Staphylococcus aureus is a bacterium that can cause severe sepsis.1 Therefore, knowing whether a patient is colonized by Staph is important. Colonization is having the bacteria in the body at a certain site. About 30% of Americans are colonized with Methicillin Sensitive Staphylococcus aureus (MSSA) and 1-4% of mothers and infants are colonized with Methicillin Resistant Staphylococcus Aureus (MRSA).2 Colonization with MSSA is usually seen with larger households, breastfeeding, and antibiotic exposure, but it is unknown if mothers can transfer the bacteria to their infants at the time of delivery (vertical transmission). 

A study tried to see if infants had a similar pattern for colonization as adults and studied the roles of different variables. This study found maternal colonization rates with MSSA to be about 30% and infant colonization rates to be 40 to 50% in the first 8 weeks of life. The researchers determined that 25% of infants were never colonized with the MSSA, and the remaining infants had a different pattern of colonization than adults. About 68% of the mother-infant pairs were colonized with the same strain of MSSA. The environmental factors that were found to be important were breastfeeding at any time and maternal colonization during 6 months. The authors suggested that MSSA colonization of the nipples may have played a role in transferring the bacteria from the mother to the infant.1 

Another study enrolled 629 pregnant women (at 34-36 weeks), and used questionnaires to determine risk factors for colonization2. Swabs of the mother (nasal and vaginal) and infant (nasal and umbilical) were tested for MSSA and MRSA during the first 4 months of life. African American infants (75% vs. 41%) and those born vaginally (86% vs. 69%) had the highest risk of colonization.2 This study had 20 maternal-infant pairs in which the mother was colonized during pregnancy and the infant was colonized within 2 hours of birth. Of these 20 pairs, only 2 of the infant isolates were colonized with the same bacterial strain as their mothers. Of the women colonized at delivery, 20 had infants that were colonized at birth and 14 at discharge; colonization peaked at 2 months of age and declined by 4 months of age. 

Although this study suggests that breastfeeding is a risk factor for colonization, it does not provide enough information to say for sure.2 This study did find a higher rate of colonization compared to other previously studied populations. Horizontal transmission (contact with mother and family members) after delivery was the most common mode of colonization.

In another study that looked at transmission around the time of delivery, the authors found that of 304 women, 43 were colonized with MSSA and 9 with MRSA.3 Of the 252 infants, 25 were colonized with MSSA at delivery or discharge from the hospital, 9 had MRSA. Only 5 of the infant-mother pairs had the same strain of MSSA. 5 infants from this study developed staphylococcal infections within the first month of life: 1 had been colonized in the perinatal period and 4 after 48 hours of life.

Another study reviewed breast milk and skin samples from 8 mothers without mastitis along with oral cavity and nasal cultures from their infants.4 The rate of Staphylococcus aureus transmission between these mother-infant pairs was 50%, 4 pairs and 1 infant were colonized.

A different study reported a case of MRSA colonization and infection in pre-term triplets. Surveillance cultures taken due to a recent outbreak of MRSA in the NICU revealed MRSA colonization starting on the 10th day of life for infant A and 18th day for infant B.5 Infant C who was not given breastmilk had negative routine cultures and passed away from premature lung disease. Infant A developed MRSA sepsis, a MRSA eye infection and pneumonia. Infant B developed a coagulase-negative staphylococcus bacteremia and a MRSA eye infection. Breastmilk stored before colonization of infants A and B was tested, and the cultures grew the same strain of MRSA. This mother had no signs or symptoms of mastitis or other infections.

These trials support that horizontal transmission is a greater risk factors than vertical transmission. However, the timing of colonization and infection varied. At this time it is not clear if mothers who are colonized with Staphylococcus aureus should stop breastfeeding their babies. Until we know more about the transmission of MSSA and MRSA, there is no reason to withhold breastmilk from mothers who are carriers. 

Adapted by Saneea Almas, MD from an original written by Hilary Rowe, PharmD


1. Peacock SJ, Justice A, Griffiths D, et al. Determinants of acquisition and carriage of Staphylococcus aureus in infancy. Journal of clinical microbiology. Dec 2003;41(12):5718-5725

2. Jimenez-Truque N, Tedeschi S, Saye EJ, et al. Relationship between maternal and neonatal Staphylococcus aureus colonization. Pediatrics. May 2012;129(5):e1252-1259

3. Pinter DM, Mandel J, Hulten KG, Minkoff H, Tosi MF. Maternal-infant perinatal transmission of methicillin-resistant and methicillin-sensitive Staphylococcus aureus. American journal of perinatology. Feb 2009;26(2):145-151

4. Kawada M, Okuzumi K, Hitomi S, Sugishita C. Transmission of Staphylococcus aureus between healthy, lactating mothers and their infants by breastfeeding. Journal of human lactation : official journal of International Lactation Consultant Association. Nov 2003;19(4):411-417

5. Behari P, Englund J, Alcasid G, Garcia-Houchins S, Weber SG. Transmission of methicillin-resistant Staphylococcus aureus to preterm infants through breastmilk. Infection control and hospital epidemiology. Sep 2004;25(9):778-780