Prevalence and Molecular Characterization of Extended-Spectrum-Beta-lactamase (ESBL) and AmpC beta-lactamase Genes among Klebsiella spp. and Escherichia coli Colonizing Isolates in Intensive Care Unit Patients

Background: Little is known about the prevalence of the different β-lactamase antibiotic-resistant genes in gram-negative organisms colonizing the gastrointestinal tract of intensive care unit (ICU) patients in the United States. Carriage of antibiotic-resistant genes in colonizing or infecting isolates has implications for empiric antibiotic therapy.

Objective: The objective of this study was to determine the prevalence of bla_ESBL and bla_AmpC genes among colonizing isolates in ICU patients over a 5-year period.

Methods: Peri-rectal surveillance cultures obtained from a cohort of patients admitted to the medical ICU (MICU) and surgical ICU (SICU) at the University of Maryland Medical Center (UMMC) between September 1, 2001 and June 30, 2005 were used for this study. Peri-rectal surveillance cultures were collected from patients on ICU admission, weekly, and upon discharge from the unit. Each peri-rectal culture was plated onto MacConkey agar with 1 μg/ml of ceftazidime. Plates were incubated at 37°C for 24 and 48 hours. Colonies were identified as Klebsiella spp. or E.coli using API 20E identification strips or the Vitek II (bioMerieux; Durham, NC). Only one isolate per species per patient was included. Susceptibility testing was performed by the disk diffusion method and all organisms underwent an ESBL screen according to the CLSI guidelines. All isolates that were positive for the ESBL screen underwent PCR for the detection of bla_TEM, bla_SHV, and bla_AmpC.

Results: There were 5,602 patients admitted to both ICUs during the study period. Of these, 193 patients (3.5%) were colonized with ESBL or AmpC-β-lactamase producing Klebsiella spp. or E.coli. The 193 patients were colonized with 202 different isolates. Of the 202 isolates, 163 (81%) were isolated from patients colonized on ICU admission. Among the 202 isolates, 97 (48%) had bla_TEM, 87 (43%) had bla_SHV and 87 (43%) had bla_AmpC . Seventy of 202 isolates (34%) had multiple bla genes; 27 isolates (13%) had both bla_TEM and bla_SHV, 23 isolates (11%) had both bla_TEM and bla_AmpC, 20 isolates (10%) had both bla_SHV and bla_AmpC. Bla_AmpC  was the predominant gene among patients colonized on ICU admission, while bla_SHV was the predominant gene acquired during ICU stay. Over the 5-year period, the bla_SHV  gene was predominant in the earlier years (2001-2002) with the bla_TEM   gene becoming more predominant in the later years (2003-2005).

Conclusions: In this study, a large percentage of colonized ICU patients were colonized with multiple bla genes that included both an ESBL and AmpC- β-lactamase gene. Understanding the prevalence and molecular characterization of bla_ESBL and bla_AmpC colonization within the ICU has implications when choosing empiric therapy strategies.