Evidence for phage-mediated gene transfer among Pseudomonas aeruginosa strains on the phylloplane

As the use of genetically engineered microorganisms for agricultural tasks becomes more frequent, the ability of bacteria to exchange genetic material in the agricultural setting must be assessed. Transduction (bacterial virus-mediated horizontal gene transfer) is a potentially important mechanism of gene transfer in natural environments. This study investigated the potential of plant leaves to act as surfaces on which transduction can take place among microorganisms. Pseudomonas aeruginosa and its generalized transducing bacteriophage F116 were used as a model system. The application of P. aeruginosa lysogens of F116 to plant leaves resulted in genetic exchange among donor and recipient organisms resident on the same plant. Transduction was also observed when these bacterial strains were inoculated onto adjacent plants and contact was made possible through high-density planting.Peer reviewedMicrobiology and Molecular Genetic

Ongoing Phenotypic and Genomic Changes in Experimental Coevolution of RNA Bacteriophage Qβ and Escherichia coli

According to the Red Queen hypothesis or arms race dynamics, coevolution drives continuous adaptation and counter-adaptation. Experimental models under simplified environments consisting of bacteria and bacteriophages have been used to analyze the ongoing process of coevolution, but the analysis of both parasites and their hosts in ongoing adaptation and counter-adaptation remained to be performed at the levels of population dynamics and molecular evolution to understand how the phenotypes and genotypes of coevolving parasite–host pairs change through the arms race. Copropagation experiments with Escherichia coli and the lytic RNA bacteriophage Qβ in a spatially unstructured environment revealed coexistence for 54 days (equivalent to 163–165 replication generations of Qβ) and fitness analysis indicated that they were in an arms race. E. coli first adapted by developing partial resistance to infection and later increasing specific growth rate. The phage counter-adapted by improving release efficiency with a change in host specificity and decrease in virulence. Whole-genome analysis indicated that the phage accumulated 7.5 mutations, mainly in the A2 gene, 3.4-fold faster than in Qβ propagated alone. E. coli showed fixation of two mutations (in traQ and csdA) faster than in sole E. coli experimental evolution. These observations suggest that the virus and its host can coexist in an evolutionary arms race, despite a difference in genome mutability (i.e., mutations per genome per replication) of approximately one to three orders of magnitude

A STUDY OF LARGE BODIES IN AZOTOBACTER AGILE

In the course of cytological studies of the genus Azotobacter, large bodies, buds, L colonies, and other peculiarities were observed in one particular strain, Azoto-bacter agile M.B. 4.4. The balloon shapes that have received attention in recent literature (Dienes, 1946; Shanahan et al., 1947; Tulasne, 1949a) were especially abundant among these morphological oddities. It should be emphasized that, unlike other strains of Azotobacter (Eisenstark et al., 1950), variety A. agile M.B. 4.4 does not go into an encysted stage. This strain is morphologically stable and rarely varies from the typical single and paired forms (figures 1 and 2) when grown on the usual laboratory nitrogen-free media. Even very old cultures contain only these regular cell types. However, when cells are transferred from a nitrogen-free medium to a medium containing beef extract or soil extract, giant cells develop in abundant numbers within 24 hours. Further study indicates that a few toxic substances may produce the same results. MATERIALS AND METHODS The organism used in this study was Azotobacter agile M.B. 4.4, obtained from Professor C. B. van Niel. This organism has been used in research on biochemical mutants (Karlsson and Barker, 1948), and it was for this reason that its cytology was of interest to the present investigators. The basal medium used in this study contained only inorganic salts, glucose, agar, and distilled water (Karlsson and Barker, 1948). Standard nutrient agar and soil extract agar (Allen, 1949) were used to obtain the large forms. Cells were examined by standard procedures of phase microscopy, electron microscopy, and nuclear staining. In order to test for possible single nutritional substances that might stimulate the production of large forms, the following carbon compounds, amino acids, and accessory growth substances were added to the basal medium during the course of experiments: glycine, aspartic acid, glutamic acid, asparagine, KNO3