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Direct-fed microbials (DFM) were originally called probiotics until 1989 when the Food and Drug Administration (FDA) required manufacturers to use the term “direct-fed microbials.”  The FDA defines DFMs as “a source of live (viable), naturally occurring microorganisms.”  FDA does not allow companies selling DFM products to make therapeutic claims, which includes the following:

• Establishing viable bacterial colonies in the gut

• Affecting structure or function of the animal

• Affecting growth or feed intake

• Increasing milk production

• Decreasing morbidity

• Reducing number of sick days

The exception to these “claims” is the approval of a new animal drug application.  Table 1 lists FDA and Association of American Feed Control Officials (AAFCO) approved microbial species for use in DFM products.

DFM Categories

Bacillus – Unique, gram-positive rods that form spores.  These spores are very stable and can withstand environmental conditions such as heat, moisture, and a range of pH.  These spores germinate into active vegetative cells when ingested by the animal and can be used in meal and pelleted diets.

Lactic Acid Bacteria – Gram-positive cocci or rods that produce lactic acid, which are antagonistic to pathogens. Since lactic acid bacteria appear to be somewhat heat-sensitive, they are not normally used in pelleted diets. Types of lactic acid bacteria include:

• Bifidobacterium

• Lactobacillus

• Streptococcus

Yeasts – Not bacteria. These microorganisms belong to the plant group fungi. Six different types of dried yeast products are defined by the AAFCO as ingredients for animal feeding (Table 2).

The concept of DFMs began in the 1950s when researchers observed a positive growth response in animals fed antibiotics.  This led scientists to theorize that intestinal microflora play an important role in the growth of animals.  Further research determined a healthy intestinal tract consists of microflora in a delicate balance between two general types of microorganisms, beneficial and potentially pathogenic.

The coexistence of beneficial and potentially pathogenic bacteria is an important factor in the general health of an animal.  If this balance is upset, the number of beneficial bacteria could decline while the number of potentially pathogenic bacteria could increase, compromising the animal’s health and growth potential.  Feeding DFMs containing live, beneficial bacteria can help to maintain this balance, which may help optimize animal health and growth performance.

Proposed DFM Modes of Action

Production of organic acids – DFMs have been found to produce a number of organic acids.  The most common are lactic, acetic, and formic acids, which inhibit intestinal pathogens.  Organic acids also serve as energy sources to the animal or other beneficial bacteria.

Production of antimicrobials – Research has reported certain strains of bacteria produce bacteriocins, antibiotics, hydrogen peroxide, and other compounds that inhibit intestinal pathogens.

Competitive exclusion – The basic idea behind competitive exclusion is that the beneficial DFM organisms occupy the attachment sites that potentially pathogenic bacteria use and thereby prevent them from colonizing the intestinal tract.

Stimulation of immune response – Research has reported that when animals are fed certain strains of bacteria, the activity of their immune systems increases.

Enzyme activity – Beneficial bacteria, especially Bacillus, produce a variety of enzymes.  Proteases, amylases, lipases, and glycosidases are just a few of the enzymes which may be produced.  This may also explain improvements in feed efficiency that have been observed when certain DFMs are fed.  Bifidobacterium bifidum produces a DNA polymerase that has been reported to be important in repairing damaged cells.

Reductions of toxic amines – Amines, produced by some intestinal microbes, are irritating and toxic, and have been associated with diarrhea.  Lactic acid bacteria have been found to reduce the level of amines in the gut and to neutralize enterotoxins.

DFM Forms and Usage

DFMs are available in a variety of product forms including powder, paste, gel, bolus, and capsules.  They may be mixed in feed, top-dressed, given as a paste, or mixed into the drinking water or milk replacer.  Usage directions vary from single-dose to continuous feeding.

A number of DFM products are currently available.  Most DFMs contain live bacteria; however, some contain only bacterial or fungal extracts or fermentation byproducts.  According to AAFCO, “fermentation product” indicates the product contains microbial cells, while “fermentation extract” indicates the product contains enzymes extracted from a microbial fermentation (cells are not contained in the product).

The effectiveness of DFMs depends on when they are used.  The addition of DFMs to an animal’s diet can assist in the replenishment of beneficial bacteria, resulting in a quicker return to balanced intestinal microflora.  The best response can be observed during the following situations:

·         When young – Normally, a newborn animal must acquire beneficial bacteria from its mother and environment.  Therefore, it is desirable to establish early colonization of the gut with beneficial bacteria.

·         During weaning or dietary changes – At weaning, a young animal’s digestive system may not be sufficiently developed to efficiently change from milk to plant-based rations.

·         Periods of stress – Handling, shipping, vaccination, and other situations can be stressful to an animal, resulting in reduced appetite, which reduces feed intake causing subsequent weight loss or reduced weight gain.

·         Antibiotic therapy – Antibiotic treatments can lower the number and growth of Lactobacillus and other beneficial microbes in the digestive tract.

Handling and Storage

The stability of live DFMs is critical because the microorganisms must be delivered live to the animal to be effective. Therefore, it is important to follow the manufacturer’s storage and handling recommendations.  Most DFMs require storage in a cool, dry area, away from heat, direct sunlight, and high levels of humidity.  After opening, the unused portion should be kept tightly closed to protect the DFMs from loss of viability.

Units of Measure for Bacteria and Yeasts

True, live-organism DFM products must provide a guaranteed number of live microorganisms present that can be substantiated using laboratory techniques.  Unfortunately, the results often depend on how the product sample was originally obtained and handled and the testing lab’s counting methodology.  Therefore, because there is no standardized format, minor differences in technique can dramatically affect final results.

The most common enumeration methods are viable plate count and direct microscopic count.  The viable count is based on the assumption that a single, viable microorganism will grow into one colony in a growth medium.  A series of dilutions are made and dispensed into a petri dish.  After incubation, the number of colonies are counted and multiplied by a dilution factor, giving the number of viable colony forming units (CFU) per gram of product.  In the direct microscopic count, the number of bacteria on a grid are counted under a microscope.  A total count of bacteria is reported, because dead and live cells cannot be distinguished.

Conclusion

Although some uncertainty exists, enough evidence is available to warrant consideration for the use of a DFM in the feeding of various classes of livestock.  Animals that have been stressed seem to respond better to DFM supplementation compared to healthy, non-stressed animals. Therefore, DFM supplementation may have greater application during stressful conditions, such as during parturition and lactation, for neonatal animals, and during disease or environmental challenges. The use of DFMs in animal nutrition will most likely continue to increase.  As our understanding of this emerging technology increases, ADM Alliance Nutrition will make appropriate program changes to enhance swine productivity and efficiency.

Table 1 Microorganisms that are approved by FDA and AAFCO for use in DFMs
Aspergillus niger	Bifidobacterium infantis	Lactobacillus reuteri
Aspergillus oryzae	Bifidobacterium longum	Leuconostoc mesenteroides
Bacillus coagulans	Bifidobacterium thermophilum	Pediococcus acidilactici
Bacillus lentus	Lactobacillus acidophilus	Pediococcus cerevisiae (damnosus)
Bacillus licheniformis	Lactobacillus brevis	Pediococcus pentosaceus
Bacillus pumilus	Lactobacillus bulgaricus	Propionibacterium freudenreichii
Bacillus subtilis	Lactobacillus casei	Propionibacterium shermanii
Bacteroides amylophilus	Lactobacillus cellobiosus	Saccharomyces cerevisiae
Bacteroides capillosus	Lactobacillus curvatus	Streptococcus cremoirs
Bacteriodes ruminicola	Lactobacillus delbrueckii	Streptococcus diacetilactis
Bacteroides suis	Lactobacillus fermentum	Streptococcus faecium
Bifidobacterium adolescentis	Lactobacillus helveticus	Streptococcus intermedius
Bifidobacterium animalis	Lactobacillus lactis	Streptococcus lactis
Bifidobacterium bifidum	Lactobacillus plantarum	Streptococcus thermophilus

 

Table 2 Summary of Dehydrated Yeast Products Defined by AAFCO
Product Name	Species of Yeast	Contains Live Cells	Contains Growth Medium	Feeding Value
Primary Dried Yeast	Saccharomyces	no	no	nutrient content
Active Dried Yeast	Saccharomyces	yes	no	fermentative action digestive aid
Irradiated Dried Yeast	Saccharomyces	no	no	vitamin D2
Brewers Dried Yeast	Saccharomyces	no	no	nutrient content
Torula or Candida Dried Yeast	Torulopsis or Candida	no	no	nutrient content
Yeast Culture	Saccharomyces	some	yes	digestive aid