The data obtained for the microbial population by DGGE were further analyzed by Jaccard’s (3) and Sorensen-Dice’s (4) indexes. Similarity indices are frequently used to study the coexistence of species or the similarity of sampling sites. A matrix of similarity coefficients, between either species or locations, may be used to analyze changes in microbial populations over time or at different locations (Real and Vargas, 1996). Jaccard’s index is one of the most useful and widely used indices to determine similarity between binary samples. Jaccard’s index may be expressed as follows:

] =—— ^—— (3)

nA+nB-nAB

where nAB is the number of bands in both samples A and B, nA is the number of bands present in sample A, and nB is the number of bands present in sample B.

Sorensen-Dice’s index, also known as Sorensen’s similarity coefficient, is also used to compare the similarity of two samples. It can also be applied to the presence/absence of data. The index is described by the expression, the terms of the equations are the same as describe above:

г 2пав

Jn —

nA+nB

2. Results