Five different microbiological markers (biotyping, antimicrobial susceptibility, heavy-metal susceptibility, klebocin susceptibility and number of extrachromosomal DNA fragments) were utilized in order to distinguish between Klebsiella ozaenae strains arising from a nosocomial infection outbreak and strains of some species isolated from hospital environment. Antimicrobial susceptibility pattern to 13 different antibiotics was the most reliable maker, because 85% of the outbreak strains shown resistance to 8-10 antibiotics and only two strains of hospital environment had similar pattern. Sensitivity for this marker was 85.0% specificity 93.5% and positive predictive value of 89.5%. Other markers were less efficient with a positive predictive value of 79.2%, for biotyping, 70.4% for klebocin susceptibility, 69.2% for number of extrachromosomal DNA fragments and 60.6% for heavy-metals susceptibility. We discuss the applicability of each one of these markers in different circumstances.