Bias caused by using different isolation media for assessing the genetic diversity of a natural microbial population

The influence of isolation medium on the biodiversity of Burkholderia cepac ia strains recovered from the rhizosphere of Zea mays was evaluated by comp aring the genetic diversity of isolates obtained by plating serial dilution s of root macerates on the two selective media TB-T and PCAT. From each med ium, 50 randomly chosen colonies were isolated. On the basis of the restric tion patterns of DNA coding for 16S rRNA (16S rDNA) amplified by means of P CR (ARDRA), all strains isolated from TB-T medium were assigned to the B. c epacia species, whereas among PCAT isolates only 74% were assigned to the B . cepacia species. Genetic diversity among the PCAT and TB-T isolates was e valuated by the random amplified polymorphic DNA (RAPD) technique. The anal ysis of molecular variance (AMOVA) method was applied to determine the vari ance component for RAPD patterns. Most of the genetic diversity (90.59%) wa s found within the two groups of isolates, but an appreciable amount (9.41% ) still separated the two groups (P < 0.001). Mean genetic distances among PCAT isolates (10.39) and TB-T isolates (9.36) were significantly different (P < 0.0001). The results indicate that the two different isolation media select for B. cepacia populations with a different degree of genetic divers ity. Moreover, a higher degree of genetic diversity was observed among stra ins isolated from PCAT medium than among those isolated from TB-T medium.