The importance of the intestinal microbiota has attracted much interest in recent years particularly with respect to ways in which the microbiota can be manipulated in order to improve health. Improving gut health through the use of probiotic microorganisms has become an area of research activity in both human and animal nutrition. However, the amount of research using companion animals is insufficient. The present review evaluates and compares the effects achieved after application of canine-derived probiotic strain Lactobacillus fermentum CCM 7421 to healthy dogs as well as to dogs suffering from gastrointestinal disorders. The experiments involved varying duration of application (4 days14 days), dosage (10(7)-10(9) CFU), form of application (fresh culture or lyophilized from) or combination with natural substances. Results from nine independent studies show the ability of probiotic strains to establish themselves in the canine gastrointestinal tract, alter the composition of intestinal microbiota and metabolites (organic acids), and modulate the physiology (serum biochemical parameters) and immunity parameters in dogs. Almost all observed effects were irrespective of dose or duration of probiotic administration.