ABSTRACT
Metchnikoff (1908) theorized that Lactobacillus delbrueckii subsp. bulgaricus could grow in the intestinal tract of humans and displace any putrefying bacteria that are present. Displacement of this group of bacteria was thought to reduce production of toxic compounds that adversely affect the human body, thus enabling humans to live longer. Research done since Metchnikoff's period has shown that Lb. delbrueckii subsp. bulgaricus neither survives nor establishes itself in the gastrointestinal tract. However, other species of lactobacilli have been reported to provide some beneficial effects through growth and action in the gastrointestinal tract. This group of bacteria and others are now often referred to as probiotics. Although there are other possibilities, cultures most often mentioned as probiotics for humans include Lb. acidophilus, Lb. casei, and Bifidobacterium species. These species along with Propionibacterium species and Lb. reuteri are the ones most often considered for use as probiotics for livestock. All these species can survive and grow in the intestinal tract, and thus have the potential to provide benefits. Certain yeast cultures also are considered as being probiotics for livestock even though the yeasts are not expected to survive and grow in the gastrointestinal tract.
