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Objective: To study the effect of selective digestive tract decontamination by erythromycin-base on the incidence of carriage and infection with MR Enterobacteriaceae producing an extended spectrum beta-lactamase (ESB)., Design: After a 10-week prospective survey to ascertain the baseline incidence in two bays (1 and 3) of the same ICU, bay 1 was compared with bay 3 during a further survey of 6 months. The patients in bay 1 received erythromycin-base., Setting: Two non-contiguous bays, 1 and 3, of 4 beds, in the same polyvalent ICU of a university hospital., Patient: Consecutive patients with unit stay longer than 2 days; 34 patients were included during the control period, 43 in bay 1 (decontamination) and 46 in bay 3 (control) during the trial period., Intervention: Erythromycin-base, 1 g t.i.d. in powder form administered by gastric tube to patients in bay 1 from admission to discharge., Measurements and Results: Digestive tract carriage was monitored by cultures of gastric and rectal swab specimens, sampled twice a week. Enterobacteriaceae were isolated on Drigalski agar with incorporated ceftazidime (4 mg/l). In bay 1 there was a decrease in ESB producing Enterobacteriaceae (23% vs 10%, p = 0.0004) from rectal swab, especially in K. pneumoniae (15% vs 2%, p = 10(-5)), during the decontamination period in comparison to the control period. During the trial period the only differences observed between bays 1 and 3 were in the gastric samples: K. pneumoniae were less often isolated in bay 1 than in bay 3 (0% vs 3%, p = 0.03). Intestinal carriage with multiresistant Enterobacteriaceae occurred in 28% patients in bay 1 and 30% patients in bay 3 during the trial period (p = 0.79). Erythromycin-base did not delay the carriage by patients in bay 1 (log rank test p = 0.42)., Conclusion: Erythromycin-base was not effective in preventing digestive tract carriage due to Enterobacteriaceae resistant to third generation cephalosporin by production of chromosomal cephalosporinase. The decrease in isolates containing K. pneumoniae in bay 1 cannot be definitively attributed to erythromycin-base, since the number of this species in bay 3 was low.