Bacteria in Human Milk: Helpful or Harmful?

Sandy Sundquist Beauman, MSN, RNC-NIC / October 2016


Bacteria exists in human milk.

In fact, it is very important, as it contributes to normal colonization of the intestines of the newborn. Even bacteria often thought of as harmful can be harmless or even beneficial to the infant.

Lin gives an eloquent summary of the benefits of these bacteria, and their role in developing brain neurotransmitters, competent gut maturity, and many more positive health impacts.1 These improve growth and development, and prevent auto-immune diseases in infants.

However, some bacteria that are protective in infants with normal immunity are not protective in extremely premature infants with compromised immunity. Reports exist of illness or death in the premature infant related to specific bacteria in the human milk.2,3 Large amounts of usually pathogenic bacteria or sometimes even what is considered normal flora can cause illness in these infants.

Neonates are often fed over a four-hour period, or fed small non-nutritive (trophic) feedings continuously. Currently, there are several recommendations concerning length of time a continuous feeding should be allowed to remain in place. The ADA recommends that feeds (formula or human milk) not be allowed to remain at room temperature longer than four hours.4

This is partially based on reported incidents of powdered formula contamination that occurred. These were reported between 1989 and as late as 2002 – specifically contamination with Cronobacter (formerly known as Enterobacter sakazakii).5-9 The FDA defines “unacceptable” levels of contamination as any aerobic agar plate growing >104 cfu/mL, three or more samples >103 cfu/mL, or any pure culture of Bacillus cereus, Listeria monocytogenes, Staphylococcus aureus or coliforms.10 This is related to formula and food feeding, not to human milk though.

What grows in human milk?

Telang et al studied the growth of bacteria in human milk, to which one sample had powdered fortifier added, as well as a sample of powdered infant formula.11 Some samples were inoculated with Enterobacter sakazakii. Bacterial growth was measured while milk was maintained at room temperature (22deg C) at 0, 2, 4 and 6 hour intervals. These researchers found that even after six hours, neither fresh human milk with fortifier nor powdered formula had a significant increase in bacterial growth.11

Jocson, Mason, Schanler evaluated bacterial growth in human milk to which a powdered fortifier had been added.12 They evaluated total bacterial growth over a four-hour simulated infusion. During the infusion period, the milk was maintained at room temperature. Bacterial count increased significantly in both the previously frozen and fortified milk, as well as fresh and fortified milk, although it was significantly greater in the fresh human milk.12

Lemons et al sampled fresh milk and frozen milk thawed under running tepid water without further warming.13 These samples were not warmed to room temperature, but samples were taken directly from refrigerated or thawed (but still cold) milk. Samples were taken at 0, 5, 6, 7, and 8 hour intervals. Frozen milk samples did not show a significant change in total colony count between zero and eight hours, but fresh milk samples revealed a significant increase in growth at the six-hour sample.13

The presence of bacteria in human milk is not in question. The rate of growth over time is the question. Careful handling is important to prevent the introduction of additional bacteria, but it is already present and may grow to unacceptable levels over time. Is this growth temperature-dependent? From the studies listed above, it appears that previously frozen milk has less bacterial growth but may also have compromised nutritional value.

Pilot Study: Warming Milk to Body Temperature

In order to attempt to answer questions about the growth of bacteria from human milk when warmed to body temperature, a pilot study (approved by the Investigational Review Board at University of New Mexico) was undertaken in which 15 milk samples were collected.

These samples had never been frozen and were refrigerated for no more than 24 hours. They were cultured at baseline, two hours and four hours after the milk was initially warmed to body temperature, with use of the Medela Waterless Milk Warmer. It was then allowed to stand at room temperature, as would occur in the NICU during a feeding. At this point, data is preliminary and has not yet been analyzed in detail.

However, at least some samples had a decrease in bacteria over time, demonstrating the power of maternal antibodies to control bacterial growth. Some samples had no growth and most had growth of normal skin flora, particularly staphylococcus aureus and staphylococcus epidermidis.

Various other factors that may affect the amount and type of bacteria present in the milk will also be analyzed, including mode of delivery and days postpartum. The science of breast milk is expanding exponentially, demonstrating many ways that human milk is the most appropriate nutrition for human babies. In spite of, or because of bacterial presence in the milk, not only nutrition but appropriate intestinal colonization is provided for the newborn. This colonization is even more important and altered in the premature or sick infant. Further analysis may aid in determining if increases in growth are clinically important.

Tell us what you think: Has this topic been a concern in your NICU, and how have you addressed it? Let us know in the comments section below.


  1. Lin, S. (2016). Why Bacteria Are the Basis of Breastfeeding. Breastfeeding Review. 24(1): 7 -9.
  2. Gastelum, D. T., Dassey, D., Mascola, L., & Yasuda, L. M. (2005). Transmission of community-associated methicillin-resistant Staphylococcus aureus from breast milk in the neonatal intensive care unit. The Pediatric infectious disease journal, 24(12), 1122-1124.
  3. Chen Z, Pan WG, Xian WY, Cheng H, Zheng JX, Hu QH, Yu ZJ, Deng QW. (2016). Identification of Infantile Diarrhea Caused by Breast Milk-Transmitted Staphylococcus aureus Infection. Curr Microbiol. 2016 Oct;73(4):498-502. doi: 10.1007/s00284-016-1088-7. Epub 2016 Jun 25.
  4. Robbins, S., Meyers, R. Infant Feedings: Guidelines for Preparation of Human Milk in Health Care Facilities, 2nd. Ed. Academy of Nutrition and Dietetics. 2011
  5. Lai KK. Enterobacter sakazakii infection among neonates, infants, children, and adults: case reports and a review of the literature. Medicine 2001;80:113-120.
  6. Biering G, Karlsson S, Clark NC, et al. Three cases of neonatal meningitis caused by Enterobacter sakazakii in powdered milk. Journal Clinical Microbiology. 1989;27:2054-2056.
  7. Simmons BP, Gelfand MS, Haas M, et al. Enterobacter sakazakii infections in neonates associated with intrinsic contamination of a powdered infant formula. Infection Control Hospital Epidemiology. 1989; 10:398-401.
  8. Van Acker J, DeSmet F, Muyldermans G, et al. Outbreak of necrotizing enterocolitis associated with Enterobacter sakazakii in powdered milk formula. Journal of Clinical Microbiology. 2001;39:293-297.
  9. Himelright K, Harris E, Lorch V, Anderson M. Enterobacter sakazakii infections associated with the use of powdered infant formula – Tennessee, 2001. MMWR. 2002;51(14):298-300.
  10. Food and Drug Administration. Compliance Program Guidance Manual. Food Composition, Standards, Labeling and Economics,Chapter 21, Compliance Program 7321.002, May 3, 1995.
  11. Telang S, Berseth CL, Ferguson PW, Kinder JM, DeRoin M, Petschow BW. Fortifying fresh human milk with commercial powdered human milk fortifiers does not affect bacterial growth during 6 hours at room temperature. Journal of the American Dietetic Association. 2005;105(10):1567-1572.
  12. Jocson MAL, Mason EO, Schanler RJ. The effects of nutrient fortification and varying storage conditions on host defense properties of human milk. Pediatrics. 1997;100:240-243.
  13. Lemons PM, Miller K, Eitzen H, Strodtbeck F, Lemons JA. Bacterial growth in human milk during continuous feeding. The American Journal of Perinatology. 1983;1(1):76-80.


About the Author

Sandy Sundquist Beauman has over 30 years of experience in neonatal nursing. In addition to her clinical work, she is very active in the National Association of Neonatal Nurses, has authored or edited several journal articles and book chapters, and speaks nationally on a variety of neonatal topics. She currently works in a research capacity to improve healthcare for neonates. Sandy is also a clinical consultant with Medela. You can find more information about Sandy and her work and interests at