Virulence of clinical Mycobacterium tuberculosis strains in Lodz, Poland

SETTING: The transmission of tuberculosis (TB) in the population and the development of disease are determined not only by the patient's immunological status, but also by the virulence of Mycobacterium tuberculosis strains. OBJECTIVE: To examine the virulence of M. tuberculosis clinical isolates with recognised transmission collected from 2006 to 2007 in a population in Lodz, Poland. METHODS: A total of 36 isolates were studied to determine their sensitivity to human neutrophil peptide 1 (HNP-1) and intracellular growth within THP-1 cells. Bacterial strains were cultured using HNP-1 at different concentrations. After incubation, the number of colony-forming units (cfu) was determined by bacteria plating. The intracellular survival was examined on days 3, 6 and 8 post-THP-1 infection by cfu enumeration. RESULTS: Overall, 69% of the isolates showed greater resistance to the highest HNP-1 concentration (15 mu g/ml) than the virulent H37Rv strain, and the growth of 10 strains was totally inhibited. On day 8, 56% of the strains displayed higher cfu numbers than the virulent H37Rv strain. CONCLUSION: The results suggest that isolates from our urban population represent highly virulent phenotypes. We could not find any significant difference in virulence between strains with unique genotypes and those in clusters.