Clinical importance of gram-negative bacteria with inducible chromosomal beta-lactamase in the medical-surgical intensive care unit

Background: The aim of the study was to determine the frequency of third-generation cephalosporins and aztreonam resistance in gram-negative bacteria with inducible chromosomal beta-lactamase (βLac-ind) after beta-lactam therapy in the medical-surgical intensive care unit (ICU) at a university-affiliated hospital. Patients and methods: We studied 34 infections in 29 patients admitted to the ICU. All were infected by strains with βLac-ind and all were treated with beta-lactam antibiotics. Susceptibility was determined by disc-diffusion. The beta-lactamase activity of those strains showing constitutive beta-lactamase overproduction were characterized by isoelectrofocusing. When this derepression occurred during the therapy, the strains were compared by genomic macrorestriction (PFGE). Results: In 29 out of 34 infections the initial strains was susceptible. In 11 cases, the culture were not negativized in spite of their susceptible pattern. In 4 cases there was derepression during therapy. In 5 cases the initial strains were derepressed. The microorganisms isolated more frequently were Pseudomonas aeruginosa (22 cases) and Enterobacter cloacae (5 cases). The beta-lactamase activity detected correspond well with a βLac-ind. In those cases with derepression during therapy, the initial susceptible strain and the resistant strain were identical by PFGE. Conclusions: The percentage of bacteria that were derepressed either initially or following therapy was 26%. Derepression was detected more frequently in patients treated with cephalosporins. These results reflect the importance of this resistance mechanism in those strains subject to strong selective pressure.