Response of the bacterial community to root exudates in soil polluted withheavy metals assessed by molecular and cultural approaches

We have used PCR based on 16S rDNA sequences followed by denaturing gradien t gel electrophoresis (PCR-DGGE) in conjunction with cultivation-based meth ods to describe the effect of artificial root exudates (ARE), of which the composition simulated maize root exudates, on the structural diversity of b acterial communities in various soils differing in the level of contaminati on with heavy metals. The aim of this study was to evaluate the effects of organic compounds of a root exudates as a potential mechanism for selective ly enhancing specific bacterial populations in contaminated soils, leading to the development of shifted communities differing in qualitative and quan titative composition. Soil microcosms were either just enriched with ARE or enriched and, additionally, flooded. To characterise the response of the s oil microflora to the enrichment, PCR-DGGE was applied for assessment of th e total bacterial community structure. Cultivation techniques were used to determine the numbers of total heterotrophic bacteria as well as of pseudom onads (which are considered to be stimulated by components of root exudates ). The community structure of culturable bacteria was studied using the con cept of r- and K-strategists, and isolates from dominant colonies growing o n King's B agar were identified by MIDI-FAME profiling. The results obtaine d showed a significant effect of root exudates on the development of bacter ial populations in soil contaminated with heavy metals. Depending on their availability and conditions prevailing in the habitat (e.g. stronger enrich ment by flooding) different bacterial populations were stimulated, resultin g in generation of different community patterns by DGGE. The most significa nt response to root exudates occurred among the culturable fraction of the soil bacteria. Distribution of bacterial classes (i.e. majority of colonies appeared after 24 h), values of EP (from 0.220 to 0.533) and CD (from 43 t o 88) indices directly showed that the culturable fraction of bacteria was highly affected by the organic mixture simulating root exudates. These exud ates reduced the bacterial diversity towards domination of r-strategists an d the reduction of diversity was greater in soil with a higher contaminatio n level. Furthermore, flooding of the soils enhanced the dominance of fast growing bacteria (over 70% formed visible colonies after 24 h even on day 6 ) and reduced the community diversity (EP and CD indices were from about 0. 291 to 0.425 and from 66 to 87, respectively). (C) 2000 Elsevier Science Lt d. All rights reserved.