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The bacteria in breast milk

The views expressed are those of the author and are not necessarily those of Scientific American.

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Bacteria are found in large numbers all over the human body where there is a channel to the outside world, for example in the gut, lungs, and surface of the skin. I’ve always thought that actually inside the human body was a bacteria-free environment unless an infection was raging so I was very excited to find that bacteria are present in breast milk. As a product which is made within the human body, breast milk was traditionally thought to be sterile.

Several recent studies have found that breast milk contains a healthy dose of commensal bacteria; all the staphylococci, streptococci, and lactic acid bacteria that are found in the infant gut. This isn’t just bacteria from the skin which have contaminated the samples, but bacteria that have come from inside the breast as an integral component of the milk.

Norse mother, by Albert Neuhuys (1844 - 1914) Image credit:

In a study of 16 women it was found that while each milk sample contained hundreds of different bacterial species around half of the microbiotic community was made up of nine species present in all samples. The other half varied from person to person. This pattern is also found in human gut microbes; a core set present in all individuals along with a large diversity of separate species to make up a unique individual microbiome.

So how do bacteria get into breast milk? Some of them may come from the mouth of the baby. During feeding the skin of both the mother and baby will be in contact with the baby’s open mouth and a certain amount of flow-back can occur between the mouth and nipple. More excitingly it’s been suggested that immune cells in the mothers gut may be able to pick up bacteria and carry them around the body using the lymphatic system. The lymphatic system is a network of vessels used to transport blood plasma. It’s a main highway for immune cells inside the body and is also involved in the absorption and transports of fats.

The cells thought to be responsible for picking up the bacteria and transporting them to the breast are called dendritic cells.  In non-breastfeeding people they are used to transport bacteria and bits of bacteria to the rest of the immune system to prepare a defence, and can sometimes carry entire live bacteria. Recent studies have shown that in mice bacteria from the gut translocate into lymph nodes and mammary glands during the late stages of pregnancy and the beginning of lactation. It’s also been shown that if breastfeeding women are given a capsule of live (and safe!) bacteria to swallow, these bacteria can be isolated later from the breast milk.

Potential sources of the bacteria present in human colostrum and milk. DC: dendritic cell. Image from reference 1, reproduced with permission from the authors

This particular model, bacteria picked up by immune cells and ferried to the breast to be added to breast milk, is looking promising although there is still plenty of research to be done. It’s not yet understood how the bacteria travel with the dendritic cells, either attached to the surface or completely engulfed by the human cell. It might even be other immune cells, such as macrophages, that are responsible for transport. It’s also not certain why the dendritic cells don’t just kill the bacteria once they’ve got hold of them (as is normal with pathogenic bacteria). A further question is exactly when the bacteria colonise the breast – are they continually ferried up from the gut or is there a specific window of opportunity for them to be introduced?

Like all humans, infants have a range of bacteria within their gut. It looks like these bacteria are initially supplied from the mother’s vaginal and skin bacteria, before being replaced by bacteria from the breast milk. Researchers also found that when babies started eating solid food a whole new range of bacteria was introduced, forming the gut microbiome that persisted into adulthood.


1: Fernández L, et al. The human milk microbiota: Origin and potential roles in health and disease. PharmacolRes (2012),

2: P.V. Jeurink, J. van Bergenhenegouwen, E. Jiménez, L.M.J. Knippels, L. Fernández, J. Garssen, J. Knol, J.M. Rodríguez and R. Martín. Beneficial Microbes, March 2013; 4(1): 17-30

Word of warning: Dealing with an infant is no easy task, and breastfeeding for many women can be both painful and difficult. While breast-milk is fascinating and marvellous stuff, formula milk can be a literal lifesaver in many circumstances, and has no major adverse effects on the child’s growth and development. Comments discussing the properties and/or benefits of breast milk are fine but any that shame, ridicule or mock women for not breastfeeding will be deleted. As will any that shame, ridicule or mock women for breastfeeding.

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Chryses 10:27 pm 12/8/2013


    “Comments discussing the properties and/or benefits of breast milk are fine but any that shame, ridicule or mock women for not breastfeeding will be deleted. As will any that shame, ridicule or mock women for breastfeeding.”

    That POV has been a long time coming. Thank you!

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  2. 2. llirbo 11:38 pm 12/10/2013

    One should not ridicule anyone for any reason.

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  3. 3. ecoligist 2:26 pm 12/11/2013

    It has been known for about 70 years that cows rapidly transport huge numbers of ingested “dairy bacteria” from grass to their milk, and for about 20 years that this was true with Swiss cheese’s propionibacteria in lactating humans, and much more recently into human female sweat during 2/3 of their menstual cycles, it is good that finally some major researchers are looking at the mechanism of transport. My undergraduate group has long suspected some sort of semi-competent phagocyte that could ingest but not digest bacteria and that may be under some sort of control by progesterone. Remembering that mammary glands are derived from sweat glands, my students have also shown similar transport of ingested bacteria into premenopausal female sweat in concert with their progesterone levels. (It must also be pointed out that pregnant mammals transport a stolen rare E. coli cell from their intestines to inoculate and colonize fetal colons starting about 3 weeks prior to normal term. But this might be a different story as only 1 or 2 bacterial cells are transported on average every three days, while in the milk case, millions must be transported within just 2 or 3 hours.) Both of these transports are highly important for the newborn’s survival as the baby is thrust out into the bacteria-laden world. What is more, it is known that the dairy bacteria (the “good” lactic acid bacteria) kill other types of bacteria and thus help minimize mastitis and infant diarrhea. It would really be nice if (a) these dairy bacteria could be found in large numbers in the circulatory system in a way that protects against the symptoms of septicemia, and (b) positively identify the transporter cells. By revealing the normal, we should gain insight into the mechanisms for such maladies as prenatal infections, and perhaps even mastitis – of benefit for both humans and their dairy livestock. As we guys know, women are mysterious. Let’s hear more about dendritic cells!

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  4. 4. S.E. Gould in reply to S.E. Gould 12:43 pm 12/13/2013

    Thanks for the comments! I’ve never come across any problems with either breast or bottle feeding in real life (most people are really supportive) but the internet can be a nasty place sometimes.

    @ecoligist: That is fascinating to hear, I suppose it’s another reason why milk should be pasteurised, if cattle-milk bacteria are present in it! I’d never suspected that live bacteria could be transported around the body so this research opened up a whole new area for me. I don’t know how many bacteria are transported versus grown in the breast, it might be that it could work in a similar way to the colon colonisation.

    And it seems churlish to point out after your great comment but I am not one of ‘we guys’ (clue in the fact that I’ve just given birth! :p) However people are indeed mysterious creatures and bacteria even more so. :)

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  5. 5. ecoligist 8:14 pm 12/18/2013

    @S.E.Gould: There seems be a difference in the transport to milk versus fetal intestines because in both the cow and human subjects, after only about 90 minutes of ingesting the bacteria, the bacteria begin coming out in the milk in large numbers. The lactic acid bacteria are not noted for rapid multiplication, and surely were only a few transported to the milk ducts, billions could not be produced in less than 2 hours.

    To be sure, there is much more to be done. For example, what about comparing the relative rates of transport of different species of dairy bacteria. Transport could be rather easily followed were the ingested strains genetically tagged in various ways that are readily detected on agar plates. E.g.: one strain might be made resistant to penicillin, another to streptomycin… (As E. coli is not normally found in milk/sweat, it obviously is not transported internally to either the sweat or milk glands. What is the means by which the body selects what it transports? Different transport cells? Different hormones?

    Lots to be learned about the comings and goings of the microbiome!

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  6. 6. rshoff2 12:17 pm 02/10/2014

    I think breastfeeding is the extremely important and support it absolutely. Apparently it not only provides superior nutrition, but helps assure a good and appropriate bacterial balance for the infant.

    Having said that, every breast feeding woman can help make this practice more accepted by remembering that her breast is also a body part that has been, and continues to be used, as a sexual stimulant for adults. Sometimes personally during sex and often times via in the entertainment media. Either way, it’s out there and used as a sex enhancer, like it or not.

    If nursing women could carry that sensitivity during breastfeeding, I’m sure all rational people would support breast feeding almost anywhere. I surely do.

    As far as employers making it possible to nurse an infant, that should be mandated by law.

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  7. 7. S.E. Gould in reply to S.E. Gould 1:22 pm 02/10/2014

    @rshoff2: Thanks for your comment! I’m a little confused by your statement “If nursing women could carry that sensitivity during breastfeeding”. The problem with breastfeeding is that until you get the hang of it, it is awkward, painful, stressful and the ‘painful’ aspect is really worth mentioning twice. I am now able to breastfeed very discretely, but I’ve spent four and a half months practising. For the first few weeks I was so tired and exhausted that in the process of getting a flailing and crying baby attached to a sore breast under several layers of clothing (this was winter!) I might have flashed a nipple or two. I certainly don’t feel this should damage the argument for breastfeeding in public.

    If breastfeeding were simple, non-painful, and quick I would agree absolutely with your statement. As it is, however, I feel that it would be far simpler if people who did find the practise unacceptable would move their eyes approximately two inches to the left to avoid seeing anything that would offend them.

    Believe me, immediately post-birth is the time a women feels at her least sexy, most exhausted, and most desperate. If you see any part of a nursing woman’s breast it’s not because she’s selfishly trying to flaunt her sexual assets with no regard for surrounding adults. It’s because the baby is being a ****ing pain and won’t latch on properly.

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