Selection of resistance during sequential use of preferential antibiotic classes

To determine the effect of antibiotic class pressure on the susceptibility of bacteria during sequential periods of antibiotic homogeneity.

Prospective study in a mixed ICU with three separated subunits of eight, eight, and ten beds.

The study examined the 1,721 patients with a length of stay longer than 2 days.

Three different antibiotic regimens were used sequentially over 2 years as first-choice empirical treatment: cephalosporins, fluoroquinolone, or a penicillin-beta-lactamase inhibitor combination. Each regimen was applied for 8 months in each subunits of the ICU, using "latin square" design.

We treated 731 infections in 546 patients (32% of patients staying more than 48 h). There were 25.5 ICU-acquired infections per 1,000 patient-days. Infecting pathogens and colonizing bacteria were found in 2,739 samples from 1,666 patients (96.8%). No significant change in global antibiotic susceptibility was observed over time. However, a decrease in the susceptibility of several species was observed for antibiotics used as the first-line therapy in the unit. Selection pressure of antibiotics and occurrence of resistance during treatment was documented within an 8-month rotation period.

Antibiotic use for periods of several months induces bacterial resistance in common pathogens.