Self-Transmissible Mercury Resistance Plasmids with Gene-Mobilizing Capacity in Soil Bacterial Populations: Influence of Wheat Roots and Mercury Addition

Abstract

A set of mercury resistance plasmids was obtained from wheat rhizosphere soil amended or not amended with mercuric chloride via exogenous plasmid isolation by using Pseudomonas fluorescens R2f, Pseudomonas putida UWC1, andEnterobacter cloacae BE1 as recipient strains. The isolation frequencies were highest from soil amended with high levels of mercury, and the isolation frequencies from unamended soil were low. With P. putida UWC1 as the recipient, the isolation frequency was significantly enhanced in wheat rhizosphere compared to bulk soil. Twenty transconjugants were analyzed per recipient strain. All of the transconjugants contained plasmids which were between 40 and 50 kb long. Eight selected plasmids were distributed among five groups, as shown by restriction digestion coupled with a similarity matrix analysis. However, all of the plasmids formed a tight group, as judged by hybridization with two whole-plasmid probes and comparisons with other plasmids in dot blot hybridization analyses. The results of replicon typing and broad-host-range incompatibility (Inc) group-specific PCR suggested that the plasmid isolates were not related to any previously described Inc group. Although resistance to copper, resistance to streptomycin, and/or resistance to chloramphenicol was found in several plasmids, catabolic sequences were generally not identified. One plasmid, pEC10, transferred into a variety of bacteria belonging to the β and γ subdivisions of the class Proteobacteria and mobilized as well as retromobilized the IncQ plasmid pSUP104. A PCR method for detection of pEC10-like replicons was used, in conjunction with other methods, to monitor pEC10-homologous sequences in mercury-polluted and unpolluted soils. The presence of mercury enhanced the prevalence of pEC10-like replicons in soil and rhizosphere bacterial populations.

The prevalence and diversity of mobile genetic elements in bacterial communities of different environmental habitats: insights gained from different methodological approaches

“…In contrast, when analysing the e¡ect of mercury pollution in bulk and rhizosphere soils on the transfer frequencies in matings between the bacterial soil community and the rif r recipient strains Enterobacter cloacae BE1, P. £uorescens R2f and P. putida UWC1, increased transfer frequencies in the rhizosphere of young wheat with increasing mercury pollution were found by Smit et al [22]. With P. putida UWC1 as the recipient, transfer frequencies were signi¢cantly enhanced in the wheat rhizosphere in contrast to bulk soil.…”

“…Host range studies revealed a preference for hosts belonging to the L-and Q-proteobacteria. Despite the diversity seen by restriction pattern analysis the plasmids seemed to be related to each other based on hybridisation studies, and thus may share a rep gene distantly related to the IncA/C rep gene [22]. The preferential capture of Hg r plasmids from the rhizosphere of young wheat roots re£ects the dynamics of the plasmid hosts in response to plant growth.…”

“…Functioning of the PCR system was shown with the two IncP-9 plasmids pMT2 and pWW0. Primer systems were also designed for the exogenously isolated plasmids pEC10 [22] and pIPO2 [35,37]. Primer systems for the exogenously isolated Hg r plasmid pEC10 were developed based on the sequence of a 300-bp fragment obtained after ampli¢cation by the IncQ oriV system [22].…”

“…Primer systems were also designed for the exogenously isolated plasmids pEC10 [22] and pIPO2 [35,37]. Primer systems for the exogenously isolated Hg r plasmid pEC10 were developed based on the sequence of a 300-bp fragment obtained after ampli¢cation by the IncQ oriV system [22]. Alignment of the sequence with sequences in the EMBL database revealed that the primer annealing sites were the only stretches of the DNA homology.…”

Stability of soil microbial structure and activity depends on microbial diversity

“…S3) that occurred only in D0 after 10 days incubation. These bands could be attributed to the development of populations that initially shared resistant/tolerant mercury traits or acquired them via mechanisms of gene transfer (Smit et al, 1998;Rasmussen and Sørensen, 2001).…”