Performance of surveillance cultures at different body sites for Staphylococcus aureus in pregnant women and neonates

Background: Dissemination of S. aureus, whether methicillin-susceptible (MSSA) or resistant (MRSA), occurs from person to person. In the hospital setting this may occur from patient to patient during patient care or through direct contact such as mother and child contact. Detection of asymptomatic carriers, especially of MRSA, has been suggested by some authors as a strategy to control nosocomial transmission. Thus asymptomatic carriers can be identified and put under contact precautions.

Objective: To evaluate the performance of surveillance cultures at different body sites in the detection of colonization by S. aureus.

Methods: From February 2009 through January 2010, pregnant women (PW) were submitted to surveillance cultures during labor (anterior nares; perineum; anus and oropharynx). Their neonates (NN) were cultured at birth, 3^rd day, one week and weekly until discharge or the 4^th week (whichever occurred first) at the sites: anterior nares; oropharynx; perineum and umbilicus. The swabs in Stuart’s medium were taken to the laboratory where they were cultured in BHI, incubated at 35^o C and after 24h plated on mannitol salt agar then incubated for 48 h. Suspected S. aureus colonies were submitted to oxacillin resistance screening test with 6 µg/ml of oxacillin. Broth microdilution was used to determine the MIC for oxacillin.PCR was used to determine mecA and coa genes. Only the first occasion on which S. aureus isolation occurred was considered for each patient.

Results: 384 pregnant women were cultured (184 were positive for S. aureus-49%) and 378 neonates (207 were positive -54%). Thirty-two NN and 15 PW carried MRSA (8% of NN and 4% of PW). The positivity of each body site can be seen in the Table. Combining cultures sites such as nares and umbilical cultures in neonates led to 85% positivity for MSSA and 91% for MRSA. For PW, combining nares with the oropharynx led to 81% positivity for MSSA. For MRSA in PW, positivity of 80 % occurred by combining cultures of the nares with any other site.

Conclusions: Surveillance cultures of one body site are insufficient to detect a large proportion of colonized neonates and pregnant women. Combining cultures of 2 body sites will improve positivity but there will be 9-20% of carriers who will remain undetected. Thus, control of S.aureus relying mainly on surveillance cultures may be jeopardized.

Table: Positivity of surveillance cultures for Staphylococcus aureus at different body sites of pregnant women and neonates.

Neonates
	Nares	Oropharynx	Perineum	Umbilicus
Methicillin-susceptible S. aureus (n: 152)	91 (60%)	65 (43%)	82 (54%)	92 (61%)
Methicillin-resistant S. aureus (n: 32)	15 (47%)	13 (41%)	23 (72%)	23 (72%)
Pregnant women
	Nares	Oropharynx	Perineum	Anus
Methicillin-susceptible S. aureus (n: 192)	110 (57%)	97 (51%)	47 (25%)	46 (25%)
Methicillin-resistant S. aureus (n: 15)	10 (67%)	7 (47%)	3 (20%)	3 (20%)

 Funding FAPESP 2009/08206-1.