Here, we review what is known about the microorganisms present in raw milk, including milk from cows, sheep, goats and humans. Milk, due to its high nutritional content, can support a rich microbiota. These microorganisms enter milk from a variety of sources and, once in milk, can play a number of roles, such as facilitating dairy fermentations (e.g. Lactococcus, Lactobacillus, Streptococcus, Propionibacterium and fungal populations), causing spoilage (e.g. Pseudomonas, Clostridium, Bacillus and other spore-forming or thermoduric microorganisms), promoting health (e.g. lactobacilli and bifidobacteria) or causing disease (e.g. Listeria, Salmonella, Escherichia coli, Campylobacter and mycotoxin-producing fungi). There is also concern that the presence of antibiotic residues in milk leads to the development of resistance, particularly among pathogenic bacteria. Here, we comprehensively review these topics, while comparing the approaches, both culture-dependent and culture-independent, which can be taken to investigate the microbial composition of milk.

The complex microbiota of raw milk

Cheese Rind Communities Provide Tractable Systems for In Situ and In Vitro Studies of Microbial Diversity

Traditional cheeses: Rich and diverse microbiota with associated benefits

There is increasing evidence to indicate that nutrition is an important factor involved in the onset and development of several chronic human diseases including cancer, cardiovascular disease (CVD), type II diabetes and obesity. Clinical studies implicate excessive consumption of medium-chain saturated fatty acids (SFA) and trans-fatty acids (TFA) as risk factors for CVD, and in the aetiology of other chronic conditions. Ruminant-derived foods are significant sources of medium-chain SFA and TFA in the human diet, but also provide high-quality protein, essential micronutrients and several bioactive lipids. Altering the fatty acid composition of ruminant-derived foods offers the opportunity to align the consumption of fatty acids in human populations with public health policies without the need for substantial changes in eating habits. Replacing conserved forages with fresh grass or dietary plant oil and oilseed supplements can be used to lower medium-chain and total SFA content and increase cis-9 18:1, total conjugated linoleic acid (CLA), n-3 and n-6 polyunsaturated fatty acids (PUFA) to a variable extent in ruminant milk. However, inclusion of fish oil or marine algae in the ruminant diet results in marginal enrichment of 20- or 22-carbon PUFA in milk. Studies in growing ruminants have confirmed that the same nutritional strategies improve the balance of n-6/n-3 PUFA, and increase CLA and long-chain n-3 PUFA in ruminant meat, but the potential to lower medium-chain and total SFA is limited. Attempts to alter meat and milk fatty acid composition through changes in the diet fed to ruminants are often accompanied by several-fold increases in TFA concentrations. In extreme cases, the distribution of trans 18:1 and 18:2 isomers in ruminant foods may resemble that of partially hydrogenated plant oils. Changes in milk fat or muscle lipid composition in response to diet are now known to be accompanied by tissue-specific alterations in the expression of one or more lipogenic genes. Breed influences both milk and muscle fat content, although recent studies have confirmed the occurrence of genetic variability in transcript abundance and activity of enzymes involved in lipid synthesis and identified polymorphisms for several key lipogenic genes in lactating and growing cattle. Although nutrition is the major factor influencing the fatty acid composition of ruminant-derived foods, further progress can be expected through the use of genomic or marker-assisted selection to increase the frequency of favourable genotypes and the formulation of diets to exploit this genetic potential.

https://hal.archives-ouvertes.fr/hal-01190242/document

Recent developments in altering the fatty acid composition of ruminant-derived foods

The bacterial genus Pantoea comprises many versatile species that have been isolated from a multitude of environments. Pantoea was delineated as a genus approximately 25 years ago, but since then, approximately 20 species have been identified having a diversity of characteristics. Isolates from water and soil have been harnessed for industrial purposes including bioremediation, and the degradation of herbicides and other toxic products. Other isolates possess nitrogen fixation and plant growth-promoting capabilities, which are currently being explored for agricultural applications. Some isolates are antibiotic producers, and have been developed into biocontrol agents for the management of plant diseases. Pantoea is also known to form host associations with a variety of hosts, including plants, insects and humans. Although often thought of as a plant pathogen, recent evidence suggests that Pantoea is being frequently isolated from the nosocomial environment, with considerable debate as to its role in human disease. This review will explore this highly versatile group and its capabilities, its known associations, and the underlying genetic and genomic determinants that drive its diversity and adaptability.

Pantoea: insights into a highly versatile and diverse genus within the Enterobacteriaceae.

The diversity of the microbial community on cow teat skin was evaluated using a culture-dependent method based on the use of different dairy-specific media, followed by the identification of isolates by 16S rRNA gene sequencing. This was combined with a direct molecular approach by cloning and 16S rRNA gene sequencing. This study highlighted the large diversity of the bacterial community that may be found on teat skin, where 79.8% of clones corresponded to various unidentified species as well as 66 identified species, mainly belonging to those commonly found in raw milk (Enterococcus, Pediococcus, Enterobacter, Pantoea, Aerococcus, and Staphylococcus). Several of them, such as nonstarter lactic acid bacteria (NSLAB), Staphylococcus, and Actinobacteria, may contribute to the development of the sensory characteristics of cheese during ripening. Therefore, teat skin could be an interesting source or vector of biodiversity for milk. Variations of microbial counts and diversity between the farms studied have been observed. Moreover, Staphylococcus auricularis, Staphylococcus devriesei, Staphylococcus arlettae, Streptococcus bovis, Streptococcus equinus, Clavibacter michiganensis, Coprococcus catus, or Arthrobacter gandavensis commensal bacteria of teat skin and teat canal, as well as human skin, are not common in milk, suggesting that there is a breakdown of microbial flow from animal to milk. It would then be interesting to thoroughly study this microbial flow from teat to milk.

https://hal.archives-ouvertes.fr/hal-01137155/document

Cow Teat Skin, a Potential Source of Diverse Microbial Populations for Cheese Production

Milk fatty acid composition and cheese texture and appearance from cows fed hay or different grazing systems on upland pastures

This study is the first that assessed the influence of farming practices and individual cow characteristics on a large number of microbial groups (n = 10) and cow samples (n = 192). Its aim was to establish how farming practices and intrinsic characteristics of dairy cows can influence the microbiota on teat skin. Microbial flora of 96 cow teat skin from 16 farms, sampled during milking and before washing, was counted on ten dairy-specific media. Gram-positive catalase-positive bacteria including coagulase-negative staphylococci, were at high level on teat skin (4.7 ± 1.5 log cfu.mL−1) whereas lactobacilli, enterococci, Gram-negative bacteria, moulds and yeasts were at a level below 3 log cfu.mL−1. Gram-positive catalase-positive bacteria and yeasts were lower in heifers and when milking hygiene practices were intensive. Higher Lactobacillus and Enterococcus counts were linked to a silage-based diet, free stalls with straw bedding and moderate milking hygiene but also to multiparous cows. This study showed that dairy cow characteristics could interact with farming practices to affect the counts of microbial flora on teat skin. It offered prospects to better control teat microbial balance taking into account the milking hygiene practices, the parturition and the type of animal housing.

/pdf/variability-of-microbial-teat-skin-flora-in-relation-to-31bcvfwzjm.pdf

F. Monsallier

Papers

Cow Teat Skin, a Potential Source of Diverse Microbial Populations for Cheese Production

Milk fatty acid composition and cheese texture and appearance from cows fed hay or different grazing systems on upland pastures

Variability of microbial teat skin flora in relation to farming practices and individual dairy cow characteristics