Table 2. Examples of longitudinal intervention studies, including randomized clinical trials, illustrating the effects of product use on the oral and skin microbiomes.

Oral hygiene interventions have been shown to shift the oral microbiome towards a healthier state compared to the absence of hygiene (as seen in experimental gingivitis studies). Similarly, cosmetic products impact the skin microbiome, with effects varying by body site but they are typically less significant than interpersonal variation. Certain antimicrobials, such as ethanol and povidone iodine, induce short-term, reversible alterations in the forearm skin microbiome. Notably, current studies disproportionately represent populations from Western countries and China, highlighting a gap in global microbiome research coverage (for recent comprehensive reviews, see [171,172]).

	Product type	Study design			Microbiome measurement		Reported outcome
		Population/size	Regimen	Test vs. control	Site	Technique	Diversity analysis and qPCR measurements	Composition (relative abundance)
Oral microbioe	Fluoride toothpaste containing enzymes [31]	Healthy, over 18 years old, 111 subjects completed,UK	Brushing twice a day for 14 weeks	Fluoride toothpaste (1,450 ppm) vs. fluoride toothpaste containing enzymes and proteins (Zendium™)	Supragingival dental plaque	16S rRNA gene sequencing (V4–V6 region)	na	Statistical analysis shows significant increases in 12 taxa associated with gum health including Neisseria spp. and a significant decrease in 10 taxa associated with periodontal disease including Treponema spp.
	Fluoride and arginine-containing dentifrices [84]	53 patients, age≥16 years old	Baseline after a 1-week washout period to brush 2×/day with a fluoride-containing dentifrice, 3 months after use of a 1,450-ppm fluoride dentifrice and 6 months after using a 1,450-ppm fluoride with 1.5% arginine dentifrice	Caries active (n=26) vs.caries free (n=27)	Supragingival dental plaque	16S rRNA gene sequencing (V4–V6 region), shotgun and meta-transcriptomics	Unaffected	After the 3 weeks of fluoride tooth brushing, several caries-associated bacteria were reduced, and there was also an increase in several health- and periodontitis-associated bacteria in both caries and caries-free sites. After the fluoride+arginine dentifrice, there was a further decrease of both caries- and periodontitis-associated organisms, and a decrease of genes from the arginine biosynthesis pathway was also observed, in addition to an increase in the expression of genes associated with the arginine deiminase pathway
	Stannous fluoride toothpaste [173]	Healthy non-smokers over 18 years old, Canada	Preinduction phase of 1 weeks with brush twice daily with a soft manual toothbrush using control dentifrice followed by use of test or control products for 2 weeks, and from day 0, 3 weeks abstaining from all oral hygiene practices followed by dental prophylaxis (day 21) and brushing with test or control products was reinstituted for 3 weeks (day 42)	Control group:(n=17) sodium monofluorophosphate 0.76% toothpastevs.test group (n=16) zinc phosphate toothpaste	Supragingival dental plaque	16S rRNA gene sequencing (V3–V4 region)	Increase of Shannon diversity with abstaining of hygiene practice (both groups); test group has a lower diversity at 0 and day 42 than the control group	Bacteroidales as well as Bacteroidota members including Porphyromonas and Tannerella were impacted by the SnF2 treatment (different abundance after gingivitis resolution at day 42); a similar trend was observed at the species level for Porphyromonas endodentalis and Tannerella forsythia, as well as in other known oral pathogens, specifically within the Treponema genus, which includes species that are highly associated with red complex and periodontitis
	Chlorhexidine mouthwash [174]	Healthy non-smoker adults, UK	10 ml of mouthwash rinse for 1 min, twice/day for 7 days after brushing with a standardized toothpaste	Mouthwash:placebo (n=36) vs. 0.2% chlorhexidine (n=36)	Non-stimulated saliva	16S rRNA gene sequencing(V1–V2region)	Chlorhexidine promoted reductions in Shannon diversity	Increases in the abundance of some genera Escherichia, Hylemonella, Capnocytophaga,Granulicatella, Streptococcus and Neisseria and decreases in Prevotella,Actinomyces, Fusobacterium, Megasphaera,Campylobacter, Lachnoanaerobaculum, Catonella,Corynebacterium, Clostridium and TG5 with chlorhexidine compared to the placebo
	Cetylpyridinium chloride mouthwash [175]	Healthy 18 to 53 years old, China	After 3 weeks of standard regimen and professional cleaning, subjects refrained from mechanical oral hygiene and rinsed twice/day with 20 ml mouthwash for 30 s, for 21 days	Cetylpyridinium chloride (CPC) (n=56) vs. water (n=36)	Supragingival plaque	16S rRNA gene sequencing (V1–V3region)	Αlpha-diversity in the control group exhibited a significant increase from baseline to day 21, whereas that in the CPC group remained stable	After 3 weeks, decrease of 17 with CPC use compared to water, including Porphyromonas, Peptostreptococcus, Prevotella, Peptococcus, Selenomonas, Solobacterium, SR1, Tannerella, TM7 genus, Uncultured_Lachnospiraceae Atopobium, Gemella, Megasphaera, Mogibacterium, Moraxella, Oribacterium and Shuttleworthia; increase of Haemophilus and Lautropia and Neisseria, Capnocytophaga and Propionibacterium
	CPC and essential oils mouthwash [176]	Healthy subjects: age 18–60 years; aminimum 20, USA	At baseline, within 2 weeks of first visit, dental prophylaxis and allocated intogroups; refraining from brushing, rinsing was performed twice daily with 20 ml of the assigned mouthrinse; sample collection after 6 and 12 weeks	Control group n=41 (water rinse) vs. test n=39 (CPC+essentialoil)	Supragingival plaque	16S rRNA gene sequencing (V1–V3region)	Observed and Shannon diversity increased in both groups, although not significant; weighted UniFrac distances in composition were different between baseline and week 12 in the test group	33 OTUs at the end of the study that were differentially depleted in the treatment group compared to the control group included Corynebacterium matruchotii, Corynebacterium durum, several Actinomyces, Fusobacterium, Leptotrichia, Capnocytophaga, Neisseria, Streptococcus, Aggregatibacter, Porphyromonas, Terrahaemophilus aromaticivorans and Lautropia; 40 OTUs were overrepresented in the treatment group at week 12
Skin microbiome	Skin care products (body wash, moisturizer, sunscreen, antiperspirant and soothing foot powder) [32]	12 healthy individuals, USA	No use of any personal care product for weeks 1–3 except a mild body wash; during weeks 4–6, in addition to the body wash, participants were asked to apply selected commercial skin care products at specific body parts; weeks 7–9, return to their normal routine by using the same personal care products as prior to the study; samples collected once a week for most participants	Test products: a moisturizer on the forearm, a sunscreen on the face, an antiperspirant on the armpits and a soothing powder on the foot	Front of the elbow and front forearm, the upper cheek bone and lower jaw,armpits, between the first and second toe and between the third and fourth toe of the foot	16S rRNA gene sequencing(V4 region)	Higher Shannon diversity in arms and face at baseline for both female and male; refraining from using beauty products leads to a significant decrease in armpits and feet; a higher diversity was observed for armpits and feet of all individuals during the use of antiperspirant and foot powder, followed by a bacterial diversity decrease in the armpits when their regular personal beauty product use was resumed	Although the microbiome was site-specific, it varied more between individuals, and this inter-individual variability was maintained over time despite the changes in personal care routine; significant increase in abundance of Gram-negative bacteria Acinetobacter and Paracoccus genera for the armpits and feet of both females and males during the use of antiperspirant, while their abundance remained stable for the arms and face during that time; decrease in abundance of Enhydrobacter in the armpits of males; Cyanobacteria, potentially originating from plant material, also increased during beauty product use, especially in males, in the armpits and face of females and males
	Skin cleansing (soaps) [98]	10 healthy subjects, USA	0.5 ml of product was spread over the volar forearm with a gloved hand for 30 s and then rinsed off with tap water while being scrubbed with a gloved hand for 60 s; 16S samples were taken prewash, 10 min after, 24 h after and 3 days after washing;quantitative PCR (qPCR) samples prewash, after 10 min, 6 h and 24 h	Tests with soft soaps: soap A, lavender and chamomile hand soap; soap B, soothing aloe vera moisturizing hand soap; soap C, rich shea butter moisturizing hand soap; soap D, aquarium series hand soap; soap E, containing benzalkonium chloride; soap F, containing triclocarbanvs.water	Forearm	16S rRNA gene sequencing (V1–V3 region), real-time qPCR for total bacteria and Staphylococcus epidermidis	No significant difference in the alpha-diversity was detected between soap D and water application at any sample time; no significant difference in total microbial abundance as measured by real-time qPCR in 24 h; the antimicrobial compounds benzalkonium chloride and triclocarban did not result in a statistically significant difference in abundance of Staphylococcus epidermidis at any sampling time	No difference in the taxonomic composition of the skin bacterial community between washing with soap D and water
	Ethanol [101]	10 healthy adults, UK	The skin area was cleaned using four 70% ethanol-soaked cotton wool pads (two for the volar and two for the dorsal) forearm by wiping; samples were taken post-wash after drying and at different times (2, 4, 6, and 24 h after wiping) at three different visits	70% ethanol	Volar and dorsal forearm	16S rRNA gene sequencing (V1–V2region), qPCR for total bacteria and Staphylococcus epidermidis	Alpha-diversity (observed and Shannon diversity) was systematically higher for females than males anddecreased with treatment for both males and females and recovered in 6 h; total eubacterial 16S decreased following ethanol wiping and recovered in 6 h; similar trend for Staphylococcus epidermidis, but recovery time was less obvious	Baseline samples taken for each individual at the three sampling visits indicate stability across time,and volar and dorsal sample profiles were highly similar within individuals; the wiping had little effect on relative composition depending on individuals
	Antiseptic agent [177]	13 healthy subjects over 21 years old, USA	A 1-in2 area was swabbed vigorously with ten swipes followed by ten additional swipes in the perpendicular direction before being administered one of four treatments for 1.5 min, using gentle swiping with a cotton pad soaked in 5 ml of the test agent; cotton pads and test agents were treated with UV for 20 min prior to use; sampling at baseline and after 1, 6, 12, 24, 36 and 72 h of administration	Alcohol (80% ethanol) vs. water (UltraPure Distilled Water, Invitrogen) – first visit and ovidone-iodine (10%) vs. chlorhexidine (chlorhexidine-gluconate 4%, results not analysed) – second visit	Volar forearm and the upper back	16S rRNA gene sequencing (V1–V3region), qPCR for total bacteria	Alpha-diversity (Shannon diversity, observed species and equitability) was significantly higher on the forearm compared to back; beta-diversity (UniFrac) indicated that interpersonal variability and site specificity were the most significant contributors to variation; water, alcohol and povidone-iodine all significantly reduced the number of observed species on the forearm compared to adjacent controls at 6 h; water and alcohol were found to decrease overall bacterial load at each body site	Back communities were dominated by Propionibacteriaceae and Staphylococcaceae, while forearm hosted additional taxa, including Streptococcaceae and Corynebacteriaceae; accounting for interpersonal variability, lowly abundant members of the skin microbiota were more likely to be displaced and subsequently replaced by the most abundant taxa prior to treatment, while members of the skin commensal family. Propionibactericeae were particularly resilient to treatment