Molecular Epidemiology of Antibiotic-Resistant Gram-Negative Bacilli in a Neonatal Intensive Care Unit During a Nonoutbreak Period

Abstract

Objective.: Gram-negative organisms that are resistant to parenteral antibiotics are a growing threat to hospitalized patients. This study was conducted to define the epidemiologic characteristics of these organisms during a nonoutbreak period in a neonatal intensive care unit (NICU). Methods.: Nasopharyngeal and rectal swab specimens were obtained 3 times a week from every infant in a tertiary care NICU during a 12-month period. Specimens were processed to identify aerobic Gram-negative species resistant to gentamicin, piperacillin-tazobactam, or ceftazidime. Selected clinical parameters were tested for their association with colonization with a resistant organism. Restriction endonuclease digests of genomic DNA were derived from isolates of the most frequently occurring species. The fragments were analyzed by pulsed-field gel electrophoresis (PFGE) to determine the genetic relatedness of the various isolates and thereby determine the length of colonization, the frequency of horizontal transmission, and the size and duration of clusters. Results.: A total of 101 infants (8.6%) of 1180 admissions were colonized with at least 1 antibiotic-resistant bacillus before NICU discharge. Multiple parameters indicating a prolonged, complicated NICU course were associated with resistant colonization, including gestational age, length of stay, and exposure to several classes of antibiotics. Colonization with resistant bacilli occurred as early as the first NICU day, but acquisition continued throughout the infants' stay. A total of 436 isolates were analyzed by PFGE. On the basis of this molecular analysis, it was determined that duration of colonization was usually very short; the median for all species tested was <1 week. In addition, cross-colonization occurred in only 12% of all PFGE-analyzed isolates. Most clusters of cross-colonized infants were small, with the majority involving only 2 patients. Conclusions.: During endemic periods, acquisition of antibiotic-resistant Gram-negative bacilli in the NICU may occur very soon after admission, but colonization continues over many weeks of NICU stay. The duration of colonization with resistant bacilli is short, and horizontal transmission is unusual. These characteristics suggest a gradual but temporary incorporation of these organisms from the NICU environment into the nascent newborn microflora over time with little cross-colonization. These observations may aid the rational development of infection-control strategies to contain the reservoir of resistant Gram-negative organisms in the NICU.antibiotic resistance, Gram-negative bacilli, neonatal intensive care, antibiotic utilization, colonization, pulsed-field gel electrophoresis.