Epidemiological study of variations in β-lactam antibiotic susceptibility of Pseudomonas aeruginosa in two intensive care units

Abstract A six-month prospective study was carried out in the medical and surgical intensive care units (ICUs) at Besancon University Hospital to assess the frequency and risk factors for β-lactam-resistant isolates of Pseudomonas aeruginosa . Clinical samples were screened for P. aeruginosa , and four antibiograms were distinguished using imipenem and ceftazidime, namely: fully susceptible (SS), imipenem-resistant (RS), ceftazidime-resistant (SR), and resistant to both (RR). Dra I restriction fragment length polymorphism of isolates from different patients or with different resistance patterns but the same serotype was assessed by pulsed-field gel electro-phoresis. One hundred and twenty-one isolates were obtained from 50 of 281 patients, 60·3% were fully susceptible. 19·8% imipenem-resistant, 13·2% ceftazidime-resistant, and 6·6% resistant to both. Antibiotic-resistance was independent of serotype. Twenty-two of 32 imipenem-resistant isolates from six patients were of the same DNA type, and six other isolates from four patients were of a second DNA type. On only one occasion did a clonally defined strain develop imipenem resistance. By contrast ceftazidime-resistant strains had differing DNA types, but had been originally ceftazidime-susceptible in seven of 12 patients. Reversion of imipenem resistant strains to susceptibility occurred in one patient, and of ceftazidime-resistant strains in five patients. Case-control studies identified prior antibiotic therapy as a risk factor in colonization with resistant strains. Resistance to imipenem followed imipenem therapy, and resistance to ceftazidime followed use of weakly anti-pseudomonal β-lactam antibiotics. The major route of spread of imipenem-resistant strains was cross-colonization. Thus, assuming appropriate isolation, a carbapenem should be preferred to an extended-spectrum cephalosporin to treat pseudomonas infections in ICU patients.