Human milk microbiome

File:Graphic of bacterial phyla and 10 commonest genera in human milk.jpg

Breast milk is a natural source of lactic acid bacteria for the newborn through breastfeeding, and may be considered a symbiotic food.{{Cite journal |display-authors=6 |vauthors=Martín R, Langa S, Reviriego C, Jimínez E, Marín ML, Xaus J, Fernández L, Rodríguez JM |date=December 2003 |title=Human milk is a source of lactic acid bacteria for the infant gut |journal=The Journal of Pediatrics |volume=143 |issue=6 |pages=754–8 |doi=10.1016/j.jpeds.2003.09.028 |pmid=14657823}} The normal concentration of bacteria in milk from healthy women was about 103 colony-forming units (CFU) per milliliter.{{Cite journal |display-authors=6 |vauthors=Hunt KM, Foster JA, Forney LJ, Schütte UM, Beck DL, Abdo Z, Fox LK, Williams JE, McGuire MK, McGuire MA |date=2011 |title=Characterization of the diversity and temporal stability of bacterial communities in human milk |journal=PLOS ONE |volume=6 |issue=6 |pages=e21313 |bibcode=2011PLoSO...621313H |doi=10.1371/journal.pone.0021313 |pmc=3117882 |pmid=21695057 |doi-access=free}} The milk's bacterial communities were generally complex. Among the hundreds of operational taxonomic units detected in the milk of every woman, only nine (Streptococcus, Staphylococcus, Serratia, Pseudomonas, Corynebacterium, Ralstonia, Propionibacterium, Sphingomonas, and Nitrobacteraceae) were present in every sample from every woman, but an individual's milk bacterial community was generally stable over time.{{Cite journal |vauthors=Soto A, Martín V, Jiménez E, Mader I, Rodríguez JM, Fernández L |date=July 2014 |title=Lactobacilli and bifidobacteria in human breast milk: influence of antibiotherapy and other host and clinical factors |journal=Journal of Pediatric Gastroenterology and Nutrition |volume=59 |issue=1 |pages=78–88 |doi=10.1097/MPG.0000000000000347 |pmc=4086764 |pmid=24590211}} Human milk is a source of live Staphylococci, Streptococci, lactic acid bacteria, Bifidobacteria, Propionibacteria, Corynebacteria, and closely related Gram-positive bacteria for the infant gut.

Breast milk was considered to be free of bacteria until about the early 2000s, when lactic acid bacteria were first described in human milk hygienically collected from healthy women. Several studies have shown that there is a mother-to-infant transfer of bacterial strains belonging, at least, to the genera Lactobacillus, Staphylococcus, Enterococcus, and Bifidobacterium through breastfeeding, thus accounting for the close relationship of bacterial composition of the gut microbiota of breastfed infants with that found in the breast milk of their respective mothers. Research has also found that there are similarities between human milk and infant gut microbial flora, suggesting that dietary exposure, such as human milk probiotics, may have a contribution in supporting infant gut microbiota and immune development.{{Cite journal |display-authors=6 |vauthors=Martín V, Maldonado-Barragán A, Moles L, Rodriguez-Baños M, Campo RD, Fernández L, Rodríguez JM, Jiménez E |date=February 2012 |title=Sharing of bacterial strains between breast milk and infant feces |journal=Journal of Human Lactation |volume=28 |issue=1 |pages=36–44 |doi=10.1177/0890334411424729 |pmid=22267318 |s2cid=7489223}}

Bacteria commonly isolated in human milk samples include Bifidobacterium, Lactobacillus, Staphylococcus, Streptococcus, Bacteroides, Clostridium, Micrococcus, Enterococcus, and Escherichia. Metagenomic analyses of human milk find it is dominated by Staphylococcus, Pseudomonas, and Edwardsiella.{{Cite journal |vauthors=Ward TL, Hosid S, Ioshikhes I, Altosaar I |date=May 2013 |title=Human milk metagenome: a functional capacity analysis |journal=BMC Microbiology |volume=13 |pages=116 |doi=10.1186/1471-2180-13-116 |pmc=3679945 |pmid=23705844 |doi-access=free}}{{Cite journal |display-authors=6 |vauthors=Jiménez E, de Andrés J, Manrique M, Pareja-Tobes P, Tobes R, Martínez-Blanch JF, Codoñer FM, Ramón D, Fernández L, Rodríguez JM |date=August 2015 |title=Metagenomic Analysis of Milk of Healthy and Mastitis-Suffering Women |journal=Journal of Human Lactation |volume=31 |issue=3 |pages=406–15 |doi=10.1177/0890334415585078 |pmid=25948578 |s2cid=24138114}} The human milk microbiome likely varies by population and between individual women,{{Cite journal |display-authors=6 |vauthors=Kumar H, du Toit E, Kulkarni A, Aakko J, Linderborg KM, Zhang Y, Nicol MP, Isolauri E, Yang B, Collado MC, Salminen S |date=2016 |title=Distinct Patterns in Human Milk Microbiota and Fatty Acid Profiles Across Specific Geographic Locations |journal=Frontiers in Microbiology |volume=7 |pages=1619 |doi=10.3389/fmicb.2016.01619 |pmc=5061857 |pmid=27790209 |doi-access=free}} however, a study based on a group of U.S. women observed the same nine bacterial taxa in all samples from all of their participants, suggesting a common "core" of the milk microbiome, at least in that population. Bacterial communities of human colostrum have been reported as being more diverse than those found in mature milk.{{Cite journal |vauthors=Cabrera-Rubio R, Collado MC, Laitinen K, Salminen S, Isolauri E, Mira A |date=September 2012 |title=The human milk microbiome changes over lactation and is shaped by maternal weight and mode of delivery |journal=The American Journal of Clinical Nutrition |volume=96 |issue=3 |pages=544–51 |doi=10.3945/ajcn.112.037382 |pmid=22836031 |doi-access=free}}

The three strains of Lactobacilli with probiotic properties that were isolated from breast milk were L. fermentum CECT5716, L. gasseri CECT5714, and L. salivarius CECT5713,{{Cite journal |vauthors=Lara-Villoslada F, Olivares M, Sierra S, Rodríguez JM, Boza J, Xaus J |date=October 2007 |title=Beneficial effects of probiotic bacteria isolated from breast milk |journal=The British Journal of Nutrition |volume=98 |issue=Suppl 1 |pages=S96-100 |doi=10.1017/S0007114507832910 |pmid=17922969 |doi-access=free}} with L. fermentum being one of the most abundant strains. Early administration of L. fermentum CECT5716 in infant formula is claimed to be safe and well tolerated for infants one to six months of age,{{Cite journal |display-authors=6 |vauthors=Maldonado J, Cañabate F, Sempere L, Vela F, Sánchez AR, Narbona E, López-Huertas E, Geerlings A, Valero AD, Olivares M, Lara-Villoslada F |date=January 2012 |title=Human milk probiotic Lactobacillus fermentum CECT5716 reduces the incidence of gastrointestinal and upper respiratory tract infections in infants |journal=Journal of Pediatric Gastroenterology and Nutrition |volume=54 |issue=1 |pages=55–61 |doi=10.1097/MPG.0b013e3182333f18 |pmid=21873895 |s2cid=29071981 |doi-access=free}} and safe for long term use.{{Cite journal |display-authors=6 |vauthors=Gil-Campos M, López MÁ, Rodriguez-Benítez MV, Romero J, Roncero I, Linares MD, Maldonado J, López-Huertas E, Berwind R, Ritzenthaler KL, Navas V, Sierra C, Sempere L, Geerlings A, Maldonado-Lobón JA, Valero AD, Lara-Villoslada F, Olivares M |date=February 2012 |title=Lactobacillus fermentum CECT 5716 is safe and well tolerated in infants of 1-6 months of age: a randomized controlled trial |journal=Pharmacological Research |volume=65 |issue=2 |pages=231–8 |doi=10.1016/j.phrs.2011.11.016 |pmid=22155106}}

While the origins of the human milk microbiome are not exactly known, several hypotheses for its establishment have been proposed. Bacteria present in human milk may be derived from the surrounding breast skin flora,{{Cite journal |vauthors=West PA, Hewitt JH, Murphy OM |date=April 1979 |title=Influence of methods of collection and storage on the bacteriology of human milk |journal=The Journal of Applied Bacteriology |volume=46 |issue=2 |pages=269–77 |doi=10.1111/j.1365-2672.1979.tb00820.x |pmid=572360}}{{Cite journal |display-authors=6 |vauthors=Grice EA, Kong HH, Conlan S, Deming CB, Davis J, Young AC, Bouffard GG, Blakesley RW, Murray PR, Green ED, Turner ML, Segre JA |date=May 2009 |title=Topographical and temporal diversity of the human skin microbiome |journal=Science |volume=324 |issue=5931 |pages=1190–2 |bibcode=2009Sci...324.1190G |doi=10.1126/science.1171700 |pmc=2805064 |pmid=19478181}} or the infant's oral cavity microbiota.{{Cite journal |vauthors=Cephas KD, Kim J, Mathai RA, Barry KA, Dowd SE, Meline BS, Swanson KS |date=August 2011 |title=Comparative analysis of salivary bacterial microbiome diversity in edentulous infants and their mothers or primary care givers using pyrosequencing |journal=PLOS ONE |volume=6 |issue=8 |pages=e23503 |bibcode=2011PLoSO...623503C |doi=10.1371/journal.pone.0023503 |pmc=3154475 |pmid=21853142 |doi-access=free}}{{Cite journal |vauthors=Nasidze I, Li J, Quinque D, Tang K, Stoneking M |date=April 2009 |title=Global diversity in the human salivary microbiome |journal=Genome Research |volume=19 |issue=4 |pages=636–43 |doi=10.1101/gr.084616.108 |pmc=2665782 |pmid=19251737}} Retrograde backflow during nursing or suckling may also lead to bacterial establishment in the mammary ducts,{{Cite journal |vauthors=Rodríguez JM |date=November 2014 |title=The origin of human milk bacteria: is there a bacterial entero-mammary pathway during late pregnancy and lactation? |journal=Advances in Nutrition |volume=5 |issue=6 |pages=779–84 |doi=10.3945/an.114.007229 |pmc=4224214 |pmid=25398740}} supported by the observation that a certain degree of flowback has been shown to occur during nursing using infrared photography.{{Cite journal |vauthors=Ramsay DT, Kent JC, Owens RA, Hartmann PE |date=February 2004 |title=Ultrasound imaging of milk ejection in the breast of lactating women |journal=Pediatrics |volume=113 |issue=2 |pages=361–7 |doi=10.1542/peds.113.2.361 |pmid=14754950}} Alternatively, bacteria may be translocated to the mammary duct from the maternal gastrointestinal tract via an entero-mammary pathway, facilitated by dendritic cells.{{Cite journal |display-authors=6 |vauthors=Jeurink PV, van Bergenhenegouwen J, Jiménez E, Knippels LM, Fernández L, Garssen J, Knol J, Rodríguez JM, Martín R |date=March 2013 |title=Human milk: a source of more life than we imagine |journal=Beneficial Microbes |volume=4 |issue=1 |pages=17–30 |doi=10.3920/bm2012.0040 |pmid=23271066 |doi-access=free}}

Several factors may influence the composition of human milk probiotics, such as maternal body mass index (BMI), infant sex, birth modality, and mode of breastfeeding.{{Cite journal |display-authors=6 |vauthors=Munblit D, Peroni DG, Boix-Amorós A, Hsu PS, Van't Land B, Gay MC, Kolotilina A, Skevaki C, Boyle RJ, Collado MC, Garssen J, Geddes DT, Nanan R, Slupsky C, Wegienka G, Kozyrskyj AL, Warner JO |date=August 2017 |title=Human Milk and Allergic Diseases: An Unsolved Puzzle |journal=Nutrients |volume=9 |issue=8 |pages=894 |doi=10.3390/nu9080894 |pmc=5579687 |pmid=28817095 |doi-access=free}}{{Cite conference |date=2017 |title=Maternal and early life factors influencing the human milk microbiota in the child cohort. |conference=Poster Session: Canadian Society of Microbiololists |location=Waterloo ON |vauthors=Moossavi S, Khafipour E, Sepehri S, Robertson B, Bode L, Becker AB, etal}} A study done by Soto et al also revealed that Lactobacilli and Bifidobacteria are more commonly found in the human milk of women who did not receive any antibiotics during pregnancy and lactation.

Human milk oligosaccharides (HMOs), a primary component of human milk, are prebiotics which have been shown to promote growth of beneficial Bifidobacterium and Bacteroides species.{{Cite journal |vauthors=Bode L |date=November 2009 |title=Human milk oligosaccharides: prebiotics and beyond |journal=Nutrition Reviews |volume=67 Suppl 2 |issue=suppl_2 |pages=S183-91 |doi=10.1111/j.1753-4887.2009.00239.x |pmid=19906222 |doi-access=free}}{{Cite journal |vauthors=Jost T, Lacroix C, Braegger C, Chassard C |date=July 2015 |title=Impact of human milk bacteria and oligosaccharides on neonatal gut microbiota establishment and gut health |journal=Nutrition Reviews |volume=73 |issue=7 |pages=426–37 |doi=10.1093/nutrit/nuu016 |pmid=26081453 |doi-access=free}}

Maternal health status is associated with changes in the bacterial composition of milk. Higher maternal BMI and obesity are associated with changes in the levels of Bifidobacterium and Staphylococcus species and overall lower bacterial diversity.{{Cite journal |vauthors=Collado MC, Laitinen K, Salminen S, Isolauri E |date=July 2012 |title=Maternal weight and excessive weight gain during pregnancy modify the immunomodulatory potential of breast milk |journal=Pediatric Research |volume=72 |issue=1 |pages=77–85 |doi=10.1038/pr.2012.42 |pmid=22453296 |doi-access=free}} Milk of women with celiac disease is observed to have reduced levels of Bacteroides and Bifidobacterium.{{Cite journal |vauthors=Olivares M, Albrecht S, De Palma G, Ferrer MD, Castillejo G, Schols HA, Sanz Y |date=February 2015 |title=Human milk composition differs in healthy mothers and mothers with celiac disease |journal=European Journal of Nutrition |volume=54 |issue=1 |pages=119–28 |doi=10.1007/s00394-014-0692-1 |pmid=24700375 |s2cid=36508893}} Women who are HIV-positive show higher bacterial diversity and increased abundances of Lactobacillus in their milk than do non-HIV-positive women.{{Cite journal |display-authors=6 |vauthors=González R, Maldonado A, Martín V, Mandomando I, Fumadó V, Metzner KJ, Sacoor C, Fernández L, Macete E, Alonso PL, Rodríguez JM, Menendez C |date=November 2013 |title=Breast milk and gut microbiota in African mothers and infants from an area of high HIV prevalence |journal=PLOS ONE |volume=8 |issue=11 |pages=e80299 |bibcode=2013PLoSO...880299G |doi=10.1371/journal.pone.0080299 |pmc=3841168 |pmid=24303004 |doi-access=free}} Mastitis has been linked to changes in human milk microbiota at the phylum level, lower microbial diversity, and decreased abundance of obligate anaerobic taxa.{{Cite journal |vauthors=Patel SH, Vaidya YH, Patel RJ, Pandit RJ, Joshi CG, Kunjadiya AP |date=August 2017 |title=Culture independent assessment of human milk microbial community in lactational mastitis |journal=Scientific Reports |volume=7 |issue=1 |pages=7804 |bibcode=2017NatSR...7.7804P |doi=10.1038/s41598-017-08451-7 |pmc=5552812 |pmid=28798374}}{{Cite journal |vauthors=Delgado S, Arroyo R, Martín R, Rodríguez JM |date=April 2008 |title=PCR-DGGE assessment of the bacterial diversity of breast milk in women with lactational infectious mastitis |journal=BMC Infectious Diseases |volume=8 |pages=51 |doi=10.1186/1471-2334-8-51 |pmc=2383900 |pmid=18423017 |doi-access=free}}

Women delivering term and preterm show differences in their milk microbiome composition, with mothers of term-births showing lower abundances of Enterococcus species and higher amounts of Bifidobacterium species in their milk compared to mothers of preterm births.

Few studies have been conducted examining the influence of maternal diet on the milk microbiome, but diet is known to influence other aspects of milk composition, such as the lipid profile,{{Cite journal |vauthors=Nishimura RY, Barbieiri P, Castro GS, Jordão AA, Perdoná G, Sartorelli DS |date=June 2014 |title=Dietary polyunsaturated fatty acid intake during late pregnancy affects fatty acid composition of mature breast milk |journal=Nutrition |volume=30 |issue=6 |pages=685–9 |doi=10.1016/j.nut.2013.11.002 |pmid=24613435}}{{Cite journal |vauthors=Peng Y, Zhou T, Wang Q, Liu P, Zhang T, Zetterström R, Strandvik B |year=2009 |title=Fatty acid composition of diet, cord blood and breast milk in Chinese mothers with different dietary habits |journal=Prostaglandins, Leukotrienes, and Essential Fatty Acids |volume=81 |issue=5–6 |pages=325–30 |doi=10.1016/j.plefa.2009.07.004 |pmid=19709866}} which in turn could affect its microbial composition. Variation in the fat and carbohydrate content of the maternal diet may influence the taxonomic composition of the milk microbiome.{{Cite journal |last=Meyer |first=Kristen M. |last2=Mohammad |first2=Mahmoud |last3=Bode |first3=Lars |last4=Chu |first4=Derrick M. |last5=Ma |first5=Jun |last6=Haymond |first6=Morey |last7=Aagaard |first7=Kjersti |name-list-style=vanc |year=2017 |title=20: Maternal diet structures the breast milk microbiome in association with human milk oligosaccharides and gut-associated bacteria |journal=American Journal of Obstetrics and Gynecology |volume=216 |issue=1 |page=S15 |doi=10.1016/j.ajog.2016.11.911 |doi-access=free}}

Both the taxonomic composition and diversity of bacteria present in human milk likely vary by maternal geographic location, however studies with more geographically diverse participants are needed to better understand variation between populations.

Maternal perinatal antibiotic use is associated with changes in the prevalence of Lactobacillus, Bifidobacterium, Staphylococcus, and Eubacterium in milk.{{Cite journal |vauthors=Witt A, Mason MJ, Burgess K, Flocke S, Zyzanski S |date=January 2014 |title=A case control study of bacterial species and colony count in milk of breastfeeding women with chronic pain |journal=Breastfeeding Medicine |volume=9 |issue=1 |pages=29–34 |doi=10.1089/bfm.2013.0012 |pmc=3903327 |pmid=23789831}}{{Cite journal |display-authors=6 |vauthors=Urbaniak C, Cummins J, Brackstone M, Macklaim JM, Gloor GB, Baban CK, Scott L, O'Hanlon DM, Burton JP, Francis KP, Tangney M, Reid G |date=May 2014 |title=Microbiota of human breast tissue |journal=Applied and Environmental Microbiology |volume=80 |issue=10 |pages=3007–14 |bibcode=2014ApEnM..80.3007U |doi=10.1128/aem.00242-14 |pmc=4018903 |pmid=24610844}}

Social network density of mother-infant dyads was found to be associated with increased bacterial diversity in the milk microbiome of mothers in the Central African Republic.{{Cite journal |display-authors=6 |vauthors=Meehan CL, Lackey KA, Hagen EH, Williams JE, Roulette J, Helfrecht C, McGuire MA, McGuire MK |date=July 2018 |title=Social networks, cooperative breeding, and the human milk microbiome |journal=American Journal of Human Biology |volume=30 |issue=4 |pages=e23131 |doi=10.1002/ajhb.23131 |pmid=29700885 |s2cid=13793532 |doi-access=free}}

Mode of delivery may influence composition of the human milk microbiome. Vaginal births is associated with high taxonomic diversity and high prevalence of Bifidobacterium and Lactobacillus, and the opposite trend being seen with birth by caesarean section,{{Cite journal |vauthors=Cabrera-Rubio R, Mira-Pascual L, Mira A, Collado MC |date=February 2016 |title=Impact of mode of delivery on the milk microbiota composition of healthy women |journal=Journal of Developmental Origins of Health and Disease |volume=7 |issue=1 |pages=54–60 |doi=10.1017/S2040174415001397 |pmid=26286040 |s2cid=10368527}}{{Cite journal |display-authors=6 |vauthors=Hoashi M, Meche L, Mahal LK, Bakacs E, Nardella D, Naftolin F, Bar-Yam N, Dominguez-Bello MG |date=July 2016 |title=Human Milk Bacterial and Glycosylation Patterns Differ by Delivery Mode |journal=Reproductive Sciences |volume=23 |issue=7 |pages=902–7 |doi=10.1177/1933719115623645 |pmid=26711314 |s2cid=38606958}} however no relationship between delivery mode and the maternal milk microbiome has also been observed.{{Cite journal |vauthors=Urbaniak C, Angelini M, Gloor GB, Reid G |date=January 2016 |title=Human milk microbiota profiles in relation to birthing method, gestation and infant gender |journal=Microbiome |volume=4 |issue=1 |pages=1 |doi=10.1186/s40168-015-0145-y |pmc=4702315 |pmid=26739322 |doi-access=free}}

The human milk microbiome varies across lactation stage, with higher microbial diversity observed in colostrum than in mature milk. Taxonomic composition of human milk also varies across the lactation period, initially dominated by Weissella, Leuconostoc, Staphylococcus, Streptococcus, and Lactococcus species, and later composed primarily of Veillonella, Prevotella, Leptotrichia, Lactobacillus, Streptococcus, Bifidobacterium, and Enterococcus.{{Cite journal |vauthors=Khodayar-Pardo P, Mira-Pascual L, Collado MC, Martínez-Costa C |date=August 2014 |title=Impact of lactation stage, gestational age and mode of delivery on breast milk microbiota |journal=Journal of Perinatology |volume=34 |issue=8 |pages=599–605 |doi=10.1038/jp.2014.47 |pmid=24674981 |s2cid=37695304}}

Breastfeeding is thought to be an important driver of infant gut microbiome establishment.{{Cite journal |display-authors=6 |vauthors=Milani C, Duranti S, Bottacini F, Casey E, Turroni F, Mahony J, Belzer C, Delgado Palacio S, Arboleya Montes S, Mancabelli L, Lugli GA, Rodriguez JM, Bode L, de Vos W, Gueimonde M, Margolles A, van Sinderen D, Ventura M |date=December 2017 |title=The First Microbial Colonizers of the Human Gut: Composition, Activities, and Health Implications of the Infant Gut Microbiota |journal=Microbiology and Molecular Biology Reviews |volume=81 |issue=4 |pages=e00036–17 |doi=10.1128/mmbr.00036-17 |pmc=5706746 |pmid=29118049}} The gut microbiome of breastfed infants is less diverse, contains higher amounts of Bifidobacterium and Lactobacillus species, and fewer potential pathogenic taxa than the gut microbiome of formula-fed infants.{{Cite journal |display-authors=6 |vauthors=Yatsunenko T, Rey FE, Manary MJ, Trehan I, Dominguez-Bello MG, Contreras M, Magris M, Hidalgo G, Baldassano RN, Anokhin AP, Heath AC, Warner B, Reeder J, Kuczynski J, Caporaso JG, Lozupone CA, Lauber C, Clemente JC, Knights D, Knight R, Gordon JI |date=May 2012 |title=Human gut microbiome viewed across age and geography |journal=Nature |volume=486 |issue=7402 |pages=222–7 |bibcode=2012Natur.486..222Y |doi=10.1038/nature11053 |pmc=3376388 |pmid=22699611}}{{Cite journal |vauthors=O'Sullivan A, Farver M, Smilowitz JT |date=2015 |title=The Influence of Early Infant-Feeding Practices on the Intestinal Microbiome and Body Composition in Infants |journal=Nutrition and Metabolic Insights |volume=8 |issue=Suppl 1 |pages=1–9 |doi=10.4137/NMI.S29530 |pmc=4686345 |pmid=26715853}}{{Cite journal |vauthors=Bezirtzoglou E, Tsiotsias A, Welling GW |date=December 2011 |title=Microbiota profile in feces of breast- and formula-fed newborns by using fluorescence in situ hybridization (FISH) |journal=Anaerobe |volume=17 |issue=6 |pages=478–82 |doi=10.1016/j.anaerobe.2011.03.009 |pmid=21497661}} Human milk bacteria may reduce risk of infection in breastfed infants by competitively excluding harmful bacteria,{{Cite journal |vauthors=Olivares M, Díaz-Ropero MP, Martín R, Rodríguez JM, Xaus J |date=July 2006 |title=Antimicrobial potential of four Lactobacillus strains isolated from breast milk |journal=Journal of Applied Microbiology |volume=101 |issue=1 |pages=72–9 |doi=10.1111/j.1365-2672.2006.02981.x |pmid=16834593 |doi-access=free}}{{Cite journal |vauthors=Heikkilä MP, Saris PE |date=2003 |title=Inhibition of Staphylococcus aureus by the commensal bacteria of human milk |journal=Journal of Applied Microbiology |volume=95 |issue=3 |pages=471–8 |doi=10.1046/j.1365-2672.2003.02002.x |pmid=12911694 |doi-access=free}} and producing antimicrobial compounds which eliminate pathogenic strains.{{Cite journal |vauthors=Beasley SS, Saris PE |date=August 2004 |title=Nisin-producing Lactococcus lactis strains isolated from human milk |journal=Applied and Environmental Microbiology |volume=70 |issue=8 |pages=5051–3 |bibcode=2004ApEnM..70.5051B |doi=10.1128/aem.70.8.5051-5053.2004 |pmc=492443 |pmid=15294850}}{{Cite journal |vauthors=Martín R, Olivares M, Marín ML, Fernández L, Xaus J, Rodríguez JM |date=February 2005 |title=Probiotic potential of 3 Lactobacilli strains isolated from breast milk |journal=Journal of Human Lactation |volume=21 |issue=1 |pages=8–17; quiz 18–21, 41 |doi=10.1177/0890334404272393 |pmid=15681631 |s2cid=23135001}}{{Cite journal |vauthors=Martín R, Jiménez E, Olivares M, Marín ML, Fernández L, Xaus J, Rodríguez JM |date=October 2006 |title=Lactobacillus salivarius CECT 5713, a potential probiotic strain isolated from infant feces and breast milk of a mother-child pair |journal=International Journal of Food Microbiology |volume=112 |issue=1 |pages=35–43 |doi=10.1016/j.ijfoodmicro.2006.06.011 |pmid=16843562}} Certain Lactobacilli and Bifidobacteria, the growth of which is stimulated by HMOs,{{Cite journal |vauthors=Bode L |date=September 2012 |title=Human milk oligosaccharides: every baby needs a sugar mama |journal=Glycobiology |volume=22 |issue=9 |pages=1147–62 |doi=10.1093/glycob/cws074 |pmc=3406618 |pmid=22513036}} contribute to healthy metabolic and immune-related functioning in the infant gut.{{Cite journal |vauthors=Zivkovic AM, German JB, Lebrilla CB, Mills DA |date=March 2011 |title=Human milk glycobiome and its impact on the infant gastrointestinal microbiota |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=108 Suppl 1 |issue=Supplement 1 |pages=4653–8 |bibcode=2011PNAS..108.4653Z |doi=10.1073/pnas.1000083107 |pmc=3063602 |pmid=20679197 |doi-access=free}}{{Cite journal |display-authors=6 |vauthors=Asakuma S, Hatakeyama E, Urashima T, Yoshida E, Katayama T, Yamamoto K, Kumagai H, Ashida H, Hirose J, Kitaoka M |date=October 2011 |title=Physiology of consumption of human milk oligosaccharides by infant gut-associated bifidobacteria |journal=The Journal of Biological Chemistry |volume=286 |issue=40 |pages=34583–92 |doi=10.1074/jbc.M111.248138 |pmc=3186357 |pmid=21832085 |doi-access=free}}{{Cite journal |display-authors=6 |vauthors=Donnet-Hughes A, Perez PF, Doré J, Leclerc M, Levenez F, Benyacoub J, Serrant P, Segura-Roggero I, Schiffrin EJ |date=August 2010 |title=Potential role of the intestinal microbiota of the mother in neonatal immune education |journal=The Proceedings of the Nutrition Society |volume=69 |issue=3 |pages=407–15 |doi=10.1017/S0029665110001898 |pmid=20633308 |doi-access=free}}

Breastfeeding is an essential component of maternal health, providing numerous benefits. It has been associated with a decreased risk of metabolic disease, improved immune function, and delayed menstrual cycles. Lactobacillus fermentum, a type of probiotic bacteria, has been identified as a means of reducing the risk of breast cancer. Research studies showed that L. fermentum could improve mastitis, a common inflammatory disease associated with lactation, by reducing the number of Streptococcus load which is believed to be the causal agent and risk factor of mastitis.{{Cite journal |vauthors=Arroyo R, Martín V, Maldonado A, Jiménez E, Fernández L, Rodríguez JM |date=June 2010 |title=Treatment of infectious mastitis during lactation: antibiotics versus oral administration of Lactobacilli isolated from breast milk |journal=Clinical Infectious Diseases |volume=50 |issue=12 |pages=1551–8 |doi=10.1086/652763 |pmid=20455694 |doi-access=free}} Additionally, notable benefits of breastfeeding have been theoretically sustained to be able to reduce metabolic diseases such as diabetes and cardiovascular disease. The lactation process requires a substantial amount of energy expenditure, which can mitigate the risk of these diseases. Lactobacillus fermentum has been shown to facilitate weight loss and reduce fat mass, as well as improve insulin sensitivity, thereby helping to prevent diabetes and obesity.Janney CA, Zhang D, Sowers M. Lactation and weight retention. Am J Clin Nutr. 1997 Nov;66(5):1116-24. {{doi|10.1093/ajcn/66.5.1116}} PMID 9356528 Moreover, hormonal changes during lactation can further improve metabolism and glucose homeostasis, suggesting reduction in potential metabolic diseases. However, it is hard to determine the exact factor affecting weight change after birth due to various confounding factors such as pre-pregnancy BMI, weight gain during pregnancy, and social support. A recent meta-analysis of 13 cohort studies have found that breast feeding has been shown to decrease inflammatory markers, such as C-reactive protein and interleukin-6, which are associated with insulin resistance and T2DM.Tahir, M. J., Haapala, J. L., Foster, L. P., Duncan, K. M., Teague, A. M., Kharbanda, E. O., McGovern, P. M., Whitaker, K. M., Rasmussen, K. M., Fields, D. A., Harnack, L. J., Jacobs, D. R., & Demerath, E. W. (2019). Association of full breastfeeding duration with postpartum weight retention in a cohort of predominantly breastfeeding women. Nutrients, 11(4), 938. {{doi|10.3390/nu11040938|doi-access=free}}

On the other hand, breastfeeding can also delay menstrual cycles, reducing the risk of iron-deficiency anemia and related health issues. Prolactin, a hormone produced during lactation, suppresses ovulation, preventing the mother from menstruating. This suppression can continue for up to 6 months postpartum, serving as a natural form of birth control.Rassie, K., Mousa, A., Joham, A., & Teede, H. J. (2021). Metabolic conditions including obesity, diabetes, and polycystic ovary syndrome: Implications for breastfeeding and breastmilk composition. Seminars in Reproductive Medicine, 39(03/04), 111–132. {{doi|10.1055/s-0041-1732365}}

It is also suggested that in addition to physical benefits, breastfeeding can reduce the risk of postpartum depression. Breastfeeding mothers report less anxiety, less negative mood, and less stress, as well as increased sleep duration and reduced sleep disturbances when compared to formula-feeding mothers. Studies on post-partum depression demonstrate that breastfeeding may protect mothers from this disorder, and researchers have strived to explain the biological processes that explain this protection. For example, lactation attenuates neuro-endocrine responses to stress, and this may be related to fewer post-partum depressive symptoms. Moreover, early breastfeeding cessation was linked to higher risk of post-partum depression. It is the psychological pressure to exclusively breastfeed that contributes to postpartum depression symptoms in mothers unable to achieve their breastfeeding intentions. In a prospective follow-up for eight weeks postpartum, mothers with breastfeeding problems (including mastitis, nipple pain, need for frequent expressing of milk, or over-supply or under-supply of milk) showed poor mental health.Cooklin, A.R.; Amir, L.H.; Nguyen, C.D.; Buck, M.L.; Cullinane, M.; Fisher, J.R.W.; Donath, S.M.; CASTLE Study Team. Physical Health, Breastfeeding Problems and Maternal Mood in the Early Postpartum: A Prospective Cohort Study. Arch. Womens Ment. Health 2018, 21, 365–374.

Breastfed children have a lower incidence of infections than formula-fed children, which could be mediated in part through modulation of the intestinal microflora by breast milk components.{{Cite book |title=Breast feeding and the intestinal microflora of the infant--implications for protection against infectious diseases |vauthors=Wold AE, Adlerberth I |date=2000 |isbn=0-306-46405-5 |series=Advances in Experimental Medicine and Biology |volume=478 |pages=77–93 |chapter=Breast Feeding and the Intestinal Microflora of the Infant — Implications for Protection Against Infectious Diseases |doi=10.1007/0-306-46830-1_7 |pmid=11065062}} Indeed, breast-fed infants seem to develop a gut microflora richer in Lactobacilli and Bifidobacteria with reduced pathogenic bacteria compared with formula-fed infants.{{Cite journal |vauthors=Mackie RI, Sghir A, Gaskins HR |date=May 1999 |title=Developmental microbial ecology of the neonatal gastrointestinal tract |journal=The American Journal of Clinical Nutrition |volume=69 |issue=5 |pages=1035S–1045S |doi=10.1093/ajcn/69.5.1035s |pmid=10232646 |doi-access=free}} Research, by Maldonado et al, found that infants receiving a follow-on formula enriched with L. fermentum demonstrated a reduction in gastrointestinal and respiratory infections, thus the administration of such formula may be useful for the prevention of community-acquired gastrointestinal and upper respiratory infections in infants.

Human milk probiotics could also act as pioneering species to increase the colonization of ‘beneficial’ bacteria and support the infant’s immature immune system.{{Cite journal |vauthors=Bergmann H, Rodríguez JM, Salminen S, Szajewska H |date=October 2014 |title=Probiotics in human milk and probiotic supplementation in infant nutrition: a workshop report |journal=The British Journal of Nutrition |volume=112 |issue=7 |pages=1119–28 |doi=10.1017/S0007114514001949 |pmid=25160058 |doi-access=free}} It is known that Lactobacilli and Bifidobacteria can suppress the growth of pathogenic microorganisms such as Salmonella typhimurium and Clostridium perfringens by colonization of a child's intestine and competing for nutrients, thus preventing their adhesion. Intestinal colonization by commensal bacteria also plays a vital role in maintaining homeostasis of immune system. These bacteria stimulate the T helper 1 response and counteract the trend towards a T helper 2 response of neonatal immune system, which in turn reducing the incidence of the inflammatory processes such as necrotizing enterocolitis.

Children with colic symptoms possibly have an imbalance in the intestinal microbiota – analyses of faecal samples found higher counts of coliform bacteria and lower counts of Lactobacilli in infants with colic symptoms compared with children not suffering from colic.{{Cite journal |vauthors=Savino F, Tarasco V |date=December 2010 |title=New treatments for infant colic |journal=Current Opinion in Pediatrics |volume=22 |issue=6 |pages=791–7 |doi=10.1097/MOP.0b013e32833fac24 |pmid=20859207 |s2cid=26003983}} On the other hand, probiotics have been shown to influence intestinal motility and sensory neurons as well as contractile activity of the intestine and to exert anti-inflammatory effects.

There is some indication of relationships between milk microbiota and other human milk components, including HMOs, maternal cells, and nutrient profiles.{{Cite journal |vauthors=Williams JE, Price WJ, Shafii B, Yahvah KM, Bode L, McGuire MA, McGuire MK |date=August 2017 |title=Relationships Among Microbial Communities, Maternal Cells, Oligosaccharides, and Macronutrients in Human Milk |journal=Journal of Human Lactation |volume=33 |issue=3 |pages=540–551 |doi=10.1177/0890334417709433 |pmid=28609134 |doi-access=free}}{{Cite journal |vauthors=Boix-Amorós A, Collado MC, Mira A |date=2016 |title=Relationship between Milk Microbiota, Bacterial Load, Macronutrients, and Human Cells during Lactation |journal=Frontiers in Microbiology |volume=7 |pages=492 |doi=10.3389/fmicb.2016.00492 |pmc=4837678 |pmid=27148183 |doi-access=free}} Specific bacterial genera have been shown to be associated with variation in levels of milk macronutrients such as lactose, proteins, and fats. HMOs selectively facilitate growth of particular beneficial bacteria, notably Bifidobacterium species.{{Cite book |last=Clancy |first=Kathryn B.H. |title=Building Babies |last2=Hinde |first2=Katie |last3=Rutherford |first3=Julienne N. |date=2013 |publisher=Springer |isbn=978-1-4614-4059-8 |series=Developments in Primatology: Progress and Prospects |location=New York |pages=233–256 |chapter=Infant Gut Microbiota: Developmental Influences and Health Outcomes |doi=10.1007/978-1-4614-4060-4_11 |name-list-style=vanc}}{{Cite journal |vauthors=Sela DA, Mills DA |date=July 2010 |title=Nursing our microbiota: molecular linkages between bifidobacteria and milk oligosaccharides |journal=Trends in Microbiology |volume=18 |issue=7 |pages=298–307 |doi=10.1016/j.tim.2010.03.008 |pmc=2902656 |pmid=20409714}} Furthermore, as Bifidobacteria genomes are uniquely equipped to metabolize HMOs,{{Cite journal |display-authors=6 |vauthors=Sela DA, Chapman J, Adeuya A, Kim JH, Chen F, Whitehead TR, Lapidus A, Rokhsar DS, Lebrilla CB, German JB, Price NP, Richardson PM, Mills DA |date=December 2008 |title=The genome sequence of Bifidobacterium longum subsp. infantis reveals adaptations for milk utilization within the infant microbiome |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=105 |issue=48 |pages=18964–9 |bibcode=2008PNAS..10518964S |doi=10.1073/pnas.0809584105 |pmc=2596198 |pmid=19033196 |doi-access=free}} which are otherwise indigestible by enzymes of the infant gut, some have suggested a coevolution between HMOs and certain bacteria common in both the milk and infant gastrointestinal microbiomes.{{Cite journal |vauthors=German JB, Freeman SL, Lebrilla CB, Mills DA |date=2008 |title=Human milk oligosaccharides: evolution, structures and bioselectivity as substrates for intestinal bacteria |journal=Nestle Nutrition Workshop Series. Paediatric Programme |series=Nestlé Nutrition Workshop Series: Pediatric Program |volume=62 |pages=205–18; discussion 218–22 |doi=10.1159/000146322 |isbn=978-3-8055-8553-8 |pmc=2861563 |pmid=18626202}}{{Cite journal |vauthors=Allen-Blevins CR, Sela DA, Hinde K |date=April 2015 |title=Milk bioactives may manipulate microbes to mediate parent-offspring conflict |journal=Evolution, Medicine, and Public Health |volume=2015 |issue=1 |pages=106–21 |doi=10.1093/emph/eov007 |pmc=4512713 |pmid=25835022}} Furthermore, relative to other mammalian milks such as primate milk, human milk appears to be unique with respect to the complexity and diversity of its oligosaccharide repertoire. Human milk is typified by greater overall HMO diversity and predominance of oligosaccharides known to promote growth of Bifidobacterium in the infant gut.{{Cite journal |display-authors=6 |vauthors=Urashima T, Odaka G, Asakuma S, Uemura Y, Goto K, Senda A, Saito T, Fukuda K, Messer M, Oftedal OT |date=May 2009 |title=Chemical characterization of oligosaccharides in chimpanzee, bonobo, gorilla, orangutan, and siamang milk or colostrum |journal=Glycobiology |volume=19 |issue=5 |pages=499–508 |doi=10.1093/glycob/cwp006 |pmid=19164487 |doi-access=free}} Milk microbiota are thought to play an essential role in programming the infant immune system, and tend to reduce the risk of adverse infant health outcomes. Differences in milk oligosaccharides between humans and non-human primates could be indicative of variation in pathogen exposure associated with increased sociality and group sizes.{{Cite journal |display-authors=6 |vauthors=Tao N, Wu S, Kim J, An HJ, Hinde K, Power ML, Gagneux P, German JB, Lebrilla CB |date=April 2011 |title=Evolutionary glycomics: characterization of milk oligosaccharides in primates |journal=Journal of Proteome Research |volume=10 |issue=4 |pages=1548–57 |doi=10.1021/pr1009367 |pmc=3070053 |pmid=21214271}} Together, these observations may indicate that milk microbial communities have coevolved with their human host, supported by the expectation that microbes which promote host health facilitate their own transmission and proliferation.{{Cite journal |vauthors=Funkhouser LJ, Bordenstein SR |date=2013 |title=Mom knows best: the universality of maternal microbial transmission |journal=PLOS Biology |volume=11 |issue=8 |pages=e1001631 |doi=10.1371/journal.pbio.1001631 |pmc=3747981 |pmid=23976878 |doi-access=free}}

Both human and macaque milks contains high abundances of Streptococcus and Lactobacillus bacteria, but differ in their respective relative abundances of these taxa.{{Cite journal |vauthors=Jin L, Hinde K, Tao L |date=February 2011 |title=Species diversity and relative abundance of lactic acid bacteria in the milk of rhesus monkeys (Macaca mulatta) |journal=Journal of Medical Primatology |volume=40 |issue=1 |pages=52–8 |doi=10.1111/j.1600-0684.2010.00450.x |pmc=3697067 |pmid=20946146}} Bacteria observed to be most common in healthy bovine milk include Ralstonia, Pseudomonas, Sphingomonas, Stenotrophomonas, Psychrobacter, Bradyrhizobium, Corynebacterium, Pelomonas, Staphylococcus, Faecalibacterium, Lachnospiraceae, Propionibacterium, Aeribacillus, Bacteroides, Streptococcus, Anaerococcus, Lactobacillus, Porphyromonas, Comamonas, Fusobacterium, and Enterococcus.{{Cite journal |display-authors=6 |vauthors=Kuehn JS, Gorden PJ, Munro D, Rong R, Dong Q, Plummer PJ, Wang C, Phillips GJ |date=April 2013 |title=Bacterial community profiling of milk samples as a means to understand culture-negative bovine clinical mastitis |journal=PLOS ONE |volume=8 |issue=4 |pages=e61959 |bibcode=2013PLoSO...861959K |doi=10.1371/journal.pone.0061959 |pmc=3636265 |pmid=23634219 |doi-access=free}}{{Cite journal |display-authors=6 |vauthors=Oikonomou G, Bicalho ML, Meira E, Rossi RE, Foditsch C, Machado VS, Teixeira AG, Santisteban C, Schukken YH, Bicalho RC |date=2014 |title=Microbiota of cow's milk; distinguishing healthy, sub-clinically and clinically diseased quarters |journal=PLOS ONE |volume=9 |issue=1 |pages=e85904 |bibcode=2014PLoSO...985904O |doi=10.1371/journal.pone.0085904 |pmc=3896433 |pmid=24465777 |doi-access=free}}{{Cite journal |vauthors=Zhang R, Huo W, Zhu W, Mao S |date=March 2015 |title=Characterization of bacterial community of raw milk from dairy cows during subacute ruminal acidosis challenge by high-throughput sequencing |journal=Journal of the Science of Food and Agriculture |volume=95 |issue=5 |pages=1072–9 |bibcode=2015JSFA...95.1072Z |doi=10.1002/jsfa.6800 |pmid=24961605}}{{Cite journal |vauthors=Addis MF, Tanca A, Uzzau S, Oikonomou G, Bicalho RC, Moroni P |date=July 2016 |title=The bovine milk microbiota: insights and perspectives from -omics studies |journal=Molecular BioSystems |volume=12 |issue=8 |pages=2359–72 |doi=10.1039/c6mb00217j |pmid=27216801 |hdl-access=free |hdl=2434/383895}}

• Human microbiota
• Breast milk
• Human milk oligosaccharide

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Category:Maternal health

Category:Breast milk

Category:Microbiomes