Influence of long-term heavy-metal contamination on microbial communities in soil

Dot blot hybridization with group specific probes and restriction fragment length polymorphism (RFLP) were used for assessing the effect of long-term sewage sludge application on the culturable bacterial flora and total bacterial community in soil. Results obtained using dot blot hybridization with phylogenetic probes on bacterial isolates showed no dramatic difference in the community structure in two soils with different amendments of heavy metal and sludge. A decrease was seen in the percentage of isolates hybridizing with the probes ALF1b, CF319a and LGCb, with increasing sludge/metal amendment, while the percentages of isolates detected with probes BET42a, GAM42a, SRB385 and HGC69a showed an increase in soil with high-sludge/high-metal amendments. Our results support the general idea that the culturable population represents a part of the total population in soil only. A much higher percentage of clones in the 16S rDNA library of bacteria from high-metal/high-sludge than from low-metal/low-sludge amended soil hybridized with the ALF1b probe. The percentage of cloned inserts hybridizing with the probes BET42a, GAM42a, SRB385, CF319a, LGCb and HGC69a were lower in high-sludge/high-metal amended soil compared to the soil with low-sludge/low-metal amendment. Cluster analysis of the RFLP patterns of the bacterial isolates resulted in a higher diversity index in the high-sludge/high-metal soil compared to the low-sludge/low-metal soil, while cluster analysis of the cloned library from the two soils gave the opposite result.