Enterobacteriaceae are Gram-negative bacteria and commensals in the human digestive system, as well as in many animal species. One of Enterobacteriaceae is encapsulated bacterium K. pneumoniae which causes many hospital infections and infections outside of the hospital. Antibiotic resistance is the ability of bacterial division nonetheless of antibiotic concertation. Apart from a genetic point of view, antibiotic resistance can be considered as biochemical mechanisms which include reduced membrane permeability, antibiotic inactivation and efflux pumps. Among the most important multiresistant bacteria are extended-spectrum beta-lactamase-producing Enterobacteriaceae which produce extended beta-lactamases. Extended beta-lactamases are bacterial enzymes with the ability to hydrolyze beta-lactam antibiotics, e.g. penicillin, cephalosporins and carbapenems. In this thesis, 33 K. pneumoniae isolates from Klinički bolnički centar Rijeka were used. Phenotype methods that were used were antibiograms, including a double-disk synergy test and a combination disk test, which, in the end, showed the production of extended-spectrum beta-lactamases. Molecular methods that were used were polymerase chain reaction and Sanger’s sequencing showing the presence of resistance genes OXA-48, TEM, SHV and CTX-M. The most common gene was OXA-48 and then CTX-M, while genes TEM and SHV were equally common. Sanger’s sequencing showed differences between subtype extended-spectrum beta-lactamase, CTX-15, analyzed in two different strains. One of them was chromosomally encoded, while the other one was plasmid with a lot of insertions.