Date of Award


Document Type

Open Access Thesis

Degree Name

Medical Doctor (MD)

First Advisor

Robert Baltimore, MD


Introduction: Nasal carriage of Staphylococcus aureus is a sensitive indicator of staphylococcal colonization and is considered a source of subsequent infection. When the incidence of Staphylococcus aureus colonization increased in our neonatal intensive care unit (NICU), resulting in 3 methicillin-resistant Staphylococcus aureus (MRSA) infections over a 3 month period, we sought to further our understanding of S. aureus epidemiology and response to infection control practice. The purpose of this investigation was to study the methicillin-susceptible Staphylococcus aureus (MSSA), and further determine the clonal spread of MSSA strain types. Since few studies have analyzed the clinical profile of MSSA in neonates, we hypothesized that the incidence of MSSA colonization would follow a mixed endemic and epidemic pattern over the period of the study. We further compared the MSSA colonization data to that of MRSA, in order to get a fuller picture of the circulating S. aureus strain pool in the NICU. Methods: This retrospective longitudinal study consisted of infants hospitalized in a Level III-IV NICU (approximately 45 beds) from April 2003 to December 2004. Nasal surveillance cultures of all infants were obtained on admission and weekly and pulsed-field gel electrophoresis (PFGE) was used to determine Staphylococcus aureus strain type. Transmission of identical strains among infants was noted. By testing for antibiotic susceptibilities, the prevalence and transmission data of MSSA was calculated. Thereafter, epidemiologic data such as birth weight, age, therapeutic modalities, length of admission, and antibiotic use was obtained from clinical summaries and used to characterize patients who had been colonized with MSSA. Results: During the 21 month study period, 1081 infants were screened for S. aureus. Of these, 877 (81.1%) tested negative, and 156 (14.4%) tested positive for MSSA. The prevalence of colonization with MSSA approached 45% by the 5th week of hospitalization for any given infant, and 70% in 9 weeks. Following the institution of routine nasal surveillance in April 2003, the incidence of MSSA cases fell from 6.5 to 1.5 per 1000 patient-days per month. Molecular typing using PFGE demonstrated three prevalent MSSA clones: clone 4 (7%), clone "15" (11%), and clone 23 (12%), corresponding to periods of increased incidence. The median length of stay was significantly longer in the intensive care infants compared to the continuing care i.e. "feed-and-grow" infants (median 77 versus 44 days, wilcoxon p=0.05). The mean length of stay was also longer in the intensive care infants, although this did not reach statistical significance (86 versus 66 days, student's t-test p=0.18). On chi square analysis infants in intensive care rooms were found to have a significantly higher prevalence of MSSA isolates than continuing care rooms in the nursery (54% vs. 35% of the total pool, P < 0.04). By comparison, 48 (4.4%) infants tested positive for MRSA, and the incidence of MRSA cases fell from 5.8 to 0.4 per 1000 patient-days per month during the study period. One predominant MRSA clone, clone 9, was identified and controlled. Conclusions: Control of MSSA is challenging because colonization is expected, endemic infections are tolerated and surveillance usually focuses on drug-resistant pathogens. During the period of study for this critically ill infant population, the incidence of both MSSA and MRSA colonization fell dramatically in response to the reinforcement of hand hygiene and contact precautions. We also identified an increased risk for MSSA colonization among intensive care infants as compared to continuing care infants, which is most likely connected to the significantly longer hospital stays among intensive care infants. The findings emphasize the need for cost-effective surveillance strategies for endemic infections in order to monitor progression from colonization. Finally, knowledge of the pattern of healthcare associated infection can contribute to the intensification of infection control measures and the updating of antibiotic usage guidelines.