Table 2.

Description of products for the regulation of the intestinal bacteria population in poultry and their principal effects.

Items	Definition	Mechanism of Action
Probiotics	Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host [21]	Competitive exclusion Production of antimicrobial substances Stimulation of immune system Increased intestinal absorption surface Increased growth performance and feed intake Modulation of respiratory and GI microbiota [22,23,24,25,26,27,28,29,30,31]
Prebiotics	A nondigestible compound that, through its metabolization by microorganisms in the gut, modulates composition and/or activity of the gut microbiota, thus conferring a beneficial physiological effect on the host [32]	Nutrient source for the selective growth of beneficial bacteria of the intestinal microbiota Stimulation of short-chain fatty acids production Inhibition of bacterial adhesion to gut lining Change in mucin production Immunity boost Improvement in intestinal health and functionality. [15,33,34,35,36]
Vitamins	Vitamins are nutritional elements which are necessary for essential activities such as development, growth, and metabolism of cells [37]	Antioxidant effect Reduction in free radicals Increase in mucosal immunity Anti-inflammatory effect Immunostimulatory effects Increase in cellular immunity [37,38,39,40,41,42]
Phytogenic feed additives (or Phytobiotics)	Compounds of plant origin incorporated into animal feed to enhance livestock productivity through the improvement of digestibility, nutrient absorption, and elimination of intestinal pathogens [43]	Increase in growth performance, nutrient digestibility and gut health Introduction into the cell membrane of pathogens and consequent destruction with consequent ions leakage Antioxidant activity Modulation of intestinal microbiota composition [44,45,46,47,48,49]
Organic acids	Primarily composed of short-chain fatty acids (SCFA), also commonly referred to as volatile short-chain fatty acids (VSCFA), such as fumaric, propionic, acetic, lactic, butyric, and others. Other organic acids consist of medium-chain fatty acids (MCFA), and long-chain fatty acids (LCFA) [50]	Lowering pH of GI tract (reduction in acid sensitive bacteria) Potential for incorporation into cell membranes of target cells and promoting the loss of protons or cell ions (such as in Gram-positive bacteria) Promotion of gut health and performance [10,51,52,53,54]