title: Whence Resistance?
abstract: BACKGROUND
  Antimicrobial resistance results from a complex interaction between pathogenic and non-pathogenic bacteria, antimicrobial pressure, and genes, which together comprise the total body of potential resistance elements. The purpose of this study is to review and evaluate the importance of antimicrobial pressure on the development of resistance in a single surgical intensive care unit.
  
  
  METHODS
  We reviewed a prospectively collected dataset of all intensive care unit (ICU)-acquired infections in surgical and trauma patients over a 6-y period at a single hospital. Resistant gram-negative pathogens (rGNR) included those resistant to all aminoglycosides, quinolones, penicillins, cephalosporins, or carbapenems; resistant gram-positive infections (rGPC) included methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant enterococci (VRE). Each resistant infection was evaluated for prior or concomitant antibiotic use, previous treatment for the same (non-resistant) organism, and concurrent infection with the same organism (genus and species, although not necessarily resistant) in another ICU patient.
  
  
  RESULTS
  Three hundred and thirty resistant infections were identified: 237 rGNR and 93 rGPC. Infections with rGNR occurred frequently while receiving antibiotic therapy (65%), including the sensitive form of the subsequent resistant pathogen (42.2%). Infections with rGPC were also likely to occur on antimicrobial therapy (50.6%). Treatment of a different patient for an infection with the same resistant pathogen in the ICU at the time of diagnosis, implying potential patient-to-patient transmission occurred more frequently with rGNR infections (38.8%).
  
  
  CONCLUSIONS
  Antimicrobial pressure exerts a substantial effect on the development of subsequent infection. Our data demonstrate a high estimated rate of de novo emergence of resistance after treatment, which appears to be more common than patient-to-patient transmission. These data support the concept that efforts to limit antimicrobial usage will be more efficacious than enhanced isolation procedures when trying to reduce antimicrobial resistance.
authors.affiliation: 1 Department of Surgery, The University of Virginia Health System , Charlottesville, Virginia.
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Guidry
authors.affiliation: 1 Department of Surgery, The University of Virginia Health System , Charlottesville, Virginia.
authors.forename: Stephen W
authors.initials: SW
authors.lastname: Davies
authors.affiliation: 1 Department of Surgery, The University of Virginia Health System , Charlottesville, Virginia.
authors.forename: Rosemarie
authors.initials: R
authors.lastname: Metzger
authors.affiliation: 1 Department of Surgery, The University of Virginia Health System , Charlottesville, Virginia.
authors.forename: Brian R
authors.initials: BR
authors.lastname: Swenson
authors.affiliation: 1 Department of Surgery, The University of Virginia Health System , Charlottesville, Virginia.|2 Division of Acute Care and Trauma Surgery, The University of Virginia Health System , Charlottesville, Virginia.
authors.forename: Robert G
authors.initials: RG
authors.lastname: Sawyer
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000900:Anti-Bacterial Agents / Q000627:therapeutic use*; D001419:Bacteria / Q000187:drug effects*; D001424:Bacterial Infections / Q000188:drug therapy* / Q000382:microbiology; D024881:Drug Resistance, Bacterial; D005260:Female; D006801:Humans; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012641:Selection, Genetic; D013530:Surgical Wound Infection / Q000188:drug therapy; D014947:Wounds and Injuries / Q000150:complications
chemical_list: D000900:Anti-Bacterial Agents