Rapid-growth mycobacteria were isolated from two cases of cow mastitis with similar clinical appearance and within a narrow time frame. Mycobacteria were isolated on blood esculine agar. The isolated mycobacteria were Gram stained, Ziehl-Nielsen stained and tested for growth at 25°C, 37°C and 42°C, iron uptake, growth on Löwenstein-Jensen (LJ) agar with and without 5% NaCl, arylsulphatase (3 days), tween 80 hydrolysis, tellurite reduction, nitrate reductase and niacin synthesis. Molecular identification was performed using the Mycobacteria GenoType CM and AS tests (Hain Diagnostika, Nehren, Germany). One isolate was additionally sequenced for the hsp65, rpoB, 16S rRNA gene sequence and transcribed spacer sequence (ITS) DNA. Susceptibility testing of isolates was performed on the Sensititre Rapmycol plate (TREK Diagnostic Systems Ltd.) for trimethoprim/sulfamethoxasole, linezolid, ciprofloxacin, imipenem, moxifloxacin, cefepime, cefoxitin, amoxicillin / clavulanic acid, amikacin, ceftriaxone, doxycycline, minocycline, tigecycline, tobramycine and clarythromycine. Gram-positive acid-resistant rods were observed in stained smears. Both strains grew at 25°C, 37°C and 42°C on LJ medium, and on LJ medium containing 5 % NaCl. The conventional biochemical tests for iron uptake, arylsulphatase (3 days), Tween 80 hydrolysis, tellurite reduction and nitrate reductase were positive, while the niacin test was negative. Both isolates were identified by the GenoType Mycobacterium CM as Mycobacterium fortuitum II/ Mycobacterium mageritense, while application of the GenoType Mycobacterium AS kit identified both isolates as belonging to the species Mycobacterium smegmatis. Analysis of the isolate sequences (strain DS) for 16S ribosomal RNA confirmed a 100% identical result with Mycobacterium smegmatis strain INHR2. According to the CLSI criteria, both strains were sensitive to sulfametoxazole/trimethoprim, linezolid, doxicycline, amikacin and tobramycin. The strains differed in their sensitivity to cefoxitim, and both strains were resistant to clarithromycin. There was a strong difference between the isolates in sensitivity toward cefoxitime and tigecycline.Brzo-rastuće bakterije iz roda Mycobacterium izdvojene se iz dva klinički slična slučaja mastitisa krava u relativno kratkom vremenu. Mikobakterije su izdvojene na krvnom agaru s dodatkom eskulina, obojene po Grammu i Ziehl-Nielsenu te im je provjerena sposobnost rasta pri 25 °C, 37 °C i 42 °C, korištenje željeza, sposobnost rasta na Löwenstein-Jensen agaru bez i s dodatkom 5 % NaCl, tvorba arilsulfataze, sposobnost hidrolize tween 80, redukcije telurita i tvorbe nitrat reduktaze i niacina. Molekularna identifikacija izolata provedena je korištenjem testova Mycobacteria GenoType CM i AS. Odsječci gena hsp65, rpoB, 16S rRNK te tzv. (engl. transcribed spacer sequence (ITS)) jednog izolata su sekvencirani. Osjetljivost izolata provjerili smo pomoću SENSITITRE RAPMYCOL ploča (TREK Diagnostic Systems Ltd.) prema trimetoprim/sulfametoksazolu, linezolidu, ciprofloksacinu, imipenemu, moksifloksacinu, cefepimu, cefoksitinu, amoksicilinu s klavulanskom kiselinom, amikacinu, ceftriaksonu, doksiciklinu, minociklinu, tigeciklinu, tobramicinu i klaritromicinu. Gram-pozitivne acido-rezistentne štapićaste bakterije uočili smo u obojenim razmascima. Oba soja rasla su pri temperaturama 25 °C, 37 °C i 42 °C te na podlozi LJ s i bez dodatka 5 % NaCl. U oba soja utvrdili smo sposobnost korištenja željeza, tvorbe arilsulfataze, hidrolize Tween 80, redukcije telurita i tvorbe reduktaze nitrata. Sojevi nisu tvorili niacin. Oba izolata su testom GenoType Mycobacterium CM identificirana kao Mycobacterium fortuitum II/ Mycobacterium mageritense dok su GenoType Mycobacterium AS testom identificirana kao Mycobacterium smegmatis. Sekvencijskom analizom odsječka gena 16S ribosomske RNK soja DS i usporedbom s javno dostupnim izolatima, utvrdili smo potpunu podudarnost sa sojem Mycobacterium smegmatis soj INHR2. Prema CLSI kriterijima, oba izolata su osjetljiva prema sulfametoksazolu s trimetoprimom, linezolidu, doksiciklinu, amikacinu i tobramicinu, a rezistentna prema klaritromicinu. Razlike između sojeva očitovale su se u osjetljivosti prema cefoksitimu i tigeciklinu
