| Abstract |
Staphylococcus aureus is a major pathogen in both community-acquired and nosocomial infections with alarming rates of resistance to antibiotics in recent years. S. aureus can colonize many sites of the human body, among which the anterior nares represent the dominant ecological niche. There seems to exist a link between S. aureus nasal carriage and infection, since nasal and infection strains commonly share the same genotype, rates of infection are higher in carriers that in non-carriers, and eradication of carriage has been shown to reduce nosocomial infection. About 30% of the population carries S. aureus at any time; this includes 20% who are persistent and 10% who are transient carriers. Infant colonization by S. aureus has not been adequately investigated, while there are very few studies worldwide for the timing and the source of colonization in infancy.
Methicillin resistance first appeared among nosocomial isolates of S aureus in 1961. Since that time, methicillin-resistant S. aureus (MRSA) has become widespread in hospitals and intensive care units around the world. This high level of resistance results from the presence of the mecA gene that encodes the production of a penicillin binding protein known as PBP2a, which has decreased affinity for most beta-lactam antibiotics. MRSA is now one of the most common causes of bacterial nosocomial infections, accounting for 40–70% of the S. aureus infections in intensive care units. In the past decade new strains of MRSA (CA-MRSA) have emerged in the community, causing aggressive infections in young, otherwise healthy people. Many strains produce the Panton-Valentine Leukocidin, (PVL) which is associated with abscesses, invasiveness, and increased morbidity and mortality.
15
S. aureus can colonize many parts of the human body, of which the anterior nares represent the dominant ecological niche. There seems to exist a link between S. aureus nasal carriage and infection, since nasal and infection strains commonly share the same genotype, rates of infection are higher in carriers that in non-carriers, and eradication of carriage has been shown to reduce nosocomial infection. About 30% of the population carries S. aureus at any one time; this includes 20% who are persistent and 10% who are transient carriers. Infant colonization by S. aureus has not been adequately investigated, while there are few studies worldwide for the timing and the source of colonization in infancy.
The objective of the present study was to explore determinants associated with the infant colonization in the first year of life, the molecular epidemiology and the resistance to antibiotics of S. aureus strains isolated from infants and their mothers in a well-defined area, i.e., the island of Crete.
The study group consisted of a cohort of 128 recruited newborn infants and their mothers who were born at Venizeleion Pananio General Hospital in Heraklion, Crete, between November 2005 and May 2006. 98 infants were from Maternity Ward and were healthy while 30 were admitted to the Neonatal Ward. Nasal swabs were obtained from mothers and infants before discharge and after 6 and 12 months, and a series of determinants related to infant, mother, father and family were investigated. The cultured S. aureus was identified on the basis of colony morphology, Gram stain, catalase test, coagulase test and API 20 Staph system, while antimicrobial susceptibility tests against oxacillin, erythromycin and clindamycin were performed. All mother-infant pairs with simultaneous isolation S. aureus, and all strains isolated in
16
the mother-infant pair at a different sampling were characterized by pulsed-field gel electrophoresis, multilocus sequence typing, spa typing, and presence of chromosomal mecA and Panton-Valentine Leukocidin (PVL) genes.
A total of 615 nasal swabs were obtained from 310 infant-mother pairs, including 10 cases of sampling in twins. Positive for S. aureus were 55 (17.7%) samples from infants and 47 (15.4%) samples from mothers. Of the 82 infants who had all 3 swabs, 47 (57.3%) were persistent S. aureus non-carriers, 23 (28%) infants were positive once (transient carriers), and 12 (14.6%) were positive at least twice (persistent carriers). Of the 79 mothers who had all 3 swabs, 53 (67.1%) were S. aureus non-carriers, 15 (19%) were transient carriers and 11 (13.9%) were persistent carriers. Carriage rates in infants at any sampling were positively related to maternal carriage rates and to non-smoking mothers. Also persistent carriage rates were further higher in infants born to non-smoking mothers and having more siblings. Pairs of infant-mother carriers were found in 15 occasions (4.8%), almost twice the rate expected by chance (8.3 pairs, 2.7%). This 15 pairs were further typed and were found to be identical at the same time point in 8 pairs (53.3%). All identical S. aureus strains were found in infant-mother pairs without history of admission to Neonatal Ward, and almost all identical pairs (7/8) were observed in the 6-month sampling. Isolates from infants principally belonged to ST types 30 (8/19) and to PFGE type 2 (6/19). Furthermore, 10/19 isolates harbored mecA gene and 2/19 isolates PVL gene. Both PVL positive and 8/10 mecA isolates from infants were paired with isolates with the same determinants in mothers. All four PVL positive isolates were mecA positive and belonged to ST type 80. Resistance rates for methicillin, erythromycin and clindamycin were 43.6%, 5.4%, and 3.6% respectively in isolates from infants and
17
42.5%, 8.5%, and 0% in isolates from mothers. Resistance to methicillin was more frequent in isolates from Neonatology Ward than Maternity Ward infants.
Our findings suggest that strains carried by young infants are commonly identical to their mothers’ strains. The transmission in infancy is from parent to child, in contrast to other pathogens where transmission is rather from child to parents. Regarding the issue of whether infants are colonized through vertical transmission or close contact, our findings favor the close contact scenario, given that very few infant-mother pairs were colonized in the first days of life, the vast majority of identical infant-mother isolates were observed in month 6 of life, infants admitted to Neonatology Ward developed a different carriage profile, and as similar carriage rates were found in infants after vaginal delivery or caesarean section. Colonization rates at months 6 and 12 were close to those already reported. In our study infants were less colonized at month 0 than elsewhere, possibly because they were swabbed earlier in their life. Persistent carriage rates were similar to those reported. Interestingly, both infants with persistent carriage of the same strain had mothers carrying identical strains. This finding has not been reported previously and points to the role of maternal colonization in the persistent carriage in the infant. We found that maternal non-smoking, maternal colonization, and many siblings were related to higher rates of S. aureus colonization in the infant, though at different time points in infancy. In our study S. aureus carriage, both intermittent and persistent, was less common in infants of smoking mothers but this finding needs careful explanation and further investigation. Presence of siblings has been related to S. aureus carriage, but in our study siblings were found to predispose infants for persistent carriage in late rather than in early infancy. Finally, concordance in carriage in mother-infant pairs in this
18
study points to both common environment and host genetics. PVL and mecA positive isolates from infants were simultaneously paired with isolates with the same genetic determinants in mothers.
Limitations of this study included the absence of exploring other potential colonization sites but the nares and of exploring the impact of colonization of other family members. Furthermore we did not investigate potential sources of hospital transmission in the group of infants admitted to the Neonatology Ward. Despite all these limitations our findings clearly point to the impact of maternal colonization in the very first months of life and contribute to the understanding the human colonization by one of the most devastating pathogens.
|
Search
