Direct-fed microbials, commonly known as DFM or probiotics, are live microorganisms that, when provided in adequate amounts in the diet, can improve gut microbial balance (Fuller, 1989). Direct-fed microbials are generally categorized into: Bacillus-based, lactic acid-producing bacteria, and yeasts (Stein and Kil, 2006).
Bacillus-based DFM are spore-forming bacteria. Spores are thermostable and survive at low pH, which makes Bacillus-based DFM stable during feed processing and gastric digestion. Bacillus-based DFM produce spores that germinate but do not proliferate in the intestine, which means a constant supply of DFM is required to maintain the microbial population. Lactic acid-producing bacteria are not spore-forming and include Lactobacillus acidophilus, Bifidobacterium bifidum, and Enterococcus faecium. Lactic acid-producing bacteria are able to proliferate in the intestine and sustain a microbial population. However, survival during feed processing is of concern because lactic acid-producing bacteria are not thermostable. Direct-fed microbials are available as a single-species or single-strain product, but most commercial products contain more than one species, strains, and even a combination with yeasts and prebiotics (Liao and Nyachoti, 2017).
Direct-fed microbials, similar to prebiotics, increase the beneficial gut bacterial population mostly by increasing short chain fatty acids (SCFA) production. Short chain fatty acids lower the pH, reduce enteric pathogens, and also stimulate intestinal cell proliferation which maintains gut integrity. The increase in the population of beneficial bacteria also controls enteric pathogens by competitive exclusion. However, the mode of action of DFM seems to be even more comprehensive (Liao and Nyachoti, 2017).
Direct-fed microbials have sometimes been associated with performance improvements when added to swine diets (Zimmermann et al., 2016). Apparently, lactic acid-producing bacteria appear to be more beneficial for weanling pigs to help on gut microbial balance after weaning, whereas Bacillus-based DFM seem to be more beneficial for growing-finishing pigs to increase the digestibility of energy and nutrients in high-fiber diets (Liu et al., 2018). However, the effects of DFM in performance are often inconsistent, probably due to the variation in microbial strains, inclusion rate, feeding duration, as well as stage of production, health status, and husbandry practices (Liao and Nyachoti, 2017). Thus, it is difficult to generalize in terms of the effects of DFM on swine diets.
A concern with use of DFM is the transfer of antibiotic resistance genes to pathogenic bacteria (Amachawadi et al., 2018). Commercial DFM products should identify the probiotics to the strain level and ensure the probiotics do not harbor any transferable antibiotic resistance genes.