Risk factors for multidrug-resistant Pseudomonas aeruginosa acquisition. Impact of antibiotic use in a double case-control study

Multidrug-resistant strains of Pseudomonas aeruginosa(MDRPA) have been increasing in some hospitals [1] andmay become a public health problem [2].The emergence of MDRPA has been related to exposureto antibiotics against P. aeruginosa [3, 4]. Most of thesestudies have focussed on particular environments such asthe intensive care unit (ICU) [5] or particular antibioticresistances, mainly quinolone-resistant P. aeruginosa andcarbapenem-resistant P. aeruginosa or specific infectionsites such ventilator-associated pneumonia or bacteraemia[6, 7]. Most studies have used case–control methodology orhave investigated outbreaks, and the case–control studieshave usually compared susceptibility to resistant micro-organisms. This methodology may overestimate the associ-ation between the resistance-defining antibiotic or may befalsely implicated as a potential risk factor for theacquisition of this pattern of susceptibility [8, 9].The aim of this study was to assess the factors related toMDRPA acquisition, especially previous antibiotic expo-sure, using a double case–control methodology [10],analysing all types of infections and all hospital wardsduring a long period of follow-up.We conducted a double case–control epidemiologicalstudy, exploring the risk factors (host characteristics,invasive procedures and, especially, previous antibioticexposure) associated with the acquisition of MDRPA inhospitalised patients from 1 January 2001 to 31 December2006 in a University Hospital with 450 beds. P. aeruginosawas isolated and identified by the microbiology laboratoryby means of routine techniques. The susceptibility of