Purpose: To develop assessment method of bacterial virulence based on UV-Vis spectroscopy analysis. The method was based on the measurement of absorbance of DNA extracted from the bacterial cultures during the growth phases. Methods: Non-pathogenic E. coli samples were taken from bacterial cultures in the exponential phase as well as the stationary phase of the bacteria. DNA extraction from the bacteria in growth phases were measured their normalized optical density (OD) at UV-Vis spectroscopy in the range of 240 - 320 nm. The same procedures were performed with pathogenic E. coli. Therefore, four groups containing 15 samples in each group were measured of their OD. Results: In non-pathogenic E. coli, the optical densities at the exponential phase were significantly lower as compared with the stationary phase, while in pathogenic E. coli the optical densities at the exponential phase were not significantly different than the stationary phase. In non-pathogenic E. coli groups, the optical densities at the exponential phase were significantly lower than in the corresponding phase in pathogenic E. coli, while the optical densities at the stationary phase of non-pathogenic E. coli were significantly lower than in the correspondence phase of pathogenic E. coli. Conclusion: DNA quantities of pathogenic E. coli were significantly higher than in non-pathogenic E. coli in the both exponential as well as stationary phases while DNA quantities of pathogenic - E. coli in the exponential phases were not different than in the stationary phase. Therefore the pattern of DNA quantities in growth phases could be considered as virulence indicator for E. coli. This method could be adopted as an alternative to other methods used for determination of bacterial virulence.