Effect of root exudates and bacterial metabolic activity on conjugal gene transfer in the rhizosphere of a marsh plant

Abstract

Genetic exchange between bacteria in natural habitats is believed to be strongly influenced by availability of growth substrates and growth rate. To test this hypothesis, conjugal transfer of an RP4 derivative between Pseudomonas fluorescens and Serratia sp. was studied in a simple sand/plant microcosm and was related to availability of root exudates and bacterial metabolic activity. The presence of the plant (Echinochlora crusgalli) greatly stimulated transfer. Average transfer ratios (T/D·R) were 8.9×10−11 in the rhizosphere and 5.5×10−15 in sand unaffected by the plant root. The bacteria used root exudates as growth substrate and depending on cell density, the metabolic activity ([3H]leucine uptake) of the cells was higher in the rhizosphere than in the sand (around 1.0×10−2 and 0.5×10−2 fmol leucine CFU−1 h−1, respectively). Thus, an apparent correlation between metabolic activity and transfer was observed. Additional experiments, however, suggested that there was no causal relationship between the two. When incubated in a sand microcosm containing root exudates, the metabolic activity of the bacteria increased while conjugal transfer ratios remained constant. Hence, contrary to the accepted view, root exudates and metabolic activity did not appear to be responsible for the stimulation of conjugal transfer in the rhizosphere.

Horizontal gene transfer of atrazine‐degrading genes (atz) from Agrobacterium tumefaciens St96‐4 pADP1::Tn5 to bacteria of maize‐cultivated soil

“…The low number of isolated transconjugants did not allow us to draw conclusions about atrazine treatment and rhizosphere effects on the transfer frequency of pADP1::Tn5. However, it has already been shown for other catabolic genes that (1) the selective pressure exerted by the xenobiotic can enhance the survival and growth of transconjugants in soil,38, 39 and that (2) the rhizosphere can act as a hotspot for plasmid transfer 40–44. In addition, no effects of either atrazine treatment or rhizosphere were observed on the nature of the isolated transconjugants.…”

On the meaning and estimation of plasmid transfer rates for surface-associated and well-mixed bacterial populations

“…There is empirical evidence (Lilley et al, 1996; Kroer et al, 1998; Normander et al, 1998; Hausner and Weurtz, 1999) that – at least for certain plasmid–bacteria combinations – the dependence of conjugation rate on nutrient concentration is not of the first-order Monod form (see Section 1) used in Simonsen et al (1990), but rather exhibiting more of a threshold dependence with little or no conjugation for low levels of nutrient concentration and a nearly constant maximum conjugation rate for intermediate and high levels. (However, see Turner, 2004 for an unusual example of a plasmid whose conjugation rate appears to decrease at high glucose concentrations.)…”

The enhancement of conjugal plasmid pBHR1 transfer between bacteria in the presence of extracellular metabolic products produced by Microcystis aeruginosa

“…In our preliminary experiment using tetrazolium salt, sodium 3 0 -{1-[(phenylamino)-car-bonyl]-3,4-tetrazolium}-bis(4-methoxy-6-nitro)benzenesulfonic acid hydrate (XTT) [27,28], the electron transport system (ETS) activity of E. coli S17-1 (pBHR1) in CBY100 medium with EMPs was significantly higher than in CBY100 medium alone (data not shown). It has been suggested that bacterial metabolic activity correlates positively with the conjugal plasmid transfer frequency [3], so the increased ETS activity that was observed in CBY100 medium with EMPs might be a key mechanism in the high conjugal plasmid transfer in the flasks containing either M. aeruginosa or its EMPs. On the other hand, it is known that the amount of space available for bacteria to colonize, except for factors that enhance bacterial activity, is important for promoting conjugal gene transfer [29].…”

“…To clarify the possibility that conjugal gene transfer can occur in the natural environment, transfer frequencies have been investigated in various terrestrial [1][2][3][4] and aquatic [5][6][7][8][9][10][11][12] environments. Conjugal gene transfer is an important mechanism of gene transfer between bacteria.…”