TABLE I.

Probiotic Species and Prebiotic Compounds Studied for Reducing the Incidence, Duration, and Severity of Respiratory Tract Infections (RTI), or Enhancing the Immune Response to Vaccinations Against RTI-causing Pathogens in Healthy Non-elderly Adults

Product class	Probiotic species and prebiotic types studied	Potential impacts on gut microbiota	Potential effects on immune functiona
Probioticsb	Bifidobacteria: animalis ssp. lactis, bifidum, longum ssp. infantis Lactobacilli: acidophilus, brevis, casei, delbreueckii ssp. bulgaricus, fermentum, gasseri, helveticus, lactis, paracasei, plantarum, rhamnosus, salivarius Enterococcus faecium Streptococcus thermophilus	Thought to be transient with minimal impact on overall diversity in healthy populations.70 May help normalize microbiota after perturbation.15	↑Adaptive immune response; ↑Innate immune response; ↑Mucosal immunity; ↓Inflammation; ↑Gut barrier health; ↑Colonization resistance; ↑Cell-surface receptor (e.g., TLR) activation; ↑Organic acids
Prebioticsc	Fructans (inulin, fructo-oligosaccharides, oligofructose); Galactans (galacto-oligosaccharides)	↑Beneficial microbes and metabolites16	↑Adaptive immune response; ↑Mucosal immunity; ↓Inflammation; ↑Gut barrier health; ↑Colonization resistance; ↑Cell-surface receptor (e.g., TLR) activation ↑SCFA production

↓; decrease; ↑, increase; SCFA, short-chain fatty acid; TLR, Toll-like receptor.

^aPotential effects are not necessarily shared across different probiotic strains or prebiotic types, nor have effects been consistently documented in human studies.^{14,15,17,18,53}

^bExamples of species limited to those investigated in randomized controlled trials of healthy non-elderly adults.

^cEstablished prebiotics which, by definition, have been shown to selectively stimulate the growth and/or activity of a limited range of beneficial microbes.¹⁶ Multiple other fermentable oligosaccharides and polysaccharides are known to stimulate the growth and/or activity of a broader population of microbes.