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Page 2 of 12 Kikukawa et al. Microbiome Res Rep 2023;2:4 https://dx.doi.org/10.20517/mrr.2022.24

Results: B. adolescentis 12451, B. adolescentis 12-111, B. boum JCM 1211, and Bifidobacterium sp. JCM 7042 showed
high C16:1 content among the tested strains. TFA extracted from Bifidobacterium sp. JCM 7042 contained C16:1 at
2.3% as the fatty acid constituent (2.4 mg/L of broth). Through GC-MS and LC-MS analyses, the C16:1 synthesized
by Bifidobacterium sp. JCM 7042 was identified as 7-cis-hexadecenoic acid (7-cis-C16:1). The authentic 7-cis-C16:1
showed strong and selective antibacterial activity against S. aureus, similar to 6-cis-C16:1, with a minimum
inhibitory concentration (MIC) of < 10 µg/mL. Components that increase C16:1 productivity were not found in the
MRS and TOS media; however, Tween 80 was shown to considerably reduce the C16:1 ratio in TFA. Antibacterial
activity against S. aureus was observed when the TFA extracted from Bifidobacterium sp. JCM 7042 contained high
level of 7-cis-C16:1 (6.1% in TFA) but not when it contained low level of 7-cis-C16:1 (0.1% in TFA).

Conclusion: The fatty acid, 7-cis-C16:1, which can selectively inhibit the S. aureus growth, is accumulated in TFA of
several bifidobacteria. The TFA extracted from cultured cells of Bifidobacterium sp. JCM 7042 demonstrated
antibacterial activity. From a practical viewpoint, our findings are important for developing an efficient method to
produce novel skin care cosmetics, functional dairy foods, and other commodities.

Keywords: Bifidobacterium, screening, hexadecenoic acid, selective antibacterial activity, Staphylococcus aureus,
Staphylococcus epidermidis

INTRODUCTION
Bifidobacteria are Gram-positive, nonspore-forming, nonmotile, and catalase-negative anaerobic bacteria
belonging to the phylum Actinobacteria . The bacteria ferment sugars primarily to lactic acid and acetic
[1]
[2]
acid via a unique metabolic pathway, the so-called bifido shunt . There is accumulating evidence showing
[3]
that bifidobacteria contribute to maintaining human health . The probiotic traits exerted by bifidobacteria
are utilized in functional food production, where fermented milk is one of the most popular foods to be
supplemented. So far, most of the studies on bifidobacteria have been conducted to understand their
diversified saccharide metabolism, which is highlighted by plant poly/oligosaccharides and human milk
[4-6]
oligosaccharide assimilation pathways . Bacteriocin production has also been reported for some
bifidobacterial strains . On the other hand, studies on lipid metabolism in bifidobacteria are limited and
[7]
[8,9]
the pathway for fatty acid biosynthesis in bifidobacteria remains poorly understood [8,10-13] .
Staphylococci constitute a group of major commensal/pathogenic microorganisms of the human skin.
Among them, Staphylococcus aureus (S. aureus) is a well-known coagulase-positive pathogen associated
with many diseases, e.g., atopic dermatitis, bacteremia, and food poisoning [14-18] . S. aureus is rarely detected
on healthy skin (an average of 1.1% of all skin bacteria), but becomes predominant (average: 65%) on the
[19]
skin of patients with atopic dermatitis . It produces several toxins and proteases, which aggravate skin
inflammation by disrupting the epidermal barrier [15,20] . In contrast, Staphylococcus epidermidis (S.
epidermidis) is a coagulase-negative commensal that is detected on the skin of both healthy individuals and
patients with atopic dermatitis. S. epidermidis has positive effects on skin health through the production of a
protein that stimulates the secretion of antimicrobial peptide β-defensin from keratinocytes, a phenol-
soluble modulin that inhibits the growth of S. aureus, a serine protease that inhibits biofilm formation by S.
aureus, and lipoteichoic acid that represses inflammatory cytokine release from keratinocytes [21-27] . Several
studies thus indicate that eradication of S. aureus from skin without affecting colonization by coagulase-
negative Staphylococcus species is one of the promising strategies for treatment of atopic dermatitis [17,18] . In
this regard, isolation of compounds with high selectivity towards harmful S. aureus warrants further
research.