Highly Resistant Microorganisms in a Teaching Hospital: The Role of Horizontal Spread in a Setting of Endemicity

OBJECTIVE. To determine the incidence density of highly resistant organisms (HROs) and the relative contribution of horizontal spread in a setting of endemicity. METHODS. Prospective surveillance was performed among hospitalized patients during an 18-month period. Enterobacteriaceae, non-fermentative gram-negative bacilli, Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecium-all considered highly resistant, according to Dutch guidelines-were included. Epidemiological linkage and nosocomial transmission were determined on the basis of molecular typing and hospital admission data. RESULTS. From 119 patients, we recovered a total of 170 unique HRO isolates, as follows: Escherichia coli, 96 isolates; Klebsiella species, 11 isolates; Enterobacter species, 8 isolates; Proteus species, 9 isolates; Citrobacter species, 5 isolates; Pseudomonas species, 5 isolates; Acinetobacter species, 3 isolates; Morganella species, 2 isolates; Salmonella species, 1 isolate; Serratia species, 1 isolate; S. pneumoniae, 20 isolates; and S. aureus, 9 isolates. No vancomycin-resistant E. faecium was found. The incidence density was 4.3 HRO isolates per 10,000 patient-days. The majority of HRO isolates were unique, and nosocomial transmission was observed 4 times for highly resistant gram-negative bacilli (case reproduction rate, 0.05) and 4 times for penicillin-nonsusceptible S. pneumoniae (case reproduction rate, 0.29). A stay on the intensive care unit was the main determinant for the recovery of an HRO. CONCLUSION. Nosocomial transmission of HROs was observed 8 times during the 18-month period. The intensive care unit was identified as the main reservoir of horizontal spread of HROs. This study shows that nosocomial transmission of HROs is largely preventable using transmission precautions.