Björklöf K, Suoniemi A, Haahtela K, Romantschuk M
Department of Biosciences, University of Helsinki, Finland.
Microbiology (Reading). 1995 Oct;141 ( Pt 10):2719-27. doi: 10.1099/13500872-141-10-2719.
The maintenance and transfer of the broad host-range plasmid RP1 in epiphytically growing populations of Pseudomonas syringae was monitored in the phyllosphere of bush bean (Phaseolus vulgaris). When foliage was inoculated with plasmid-containing bacteria, the plasmid was lost from the majority of the cells within 2 d but was stably maintained in 0.8% of the population. A high frequency of conjugation between added donors and recipients was observed under high humidity conditions. In 1 d, the number of transconjugants rose to 10(-1) of the donors and the proportional level of transconjugants continued to increase until 3 d after inoculation. Under these conditions the proportion of plasmid-containing bacteria stabilized at about 0.8% of the total population. The conjugation rate appeared to be in equilibrium with plasmid loss and the slower growth of the plasmid-carrying cells. A factor that influenced the high conjugation frequency observed was the available nutrients provided by the leaf and also, to a lesser extent, the leaf surface itself. Transfer of the plasmid from added donors to indigenous bacteria was also studied, using a donor-specific bacteriophage for counterselection of the donor. Transfer was observed to 10 different species of Gram-negative epiphytically growing bacteria. The bean leaf surface appears to function as a hotspot at least for intraspecific transfer of plasmids in high humidity. The frequency of transfer was higher than in soil or in rhizosphere habitats. This is likely to be the result of an environment that is nutritionally rich in combination with a limited colonizable surface area which permits close contact between the bacterial cells.
在菜豆（Phaseolus vulgaris）叶际中监测了广宿主范围质粒RP1在丁香假单胞菌附生生长群体中的维持和转移情况。当用含质粒的细菌接种叶片时，质粒在2天内从大多数细胞中丢失，但在0.8%的群体中稳定维持。在高湿度条件下观察到添加的供体和受体之间有高频的接合现象。在1天内，接合子数量上升到供体数量的10⁻¹ ，并且接合子的比例水平持续增加，直到接种后3天。在这些条件下，含质粒细菌的比例稳定在总人口的约0.8%。接合率似乎与质粒丢失以及携带质粒细胞的较慢生长处于平衡状态。影响观察到的高接合频率的一个因素是叶片提供的可用养分，在较小程度上还有叶片表面本身。还使用供体特异性噬菌体对供体进行反选择，研究了质粒从添加的供体转移到本地细菌的情况。观察到质粒转移到10种不同的革兰氏阴性附生生长细菌中。菜豆叶表面似乎至少在高湿度下对于质粒的种内转移起到了热点的作用。转移频率高于在土壤或根际生境中的频率。这可能是营养丰富的环境与有限的可定殖表面积相结合的结果，这种环境允许细菌细胞紧密接触。
