Dynamics of colistin and tobramycin resistance among during prolonged use of selective decontamination of the digestive tract.

Background: A high prevalence of colistin resistance among isolates in two intensive care units (ICU) (of 16 and 6 beds) using selective digestive decontamination (SDD) since 1990 instigated a retrospective and prospective investigation to quantify the role of clonal transmission. SDD is topical application of colistin and tobramycin and systemic use of cefotaxime during the first days of ICU-admission.

Methods: Multi-resistant (MREb) was defined as ESBL production and/or tobramycin non-susceptibility and/or colistin non-susceptibility. Incidence of acquisition and prevalence of carriage with MREb was determined from microbiological culture results.

Results: Colistin-resistant was first detected in November 2009 and carriage was demonstrated in 141 patients until October 2014. Mean incidence of MREb acquisition was 4.61 and 1.86 per 1000 days at risk in ICUs 1 and 2, respectively, and the mean monthly prevalence of MREb in both ICUs was 7.0 and 3.1%, respectively, without a discernible trend in time. Conversion rates from carriage of colistin-susceptible to resistant were 0.20 and 0.13 per 1000 patient days, respectively. Whole genome sequencing of 149 isolates revealed eight clusters, with the number of SNPs of the largest two clusters ranging between 0 and 116 for cluster 1 ( = 49 isolates), and 0 and 27 for cluster 2 ( = 36 isolates), among isolates derived between 2009 and 2014.

Conclusions: This study demonstrates a stable low-level endemicity of MREb in two Dutch ICUs with prolonged use of SDD, which was characterized by the persistent presence of two clusters, suggesting incidental clonal transmission.