Piglets with the nonspecific gastroenteritis received Colistin 12000000 with drinking water for 5 days is the doses 0.6 g/100 kg in Group 1 and 0.8 g/100 kg in Group 2. This provided the complete therapeutic efficacy (the clinical recovery) on Day 4 in both treatment groups. The dynamics of the erythrocytes and leukocytes suggested the recovery of the morphofunctional homeostasis after the disease in both treatment groups. After the treatment with the Colistin 12000000, the prevalence of the Streptococcus gallolyticus reduced by 8.3% in the Group 1. Klebsiella pneumoniae prevalence reduced by 33.3%. In Group 2, the prevalence of Streptococcus gallolyticus reduced by 41.7% after the 5-day therapy and was 25.0% at the end of the study. Klebsiella pneumoniae prevalence reduced twice, and it was ultimately observed in 16.7% of cases. Other bacteria presented in the washings at the baseline in both groups were no more observed at the end of the treatment. The symbiotic flora was found in both groups at the end of the experiment in a sufficient number. Thus, the efficient dose of the test drug was 0.6 g/100 kg provided the desirable clinical effect comparable to the higher dosing regimen. The drug well tolerated, with no side effects or adverse events reported throughout the study.