The main aim of the dissertation thesis was to obtain a new knowledge about the prevalence and mechanisms of antimicrobial resistance of indicator microorganism Escherichia coli by interpreted reading of the minimum inhibitory concentration and by PCR. The strains were obtained by rectal swabs of calves, pigs, poultry broilers, wild birds, dogs and sheep cheese. Some of the samples from poultry and calves were from clinical cases. Higher values of the minimum inhibitory concentration of fluoroquinolones were recorded in isolates from pigs, poultry, wild birds and dogs. The most common genes encoding plasmid-mediated resistance were qnrB (dogs) and qnrS (avian strains). The presence of the ciprofloxacin variant of the aminoglycoside transferase aac (6 ́)Ib-cr (dogs, poultry) was captured. Cit ampicillinase was detected in 7 poultry E. coli isolates, CTX-M- 1 cefotaximase (one isolate), CTX-M-2 (one isolate). The genes encoding other types of resistance were detected too, concretely: tetracycline resistance – tetA, tetB; sulfonamides resistance - sul1, sul2, sul3; trimethoprim resistance – dfrA, dfrB; streptomycin resistance - aadA; florfenicol resistance- floR. Ampicillin-resistant E. coli obtained from calves had the basic beta-lactamases TEM-1, -2, SHV-1, and tetracycline-resistant isolates had frequent occurrence of the tetA gene, along with integron 1 and transposon 3. The presence of genes encoding virulence and iron metabolism, the ability to form a biofilm were found. The diversity of calf- diarrhea-causing strains were positive to intimine (eaeA), which is often obtained in diarrhea animals, ST1 (thermostable enterotoxin), papC (fimbriae), VT2 (verotoxin) or CNF1 / CNF2 (cytotoxic necrotizing factors) genes. The most frequently detected genes from sheep cheese were papC (fimbriae) and fyuA (yersiniabactin siderophor), one isolate was positive for 3 virulence genes (iss - increased serum survival, cvaC - colicin V, papC). Due to the similarity between avian (APEC) and human urinary tract infections (UPEC), we focused on the detection of common genes and the results showed the most common presence of feoB (iron uptake), sitA (iron / manganese transport), iroN (salmochelin siderophor), iss, iutA (aerobactin receptor) and fyuA. Phylogenetic classification of E. coli to pathogens and commensals indicated more frequent detection of strains belonging to the intestinal commensal groups (B1, A, C). The results may contribute to a better strategy of the use of antimicrobial in veterinary medicine and the detected genes encoding virulence factors to a new insight of the clinical course of the disease.