The human breast milk microbiota plays a key role in newborn microbial colonization. Its composition varies throughout the postpartum period and is modulated by maternal and perinatal factors. This scoping review aimed to systematically map current knowledge of the breast milk bacterial microbiota in healthy lactating women and describe its compositional dynamics in relation to identified modulating factors. A systematic search was conducted across five databases (PubMed, ScienceDirect, Scopus, Google Scholar, and Web of Science) following the PRISMA-ScR guidelines targeting studies that used 16S rRNA sequencing. Twenty-three studies from America (6), Europe (5), Asia (11), and Oceania (1) published between 2019 and 2025 were selected, with sample sizes ranging from five to 334 breast milk samples per study. The most consistently reported genera were Streptococcus, Staphylococcus, Lactobacillus, Pseudomonas, and Acinetobacter. The most frequently reported and analyzed associated factors across the 23 studies were lactation stage, maternal body mass index (BMI), and mode of delivery. However, the methodological heterogeneity across studies, particularly regarding the hypervariable region analyzed, represents the main source of variability in the results. These findings enhance our understanding of breast milk microbiota dynamics and highlight how maternal and perinatal factors influence its composition.

Vaginal dysbiosis is associated with an increased risk of urogenital infections and reproductive complications, driving the search for biotherapeutic alternatives based on autochthonous bacteria. The objective of this study was to isolate, identify, and characterize in vitro Lactobacillus strains with probiotic potential from the vaginal microbiota of healthy women in Arequipa, Peru. Methods: A total of 100 healthy women between 18 and 64 years old performed vaginal self-sampling using eSwab^® (Copan Italia S.p.A.). Isolates were cultured on MRS agar and identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and complemented with 16S rRNA gene sequencing when identification was not achieved by mass spectrometry. Lactobacillus strains were subjected to preliminary phenotypic safety evaluations (hemolysin, catalase, and gelatinase activities, along with antimicrobial susceptibility via disk diffusion) and functional assessments (in vitro: lactic acid production, acidifying capacity, and inhibitory activity against Gardnerella vaginalis, Candida albicans, Escherichia coli, and Streptococcus agalactiae). Results: A total of 231 bacterial isolates were obtained, 142 of which corresponded to Lactobacillus, with Lactobacillus crispatus as the predominant species (113 strains). Based on preliminary phenotypic safety screening, strains were selected for functional characterization. The strains L. jensenii LMH-0038/LMH-0039 and L. crispatus LMH-0070 demonstrated acidifying capacity and inhibitory activity against G. vaginalis and C. albicans. Widespread ciprofloxacin resistance was detected across the evaluated species. Conclusions: Autochthonous Lactobacillus strains with acceptable preliminary safety profiles and notable in vitro bioactivity against key urogenital pathogens were isolated and identified. In particular, L. jensenii and L. crispatus emerge as promising probiotic candidates.

Los lactobacilos constituyen un componente predominante de la microbiota vaginal, contribuyendo al mantenimiento del pH ácido y a la protección frente a microorganismos patógenos. Estas bacterias son reconocidas como generalmente seguras; sin embargo, se ha reportado la presencia de resistencia a antibióticos, la cual podría transferirse a otros lactobacilos o bacterias patógenas. El objetivo de este estudio transversal observacional fue evaluar la susceptibilidad a antibióticos de lactobacilos vaginales aislados de mujeres sanas en edad reproductiva de la ciudad de Arequipa. Se recolectaron muestras de 74 mujeres en edad reproductiva y se obtuvieron 417 aislados de los cuales 207 correspondieron a bacilos Gram positivos pertenecientes a cuatro familias bacterianas Lactobacillaceae, Bifidobacteriaceae, Actinomycetaceae, Corynebacteriaceae. Asimismo, las especies de lactobacilos identificadas fueron L. crispatus, L. acidophilus/gasseri, L. jensenii, L. reuteri, L. rhamnosus y Lactobacillus sp. Tras la evaluación de seguridad mediante pruebas de actividad catalasa, hemolítica y gelatinosa, se seleccionaron 189 lactobacilos para determinar su perfil de susceptibilidad frente a ampicilina, eritromicina, tetraciclina, vancomicina, ciprofloxacino, rifampicina y cloranfenicol. Los lactobacilos presentaron mayor resistencia frente a ciprofloxacino, seguido por cloranfenicol, eritromicina, tetraciclina, vancomicina y rifampicina. Solo ocho aislados no presentaron resistencia a los antibióticos evaluados. Este estudio evidencia la presencia de resistencia a antibióticos en lactobacilos vaginales aislados de mujeres sanas residentes en la ciudad de Arequipa.

Human milk is a low-biomass fluid whose bacterial community may contribute to infant gut colonization; however, the milk microbiota of women from southern Peru remains uncharacterized. This study aimed to describe the bacterial composition and diversity of human milk at one month postpartum (days 30–33 of lactation) in women from Arequipa, Peru, using full-length 16S rRNA gene nanopore sequencing. In this cross-sectional study, milk was collected from 29 mothers, and 21 samples were analyzed after excluding eight with a relative abundance of Enterobacterales ≥30%. The microbiota was dominated by the phylum Bacillota (formerly Firmicutes; mean 92%), with Streptococcus (62.9%) and Staphylococcus (21.8%) forming the microbial core. Presumptive species-level assignments corresponded mainly to oral and skin commensals (Streptococcus mitis, Streptococcus salivarius, and Staphylococcus epidermidis). Alpha diversity was low (medians: Shannon index 0.66; Simpson index 0.30; observed richness 11 genera). In the principal coordinates analysis (Bray–Curtis), samples clustered according to their dominant genus. Observed richness was nominally higher in mothers with normal body mass index (p = 0.0071), whereas overall community composition was not associated with maternal, obstetric, or dietary variables (PERMANOVA, all p ≥ 0.073). This study provides the first characterization of the human milk microbiota in women from Arequipa, establishing a baseline for future studies.