, 2013DOI:https://doi.org/10.1016/j.ijid.2012.12.013

      Summary

      Objective

      To compare demographic and clinical characteristics, and methicillin-resistant Staphylococcus aureus (MRSA) strain characteristics, in patients with early-onset (EO) and late-onset (LO) MRSA nosocomial pneumonia.

      Methods

      This was a retrospective analysis of data from a multicenter observational study of nosocomial pneumonia patients admitted between November 2008 and July 2010. Laboratory analyses performed on MRSA isolates included confirmation of antimicrobial susceptibility and heteroresistance to vancomycin, USA typing, staphylococcal cassette chromosome (SCC) mec typing, and detection of Panton–Valentine leukocidin (PVL) genes.

      Results

      We identified 134 patients; 42 (31%) had EO MRSA pneumonia and 92 (69%) had LO MRSA pneumonia. The patients in the LO group were more likely to have risk factors for multidrug-resistant pathogens (98% vs. 76%, p < 0.001). The MRSA USA300 strain was found with equal frequency in the EO and LO groups. Likewise, both groups had similar frequencies of isolates exhibiting PVL and SCCmec type IV.

      Conclusions

      Our findings provide further evidence of the continued migration of community-associated MRSA into the healthcare setting in the USA. MRSA USA300 genotype has emerged as a significant cause of LO nosocomial pneumonia in intensive care units. Appropriate anti-MRSA antimicrobial therapy should be considered for both EO and LO hospital-acquired pneumonia and ventilator-associated pneumonia.

      Keywords

      1. Introduction

      Nosocomial pneumonia is the second most common hospital-acquired infection in the USA and is associated with significant morbidity and mortality. Consensus guideline recommendations suggest the time of onset of pneumonia is an important epidemiologic variable and a key risk factor for specific pathogens and outcomes.
      American Thoracic Society, Infectious Diseases Society of America
      Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia.
      Early-onset pneumonia (at ≤4 days of hospitalization) has a better prognosis and is more likely to be caused by antibiotic-susceptible bacteria, whereas late-onset pneumonia (at ≥5 days) is more likely to be caused by multidrug-resistant (MDR) bacteria and is associated with increased morbidity and mortality.
      American Thoracic Society, Infectious Diseases Society of America
      Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia.
      The unrecognized presence of MDR pathogens in intensive care unit (ICU) patients with pneumonia can threaten the adequacy of initial empiric antibiotic therapy and can be associated with an increase in morbidity and mortality.
      Methicillin-resistant Staphylococcus aureus (MRSA) has become one of the most common hospital-acquired pathogens, accounting for >60% of the clinical S. aureus isolates recovered in ICUs in the USA.
      American Thoracic Society, Infectious Diseases Society of America
      Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia.
      Additionally, nosocomial MRSA strains are generally resistant to several classes of antibiotics and are therefore difficult and costly to treat. An early study by Pujol et al. compared methicillin-susceptible Staphylococcus aureus (MSSA) and MRSA in patients with ventilator-associated pneumonia (VAP). MRSA was found to occur exclusively in patients with late-onset VAP, defined in their study as pneumonia occurring after 3 days of intubation.
      • Pujol M.
      • Corbella X.
      • Pena C.
      • Pallares R.
      • Dorca J.
      • Verdaguer R.
      • et al.
      Clinical and epidemiological findings in mechanically-ventilated patients with methicillin-resistant Staphylococcus aureus pneumonia.
      The worldwide emergence of community-associated MRSA (CA-MRSA) in the last decade represents a significant change in the epidemiology of MRSA infections. CA-MRSA strains have molecular characteristics that differ from the traditional healthcare-associated MRSA (HA-MRSA) strains. In the USA, CA-MRSA strains typically belong to the pulsed-field gel electrophoresis (PFGE) type USA300, while HA-MRSA strains belong to the USA100 PFGE profile.
      • Tsuji B.T.
      • Rybak M.J.
      • Cheung C.M.
      • Amjad M.
      • Kaatz G.W.
      Community- and health care-associated methicillin-resistant Staphylococcus aureus: a comparison of molecular epidemiology and antimicrobial activities of various agents.
      CA-MRSA strains are associated with carriage of the Panton–Valentine leukocidin (PVL) gene, a cytotoxin associated with severe necrotizing infections, including necrotizing skin lesions as well as necrotizing pneumonia.
      • Tsuji B.T.
      • Rybak M.J.
      • Cheung C.M.
      • Amjad M.
      • Kaatz G.W.
      Community- and health care-associated methicillin-resistant Staphylococcus aureus: a comparison of molecular epidemiology and antimicrobial activities of various agents.
      • Morgan M.S.
      Diagnosis and treatment of Panton–Valentine leukocidin (PVL)-associated staphylococcal pneumonia.
      CA-MRSA strains also have microbiologically distinct characteristics including less antibiotic resistance and lower minimum inhibitory concentrations (MIC) for several antimicrobial agents.
      • Tsuji B.T.
      • Rybak M.J.
      • Cheung C.M.
      • Amjad M.
      • Kaatz G.W.
      Community- and health care-associated methicillin-resistant Staphylococcus aureus: a comparison of molecular epidemiology and antimicrobial activities of various agents.
      The recent medical literature indicates that CA-MRSA strains are being increasingly reported as etiologic agents of nosocomial infections.
      • Maree C.L.
      • Daum R.S.
      • Boyle-Vavra S.
      • Matayoshi K.
      • Miller L.G.
      Community-associated methicillin-resistant Staphylococcus aureus isolates causing healthcare-associated infections.
      • Popovich K.J.
      • Weinstein R.A.
      • Hota B.
      Are community-associated methicillin-resistant Staphylococcus aureus (MRSA) strains replacing traditional nosocomial MRSA strains?.
      • Gonzalez B.E.
      • Rueda A.M.
      • Shelburne S.A.
      • Musher D.M.
      • Hamill R.J.
      • Hulten K.G.
      Community-associated strains of methicillin-resistant Staphylococcus aureus as the cause of healthcare-associated infection.
      • Klevens R.M.
      • Morrison M.A.
      • Fridkin S.K.
      • Reingold A.
      • Petit S.
      • Gershman K.
      • et al.
      Community-associated methicillin-resistant Staphylococcus aureus and healthcare risk factors.
      • Wang J.T.
      • Wang J.L.
      • Fan C.T.
      • Chie W.C.
      • Lai M.S.
      • Lauderdale T.S.
      • et al.
      Risk factors for mortality of nosocomial methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infection: with investigation of the potential role of community-associated MRSA strains.
      The spread of CA-MRSA into the nosocomial setting may limit the utility of using epidemiological or clinical risk factors to assess the likelihood of infection with CA-MRSA vs. HA-MRSA when making a decision to start empiric antibiotic therapy in hospitalized patients, thereby threatening the adequacy of the initial antibiotic selection.
      In our institutions, we have recently identified patients with nosocomial pneumonia produced by MRSA strains carrying the PVL gene. We hypothesize that as CA-MRSA clones become more established in the hospital milieu they will begin to supplant traditional HA-MRSA strains as causes of late-onset healthcare-associated infections. To test our hypothesis, we designed a study with the objective of comparing patient clinical characteristics and MRSA strain characteristics in patients with early-onset (EO) and late-onset (LO) MRSA nosocomial pneumonia.

      2. Methods

      2.1 Study design

      This was a retrospective analysis of data collected for the Improving Medicine through Pathway Assessment of Critical Therapy in Hospital-Acquired Pneumonia (IMPACT-HAP) study. IMPACT-HAP was a multicenter, observational study aimed at improving the care of ICU patients with nosocomial pneumonia, including hospital-acquired pneumonia (HAP), VAP, and healthcare-associated pneumonia (HCAP). The study has been described in detail previously.
      • Mangino J.E.
      • Peyrani P.
      • Ford K.D.
      • Kett D.H.
      • Zervos M.J.
      • Welch V.L.
      • et al.
      the IMPACT-HAP Study Group
      Development and implementation of a performance improvement project in adult intensive care units: overview of the Improving Medicine Through Pathway Assessment of Critical Therapy in Hospital-Acquired Pneumonia (IMPACT-HAP) study.
      The objective of this analysis was to compare patient clinical characteristics and MRSA strain characteristics between patients with EO and LO nosocomial pneumonia caused by MRSA. Patients with HCAP were excluded from the analysis as they were confined to the EO group. Centers contributing cases for this analysis included: the University of Louisville Medical Center (Louisville, KY), Ohio State University Medical Center (Columbus, OH), Henry Ford Health System (Detroit, MI), University of Miami/Jackson Memorial Hospital (Miami, FL), and Summa Health System (Akron, OH). The study was approved by the institutional review board at each participating hospital, which all waived the need for informed consent. University of Louisville was the coordinating center and validated the quality of the data by checking for discrepancies and inconsistencies. Upon validation, the case was accepted for analysis. Patients included in this study were treated during the period from November 2008 to July 2010.

      2.2 Patient population and assessment

      Adult patients (≥18 years of age) in participating ICUs were eligible for inclusion if there was a clinical suspicion of evolving pneumonia with new or progressive infiltrates on chest radiograph and at least two of the following: new or increased cough or sputum production, fever, hypothermia, leukocytosis, left shift, leukopenia, deterioration of pulmonary function.

      Centers for Disease Control and Prevention. National Healthcare Safety Network. Patient safety component manual. Device-associated module, ventilator-associated pneumonia (VAP) event. Atlanta, GA: CDC; 2011.

      We used the American Thoracic Society (ATS)/Infectious Diseases Society of America (IDSA) definitions for HAP, VAP, and HCAP.
      American Thoracic Society, Infectious Diseases Society of America
      Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia.
      At the time of diagnosis of pneumonia (day 0), we collected data on patient demographic and baseline characteristics, severity of illness including the Acute Physiology and Chronic Health Evaluation (APACHE) II score
      • Knaus W.A.
      • Draper E.A.
      • Wagner D.P.
      • Zimmerman J.E.
      APACHE II: a severity of disease classification system.
      and the Clinical Pulmonary Infection Score (CPIS, also repeated on day 3),
      • Pugin J.
      • Auckenthaler R.
      • Mili N.
      • Janssens J.P.
      • Lew P.D.
      • Suter P.M.
      Diagnosis of ventilator-associated pneumonia by bacteriologic analysis of bronchoscopic and nonbronchoscopic ‘blind’ bronchoalveolar lavage fluid.
      diagnostic procedures, and treatment data including antibiotics received from day 0. Risk factors for MDR pathogens (i.e., antibiotics within the prior 30 days; hospitalized for ≥5 days before antibiotics for HAP were started; bronchiectasis; known family member with MDR pathogen; colonization with MRSA, Pseudomonas aeruginosa, Acinetobacter, and/or vancomycin-resistant Enterococcus (VRE); hospitalized for >2 days within the prior 90 days; nursing home or extended care facility; chronic dialysis within the prior 30 days; home infusion therapy; home wound care; and immunosuppressive disease and/or therapy) were collected. Patients were considered at risk of MDR pathogens if they had one or more factors present. Colonization was ascertained passively, based on information included in the patient's medical records at the time of diagnosis. Patients were followed until hospital discharge, death, or 28 days after pneumonia diagnosis, whichever occurred first. Laboratory values were collected during the index hospitalization. Microbiology laboratories at each center provided semiquantitative cultures of tracheal aspirates and either semiquantitative or quantitative cultures of bronchoalveolar lavage specimens. All culture results were reviewed at every site by the study coordinator and principal investigator in order to classify identified microorganisms as pathogenic or not.

      2.3 Study definitions

      All patients in this analysis met the ATS/IDSA definitions for HAP or VAP and had a positive respiratory culture for MRSA.
      Pneumonia occurring ≥5 days after the hospital admission was defined as LO pneumonia;
      American Thoracic Society, Infectious Diseases Society of America
      Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia.
      all other patients were considered to have EO pneumonia.
      All-cause mortality was assessed at day 14 and day 28 after pneumonia diagnosis.

      2.4 Laboratory analysis of MRSA isolates

      Initial identification of MRSA isolates collected on day 0 and in vitro antibiotic susceptibilities were determined using clinical microbiology laboratory tests with automated susceptibility testing methods at each participating center. Isolates were shipped to the IMPACT-HAP microbiology reference laboratory at Henry Ford Hospital for further evaluation. In vitro susceptibilities to vancomycin were determined using the standard Etest method, following the instructions provided by the manufacturer (bioMérieux, Marcy l’Etoile, France). MRSA isolates were further characterized by PFGE for USA typing. Genomic DNA for PFGE was prepared, digested with SmaI (New England BioLabs, Beverly, MA, USA), and run on a CHEF-DR III (Bio-Rad Laboratories, Hercules, CA, USA) using a previously described method.
      • McDougal L.K.
      • Steward C.D.
      • Killgore G.E.
      • Chaitram J.M.
      • McAllister S.K.
      • Tenover F.C.
      Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: establishing a national database.
      PFGE patterns were compared using BioNumerics software (version 3.5; Applied Maths, Belgium). Isolates were determined to be of the same PFGE strain group if their SmaI restriction patterns were ≥80% similar using the Dice coefficient.
      • Singh A.
      • Goering R.V.
      • Simjee S.
      • Foley S.L.
      • Zervos M.J.
      Application of molecular techniques to the study of hospital infection.
      Multiplex PCR amplification was performed to determine the staphylococcal cassette chromosome mec (SCCmec) type (types I– V) using a previously described method.
      • Zhang K.
      • McClure J.A.
      • Elsayed S.
      • Louie T.
      • Conly J.M.
      Novel multiplex PCR assay for characterization and concomitant subtyping of staphylococcal cassette chromosome mec types I to V in methicillin-resistant Staphylococcus aureus.
      Accessory gene regulator (agr) typing was performed for all isolates by PCR using agr group-specific primers.
      • Strommenger B.
      • Cuny C.
      • Werner G.
      • Witte W.
      Obvious lack of association between dynamics of epidemic methicillin-resistant Staphylococcus aureus in central Europe and agr specificity groups.
      MRSA strains were screened for the PVL toxin genes, lukS-PV and lukF-PV, by PCR.
      • Lina G.
      • Piémont Y.
      • Godail-Gamot F.
      • Bes M.
      • Peter M.O.
      • Gauduchon V.
      • et al.
      Involvement of Panton–Valentine leukocidin-producing Staphylococcus aureus in primary skin infections and pneumonia.
      Isolates were screened and tested for vancomycin heteroresistance using the macrodilution Etest method (bioMérieux, Durham, NC, USA), as previously described.
      • Wootton M.
      • MacGowan A.P.
      • Walsh T.R.
      • Howe R.A.
      A multicenter study evaluating the current strategies for isolating Staphylococcus aureus strains with reduced susceptibility to glycopeptides.

      2.5 Statistical analysis

      Patients with EO MRSA nosocomial pneumonia were compared against those patients with LO disease. Categorical variables were compared using the Chi-square test or Fisher's exact test, and continuous variables were compared using the Student's t-test and the Mann–Whitney U-test, when appropriate. A similar approach was followed when comparing laboratory characteristics of MRSA isolates recovered from patients with EO and LO pneumonia. p-Values of ≤0.05 were considered statistically significant. All analyses were performed using SAS software, version 9.2 (SAS Inc., Cary, NC, USA).

      3. Results

      3.1 Patient characteristics

      A total of 134 ICU patients with MRSA nosocomial pneumonia were identified in our database. Forty-two patients (31%) had EO MRSA nosocomial pneumonia and 92 patients (69%) had LO MRSA nosocomial pneumonia.
      A greater frequency of risk factors for MDR pathogens was found among patients in the LO group (Table 1). However, it is important to note that risk factors for MDR bacterial pathogens were documented in a majority of ICU patients with EO and LO disease (76% and 98%, respectively). Patients with LO MRSA pneumonia were more frequently diagnosed with HAP, although the difference was not statistically significant. We observed no differences between the two groups in relation to gender, age, CPIS, APACHE II score, severe sepsis diagnosis, or underlying medical conditions. While multilobar infiltrates were more frequent among patients in the LO group (66% vs. 33%), the difference did not reach statistical significance.
      Table 1Characteristics of patients with methicillin-resistant Staphylococcus aureus (MRSA) nosocomial pneumonia, by time of onset
      VariableEarly-onset

      n = 42
      Late-onset

      n = 92
      p-Value
      Male gender26 (62%)61 (66%)0.6205
      Age, mean (SD)58 (19.2)59.2 (17.7)0.7329
      APACHE II score, day 0, mean (SD)20.4 (6.9)20.3 (6.5)0.9076
      CPIS, day 0, mean (SD)6.3 (1.8)6.4 (1.8)0.6265
      CPIS, day 3, mean (SD)6.8 (2.6)6.4 (2.6)0.3866
      Risk factors of MDRO32 (76%)90 (98%)< 0.0001
      Multilobar infiltrate2 (33%)21 (66%)0.1375
      Patients with severe sepsis33 (79%)66 (72%)0.4036
      Type of pneumonia0.056
       HAP11 (26%)40 (43%)
       VAP31 (74%)52 (57%)
      Underlying medical condition
       Cardiac disease16 (38%)30 (33%)0.5349
       Renal disease3 (7%)16 (17%)0.1147
       Vascular disease15 (36%)33 (36%)0.9861
       Diabetes10 (24%)31 (34%)0.2493
       Respiratory disease12 (29%)24 (26%)0.7634
      Immunosuppressive disease and/or therapy
       Cancer3 (7%)14 (15%)0.1926
       End-stage renal disease
      Clinical judgment of attending physician.
      0 (0%)5 (5%)0.1236
       End-stage liver disease
      Clinical or histological diagnosis of cirrhosis, or another form of chronic liver disease.
      1 (2%)3 (3%)0.7738
       End-stage lung disease
      Chronic obstructive pulmonary disease with forced expiratory volume in 1 s (FEV1) <30% or O2-dependent.
      4 (10%)11 (12%)0.6786
       Steroids
      Prednisone >10mg/day for >7 days.
      5 (12%)5 (5%)0.1925
       Active chemotherapy
      Within last 30 days.
      1 (2%)1 (1%)0.5666
       Active radiotherapy
      Within last 30 days.
      1 (2%)0 (0%)0.1374
      Results are expressed as n (%), unless specified otherwise. APACHE II, Acute Physiology and Chronic Health Evaluation II; CPIS, Clinical Pulmonary Infection Score; HAP, hospital-acquired pneumonia; MDRO, Multi-drug resistant organism; SD, standard deviation; VAP, ventilator-associated pneumonia.
      a Clinical judgment of attending physician.
      b Clinical or histological diagnosis of cirrhosis, or another form of chronic liver disease.
      c Chronic obstructive pulmonary disease with forced expiratory volume in 1 s (FEV1) <30% or O2-dependent.
      d Prednisone >10 mg/day for >7 days.
      e Within last 30 days.
      When we compared the presence of the most common risk factors identified for MDR pathogens between the two groups (Table 2), we observed that patients with LO MRSA nosocomial pneumonia were more likely to have received antibiotics in the prior 30 days, while EO MRSA nosocomial pneumonia patients were more likely to be colonized with VRE.
      Table 2Evaluation of the most common risk factors identified for multidrug-resistant (MDR) pathogens in patients with methicillin-resistant Staphylococcus aureus (MRSA) nosocomial pneumonia, by time of onset
      Risk factorsEarly onset

      n = 42
      Late onset

      n = 92
      p-Value
      Prior antibiotic use20 (48%)63 (69%)0.0211
      MRSA colonization9 (21%)23 (25%)0.6528
      VRE colonization36 (86%)61 (66%)0.0197
      Acinetobacter colonization0 (0%)3 (3%)0.2366
      Results are expressed as n (%). VRE, vancomycin-resistant Enterococcus.

      3.2 Laboratory analysis of MRSA isolates

      As depicted in Table 3, there were significant differences in MRSA USA subtypes between the EO and LO MRSA nosocomial pneumonia study groups. USA100 strains (the traditional healthcare-associated MRSA) were more frequent among patients with LO MRSA pneumonia, while USA600 strains (rarely seen in patients in the USA, associated with higher failure rates in patients with bacteremia) were found predominantly among patients with EO MRSA pneumonia.
      • Moore C.L.
      • Osaki-Kiyan P.
      • Perri M.
      • Donabedian S.
      • Haque N.Z.
      • Chen A.
      • Zervos M.J.
      USA600 (ST45) methicillin-resistant Staphylococcus aureus bloodstream infections in urban Detroit.
      The USA300 subtype was found in 42% of EO and in 38% of LO MRSA nosocomial pneumonia cases, however this particular difference is not significant (Chi-square standardized residual = −0.20). Similarly, no significant differences were found between the two study groups in terms of the presence of the PVL gene (p = 0.642), or SCCmec type IV (p = 0.365). We observed more patients with a vancomycin MIC ≥2 μg/ml in the LO group, though this difference did not reach statistical significance (p = 0.09).
      Table 3Characteristics of 134 methicillin-resistant Staphylococcus aureus (MRSA) strains causing nosocomial pneumonia, by time of onset
      VariableEarly onset n = 42Late onset n = 92p-Value
      USA strain0.0091
       USA10015 (40%)49 (60%)
       USA2002 (5%)0 (0%)
       USA30016 (42%)31 (37.8%)
       USA6005 (13%)2 (2%)
      PVL-positive PCR14 (33%)27 (29%)0.6423
      SCCmec type II21 (88%)53 (79%)0.3652
      SCCmec type IV3 (13%)14 (21%)0.3652
      agr type I21 (50%)35 (38%)0.1219
      agr type II20 (48%)57 (62%)0.1219
      agr type III1 (2%)0 (0%)0.1219
      Heteroresistance5 (12%)10 (11%)0.8601
      Vancomycin MIC0.0907
       <2 μg/ml31 (74%)52 (57%)
       ≥2 μg/ml11 (26%)40 (43%)
      MIC, minimum inhibitory concentration; PCR, polymerase chain reaction; PVL, Panton–Valentine leukocidin; SCC, staphylococcal cassette chromosome; agr, accessory gene regulator.

      4. Discussion

      Our results indicate that USA300 strains were isolated with similar frequency in ICU patients with EO and LO MRSA nosocomial pneumonia, a finding suggesting USA300 strains have become established nosocomial pneumonia pathogens in our institutions.
      Earlier studies had demonstrated EO and LO nosocomial pneumonias to be caused by different groups of pathogens, with MDR organisms predominantly associated with LO infections.
      • Trouillet J.L.
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      • Combaux D.
      • Dombret M.C.
      • et al.
      Ventilator associated pneumonia caused by potentially drug resistant bacteria.
      • Baker A.M.
      • Meredith J.W.
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      Pneumonia in intubated trauma patients: microbiology and outcomes.
      Guidelines have thus recommended dividing nosocomial pneumonia into EO and LO pneumonia to help guide the selection of appropriate empiric antimicrobial therapy.
      American Thoracic Society, Infectious Diseases Society of America
      Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia.
      However, more recent studies suggest MDR pathogens could be etiologic agents of nosocomial pneumonia in patients with early onset of infection. Giantsou et al. found no difference in the contribution of potentially multiresistant pathogens (79% vs. 85%), P. aeruginosa (42% vs. 47%), or MRSA (33% vs. 30%) between EO and LO VAP patients in a prospective evaluation of 408 VAP patients.
      • Giantsou E.
      • Liratzopoulos N.
      • Efraimidou E.
      • Panopoulou M.
      • Alepopoulou E.
      • Kartali-Ktenidou S.
      • et al.
      Both early-onset and late-onset ventilator-associated pneumonia are caused mainly by potentially multiresistant bacteria.
      In a prospective cohort study, Verhamme and colleagues described 298 patients with ICU-acquired pneumonia, and pathogens potentially resistant to multiple drugs including MRSA were isolated from 52% of patients with EO pneumonia. Risk factors for the isolation of these pathogens included older age and receipt of antibiotic prophylaxis.
      • Verhamme K.M.
      • De Coster W.
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      • De Beenhouwer H.
      • Nollet G.
      • Verbeke J.
      • et al.
      Pathogens in early-onset and late-onset intensive care unit-acquired pneumonia.
      In accordance with these observations we found a significant proportion of patients in both the EO and LO pneumonia groups to have documented risk factors for MDR pathogens, including previous receipt of antibiotics. As more patients with complicated medical conditions are routinely cared for outside the hospital, the distinction between EO and LO pneumonia may not be as clinically relevant as in the past. Our data suggest that MDR pathogens, MRSA in particular, should be an important consideration in all patients with nosocomial pneumonia irrespective of time of onset.
      Likewise, the epidemiologic distinction between CA-MRSA and HA-MRSA is blurring as MRSA strains of community origin are increasingly identified causing healthcare-associated disease. Klevens et al. at the US Centers for Disease Control and Prevention (CDC) reported data from a multistate surveillance system for invasive MRSA infections and found that 18–28% of patients with healthcare-related risk factors had infection caused by a ‘community-associated’ strain, primarily USA300.
      • Klevens R.M.
      • Morrison M.A.
      • Fridkin S.K.
      • Reingold A.
      • Petit S.
      • Gershman K.
      • et al.
      Community-associated methicillin-resistant Staphylococcus aureus and healthcare risk factors.
      In a single-center report, Seybold and colleagues at Grady Memorial Hospital, a 1000-bed, urban, public hospital in Atlanta, Georgia, USA, found USA300 strains accounted for 28% of the MRSA healthcare-associated bloodstream infections and 20% of nosocomial MRSA bloodstream infections.
      • Seybold U.
      • Kourbatova E.V.
      • Johnson J.G.
      • Halvosa S.J.
      • Wang Y.F.
      • King M.D.
      • et al.
      Emergence of community-associated methicillin-resistant Staphylococcus aureus USA300 genotype as a major cause of health care-associated blood stream infections.
      Most recently, a multicenter ICU epidemiology study in the USA reported 14% of patients admitted to ICUs were colonized with a CA-MRSA genotype.
      • Nair N.
      • Kourbatova E.
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      • Huskins W.C.
      • et al.
      Molecular epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) among patients admitted to adult intensive care units: the STAR*ICU trial.
      Outside the USA, 31.7% (39/123) of hospital-onset MRSA cases identified over 1 year in two urban hospitals in Vancouver, Canada were caused by the USA300 strain, when previously only 6% of HA-MRSA infections or colonizations reported from Canadian hospitals were attributable to USA300 or USA400 strains.
      • Wilmer A.
      • Lloyd-Smith E.
      • Romney M.
      • Hoang L.
      • Hull M.
      • Champagne S.
      Methicillin-resistant Staphylococcus aureus strain USA300 is prevalent among hospital-onset cases in an urban Canadian setting.
      Our findings confirm the prior observations and expand them to a unique and specific group of patients, ICU patients with nosocomial pneumonia. We believe this is the first study to specifically compare the genotype and phenotype MRSA strain characteristics of EO vs. LO in ICU patients with MRSA nosocomial pneumonia. In our large, multicenter collection of MRSA nosocomial patients, we found a similar distribution of USA300 MRSA strains in ICU patients irrespective of EO or LO pneumonia. Ours was a well-characterized study population with clinical and laboratory information available for analysis. Furthermore, we explored the effect of a variety of pathogen factors as well as host-related factors.
      It is not clear why CA-MRSA has been successful in transitioning from community into hospital settings, displacing HA-MRSA as a cause of some healthcare-associated infections. Possible causes include CA-MRSA as a more ‘fit’ isolate than HA-MRSA.
      • Maree C.L.
      • Daum R.S.
      • Boyle-Vavra S.
      • Matayoshi K.
      • Miller L.G.
      Community-associated methicillin-resistant Staphylococcus aureus isolates causing healthcare-associated infections.
      It has been reported that CA-MRSA isolates carrying SCCmec type IV multiply much faster than traditional HA-MRSA strains and have an enhanced ecologic fitness.
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      • et al.
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      The transition of CA-MRSA into hospital settings will limit our ability to use clinical and epidemiological risk factors when determining the risk for specific MRSA infection types, in particular as molecular-based means of classification become more available.
      The clinical implications of finding MRSA USA300 strains to have become an established cause of nosocomial pneumonia in our hospital intensive care units remain to be defined. In vitro susceptibilities to non-beta-lactam antibiotics for CA-MRSA strains may differ from HA-MRSA strains, providing clinicians with more available treatment options. USA300 strains have been reported in the literature to be associated with a variety of virulence factors (including the PVL gene), theoretically impacting the selection of anti-MRSA therapy.
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      Severe community-acquired pneumonia due to Staphylococcus aureus, 2003–04 influenza season.
      • Francis J.S.
      • Doherty M.C.
      • Lopatin U.
      • Johnston C.P.
      • Sinha G.
      • Ross T.
      • et al.
      Severe community-onset pneumonia in healthy adults caused by methicillin-resistant Staphylococcus aureus carrying the Panton–Valentine leukocidin genes.
      Infections caused by MRSA strains carrying the PVL gene have previously been reported to be associated with poor clinical outcomes,
      • Hageman J.C.
      • Uyeki T.M.
      • Francis J.S.
      • Jernigan D.B.
      • Wheeler J.G.
      • Bridges C.B.
      • et al.
      Severe community-acquired pneumonia due to Staphylococcus aureus, 2003–04 influenza season.
      • Francis J.S.
      • Doherty M.C.
      • Lopatin U.
      • Johnston C.P.
      • Sinha G.
      • Ross T.
      • et al.
      Severe community-onset pneumonia in healthy adults caused by methicillin-resistant Staphylococcus aureus carrying the Panton–Valentine leukocidin genes.
      • Vardakas K.Z.
      • Matthaiou D.K.
      • Falagas M.E.
      Incidence, characteristics and outcomes of patients with severe community acquired MRSA pneumonia.
      • Gillet Y.
      • Vanhems P.
      • Lina G.
      • Bes M.
      • Vandenesch F.
      • Floret D.
      • et al.
      Factors predicting mortality in necrotizing community-acquired pneumonia caused by Staphylococcus aureus containing Panton–Valentine leukocidin.
      • Labandeira-Rey M.
      • Couzon F.
      • Boisset S.
      • Brown E.L.
      • Bes M.
      • Benito Y.
      • et al.
      Staphylococcus aureus Panton–Valentine leukocidin causes necrotizing pneumonia.
      but the definitive role of PVL is unclear based on observations from more recent studies.
      • Bae I.G.
      • Tonthat G.T.
      • Stryjewski M.E.
      • Rude T.H.
      • Reilly L.F.
      • Barriere S.L.
      • et al.
      Presence of genes encoding the Panton–Valentine leukocidin exotoxin is not the primary determinant of outcome in patients with complicated skin and skin structure infectious due to methicillin-resistant Staphylococcus aureus: results of a multinational trial.
      • Hota B.
      • Lyles R.
      • Rim J.
      • Popovich K.J.
      • Rice T.
      • Aroutcheva A.
      • et al.
      CDC Prevention Epicenters
      Predictors of clinical virulence in community-onset methicillin-resistant Staphylococcus aureus infections: the importance of USA300 and pneumonia.
      • Peyrani P.
      • Allen M.
      • Wiemken T.L.
      • Haque N.Z.
      • Zervos M.J.
      • Ford K.D.
      • et al.
      IMPACT-HAP Study Group
      Severity of disease and clinical outcomes in patients with hospital-acquired pneumonia due to methicillin-resistant Staphylococcus aureus strains not influenced by the presence of the Panton–Valentine Leukocidin gene.
      • Bubeck Wardenburg J.
      • Palazzolo-Ballance A.M.
      • Otto M.
      • Schneewind O.
      • DeLeo F.R.
      Panton–Valentine leukocidin is not a virulence determinant in murine models of community-associated methicillin-resistant Staphylococcus aureus disease.
      Yet the finding of a mixture of MRSA strains, including HA-MRSA and USA300 isolates, as common pathogens in both EO and LO nosocomial pneumonia implies a need for clinicians to consider appropriate anti-MRSA therapy for all ICU pneumonia cases regardless of the time of onset in relation to hospital admission.
      Although we did not find a difference in the proportion of cases of MRSA nosocomial pneumonia caused by USA300 strains between EO and LO MRSA nosocomial pneumonia patients, we did observe a trend of LO group patients being infected with CA-MRSA with higher vancomycin MICs. The treatment implications of this finding are unclear at present. Several studies have demonstrated poor clinical outcomes associated with vancomycin use in bacteremia as well as pneumonia patients infected with CA-MRSA with high vancomycin MICs of 1.5 μg/ml or greater.
      • Choi E.Y.
      • Huh J.W.
      • Lim C.M.
      • Koh Y.
      • Kim S.H.
      • Choi S.H.
      • et al.
      Relationship between the MIC of vancomycin and clinical outcome in patients with MRSA nosocomial pneumonia.
      • Haque N.Z.
      • Zuniga L.C.
      • Peyrani P.
      • Reyes K.
      • Lamerato L.
      • Moore C.L.
      • et al.
      IMPACT-HAP Study Group
      Relationship of vancomycin minimum inhibitory concentration to mortality in patients with methicillin-resistant Staphylococcus aureus hospital-acquired, ventilator-associated, or health-care-associated pneumonia.
      This suggests alternative therapies may be a better choice.
      Our study has several limitations. First, the diagnosis of pneumonia was based on clinical criteria despite the fact that lower respiratory tract cultures were collected from all patients before antibiotic therapy; using clinical criteria for the diagnosis of HAP, VAP, and HCAP may lead to overdiagnosis. Second, the etiologic diagnosis of MRSA pneumonia was based on respiratory samples including sputum and tracheal aspirates, and the use of samples other than bronchoalveolar lavage to characterize the etiology of pneumonia may cause the inclusion of patients colonized but not infected with MRSA. Third, IMPACT-HAP was not a randomized controlled trial, and a major weakness of every observational study of nonconsecutive patients is sampling error. By including patients from different study sites, errors in the selection of patients and data collection might have had less of an influence on the results.
      • Wiemken T.
      • Peyrani P.
      • Arnold F.W.
      • Ramirez J.
      The use of large databases to study pneumonia: what is their value?.
      A strength of our data is the enrollment of patients representing different geographic areas in the USA, making our results more generalizable. The IMPACT-HAP database includes a well-characterized study population with detailed clinical and laboratory information available for analysis. In addition, we used a central reference laboratory for microbiology and molecular testing of the available MRSA isolates.
      In conclusion, the results of our study provide further evidence of the continued migration of CA-MRSA into healthcare settings in the USA, causing nosocomial infections and supplanting traditional HA-MRSA strains as a cause of nosocomial pneumonia in the ICU. The distinction of EO vs. LO ICU pneumonia is not as relevant as in the past, as more patients with complicated medical conditions cared for in outpatient settings are at risk of infection with MDR pathogens, irrespective of the timing of the pneumonia. Clinicians should consider starting anti-MRSA therapy empirically for all ICU patients with pneumonia, regardless of the time of onset in relation to hospital admission.
      Funding: This study was supported by Pfizer Inc., US Medical. The University of Louisville Foundation was responsible for project oversight and distribution of funds to participating institutions.
      Conflict of interest: TRP: research funding from Cubist, Forest; speaker honorarium from Cubist, Pfizer, Optimer, Merck. SJS: no conflict. BJ: no conflict. TW: no conflict. PP: research funding from Pfizer; travel support from Pfizer. NH: no conflict. MJZ: research funding from Astellas, Cubist, Pfizer; speaker honorarium from Astellas, Cubist, Pfizer; scientific advisory board or consultant for Cubist, Astellas. JAR: research funding from Astellas, Cubist, Pfizer; speaker honorarium from Astellas, Cubist, Forest, Pfizer; scientific advisory board or consultant for Cubist, Astellas. TMF: research funding from Cempra, Pfizer, The Medicine's Company; scientific advisory board or consultant for Astellas, Bayer, DaiichiSankyo, Forest, GSK, Merck, Nabriva, Pfizer, Tetraphase. EGS and KDF are Pfizer employees.

      4.1 IMPACT-HAP Study Group Investigators

      Julie E. Mangino, Carol Myers, David Taylor, Lindsay Pell, Kari Mount (Ohio State University, Columbus, OH, USA); Daniel Kett, Andrea S. Castelblanco, G. Fernando Cubillos (University of Miami Miller School of Medicine, Jackson Memorial Hospital, and Veterans Affairs Medical Center, Miami, FL, USA); Susan Donabedian, Carol Moore, Paola Osaki-Kiyan, Mary B. Perri, (Henry Ford Health System, Detroit, MI, USA); Forest Arnold, Raul Nakamatsu (University of Louisville, Louisville, KY, USA).

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