Nasal carriage of Staphylococcus aureus is a major risk factor for invasive S. aureus disease. The aim of this study was to define factors associated with carriage. We conducted a prospective, longitudinal communitybased study of infants and their mothers for a period of 6 months following delivery. The epidemiology of carriage was examined for 100 infant-mother pairs. Infant carriage varied significantly with age, falling from 40 to 50% during the first 8 weeks to 21% by 6 months. Determinants of infant S. aureus carriage included maternal carriage, breastfeeding, and number of siblings. Bacterial typing of S. aureus was performed by pulsed-field gel electrophoresis and multilocus sequence typing. The majority of individuals carried a single strain of S. aureus over time, and the mother was the usual source for colonizing isolates in infants. The effect of other components of the normal nasal flora on the development of S. aureus carriage was examined in 157 consecutive infants. Negative associations (putative bacterial interference) between S. aureus and other species occurred early in infancy but were not sustained. An increasing antistaphylococcal effect observed over time was not attributable to bacterial interference. S. aureus carriage in infants is likely to be determined by a combination of host, environmental, and bacterial factors, but bacterial interference does not appear to be an ultimate determinant of carrier status.Staphylococcus aureus is a major cause of community-acquired and nosocomial sepsis (7, 23). The rising prevalence of antibiotic-resistant strains, such as methicillin-resistant S. aureus and S. aureus with reduced susceptibility to glycopeptides (15,17,22), increases the need to prevent disease through the identification and modification of risk factors. Three sets of observations indicate that nasal carriage of S. aureus is an important risk factor for sepsis with this organism: carriers have higher rates of infection than noncarriers (16,29,30,31); the strain causing infection is usually the carriage strain in a given individual (16,18,28,31); and eradication of carriage reduces nosocomial infection (2,10,20,31). Temporary eradication of S. aureus in those at high risk of sepsis is desirable but relies on the unlikely premise of sustained susceptibility to antibiotics such as mupirocin. There is a need to develop alternative methods of S. aureus eradication, the success of which may depend on a detailed understanding of the determinants of carriage. The relative importance of host, bacterial, and environmental factors in determining S. aureus carriage is currently unknown. The aim of this study was to explore these factors during a prospective, longitudinal community-based study of carriage in mothers and their infants for a period of 6 months after delivery.
MATERIALS AND METHODSStudy design. The study was conducted in two parts. Epidemiological factors influencing infant carriage were examined for a group of 100 consecutively recruited infants and their mothers who were derived fr...