Molecular epidemiology of antibiotic-resistant Gram-negative bacilli in a neonatal intensive care unit during a nonoutbreak period

Objective. Gram-negative organisms that are resistant to parenteral antibio tics are a growing threat to hospitalized patients. This study was conducte d to define the epidemiologic characteristics of these organisms during a n onoutbreak period in a neonatal intensive care unit (NICU). Methods. Nasopharyngeal and rectal swab specimens were obtained 3 times a w eek from every infant in a tertiary care NICU during a 12-month period. Spe cimens were processed to identify aerobic Gram-negative species resistant t o gentamicin, piperacillin-tazobactam, or ceftazidime. Selected clinical pa rameters were tested for their association with colonization with a resista nt organism. Restriction endonuclease digests of genomic DNA were derived f rom isolates of the most frequently occurring species. The fragments were a nalyzed by pulsed-field gel electrophoresis (PFGE) to determine the genetic relatedness of the various isolates and thereby determine the length of co lonization, the frequency of horizontal transmission, and the size and dura tion of clusters. Results. A total of 101 infants (8.6%) of 1180 admissions were colonized wi th 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, an d exposure to several classes of antibiotics. Colonization with resistant b acilli occurred as early as the first NICU day, but acquisition continued t hroughout the infants' stay. A total of 436 isolates were analyzed by PFGE. On the basis of this molecular analysis, it was determined that duration o f colonization was usually very short; the median for all species tested wa s <1 week. In addition, cross-colonization occurred in only 12% of all PFGE -analyzed isolates. Most clusters of cross-colonized infants were small, wi th the majority involving only 2 patients. Conclusions. During endemic periods, acquisition of antibiotic-resistant Gr am-negative bacilli in the NICU may occur very soon after admission, but co lonization continues over many weeks of NICU stay. The duration of coloniza tion with resistant bacilli is short, and horizontal transmission is unusua l. These characteristics suggest a gradual but temporary incorporation of t hese organisms from the NICU environment into the nascent newborn microflor a over time with little cross-colonization. These observations may aid the rational development of infection-control strategies to contain the reservo ir of resistant Gram-negative organisms in the NICU.