Table 1. Common microorganisms used as microbial feed additives [18].

Product	Genus	Species	Respective effects	Reference
LAB	Lactobacillus	L. acidophilus	Reduces mortality. Improves growth. Improves immune defense mechanisms. Produces active dietary enzymes including protease amylase, lipase, phytase, and protease.	[19]–[22]
		Lactiplantibacillus plantarum
		Lacticaseibacillus casei
		L. gallinarum
		Ligilactobacillus salivarius
		Limosilactobacillus reuteri
		L. delbrueckii subsp. bulgaricus
	Bifidobacterium	B. pseudolongum	Maintains the balance of the intestinal microflora. Limits the risk of infections. Reduces coccidiosis in gut. Protective activity against Salmonella, Listeria and E. coli. Inhibits certain pathogens. Reduces diarrhea.	[22]–[24]
		B. thermophilum
		B. longum
		B. lactis
	Streptococcus	S. bovis	Creates a healthier and more efficient digestive system. Improves fiber digestion. Stabilizes rumen pH. Enhances microbial populations. Improves production performance. Reduces pathogenic bacteria. Improves immune function.	[24]–[27]
		S. faecium
	Enterococcus	E. faecalis	Breaks down the carbohydrates in the feed to produce lactic acid. Improves production performance. Improves animal immunity. Improves the intestinal environment.	[28]–[31]
		E. faecium
		E. faecal
LUB	Megasphaera	M. elsdenii	Improves ruminant performance. A causative agent of milk fat depression in high-producing dairy cows. Stimulates ruminal development in pre-weaning animals. Prevents milk fat depression. Stimulates ruminal development. Controls lactic acidosis. Stimulates production of ruminal VFA.	[32]–[34]
	Propionibacterium	P. shermanii	Increases production of glucose (gluconeogenesis). Spares glucogenic amino acids. Inhibits hepatic lipid oxidation. Reduces CH4 production. Impacts feed intake and feed efficiency. Increases ruminal synthesis of propionate. Increases weight gain and feed efficiency. Decreases the incidence of the metabolic disorders like acidosis and ketosis.	[35],[36]
		P. freudenreichii
		P. acidipropionici
		P. jensenii
Other bacteria	Prevotella	P. bryantii	Diverts the hydrogen flow in glycolysis away from methanogenesis. Favors propionic acid production. Improves lignocellulose processing. Competes with methanogenesis and archaea for hydrogen utilization. Reduces CH4 emissions.	[36]–[38]
	Bacillus	B. subtilis	Supports adequate health, nutrient digestibility. Improves performance of lactating dairy cows. Improves milk lactose yield, and total solids yield of lactating dairy cows. Changes rumen fermentation and bacterial profiles. Improves growth performance. Stimulates GH/IGF-1, and regulates the gut microbiota. Produces active dietary enzymes such as amylase, lipase, phytase and protease.	[19],[21],[39]–[43]
		B. licheniformis
		B. coagulans
Fungi	Saccharomyces	S. cerevisiae	Reduces CH4 production. Enhances nutrient digestion. Induces inflammatory responses. Increases fiber digestion. Increases initial rates of fiber digestion. Increase milk production in dairy cows. Improves ruminal microbial activities. Increases the number of total anaerobic and cellulolytic bacteria. Stimulates lactate uptake. Provides important nutrients and nutritional cofactors that stimulate microbial activities. Provides vitamins such as biotin and thiamine.	[30],[44]–[50]
		S. boulardii
Fungi	Aspergillus	A. oryzae	Improved the feed dry matter digestibility. Improves energy supply of VFA Concentrations. Powerful antioxidative functions. Contains multiple enzymes, especially cellulase and hemicellulose. Improves the fiber type in feed. Enhances nutrient absorption. Produces phytate.	[51]–[53]
		A. niger

LAB = lactic acid-producing bacteria, LUB, Lactic acid utilizing bacteria.