الفهرس 
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Abstract
An identified isolate of B. megaterium was used as a phosphate dissolving bacterial inoculum and as a host of phages. This isolate was kindly supplied by the Laboratory of Microbiology at the Faculty of Agriculture, Minia University, Minia, Egypt. The efficiency of this isolate of B. megaterium in dissolving phosphate was measured. Low pH values were recorded for the liquid culture of this B. megaterium isolate, and hence high concentrations of soluble phosphate were observed. Bacteriophages specific to the B. megaterium isolate were isolated from a soil sample. The spot test was used for the detection of phages. Bacteriophages of B. megaterium were found to be of widespread occurrence in soils from where the sample had been taken. The single plaque isolation technique was used to purify phages. Ten single plaques of phages specific to B. megaterium, each having a different morphology, were picked and kept as single phage isolates. For each phage isolate in this investigation, 500 ml of high-titer phage suspension was prepared. Titers of B. megaterium phages varied from 23.11x1012 pfu/ml to 4.82x1012 pfu/ml. The characteristics of the ten phage isolates were investigated to see whether they were different types or similar. The infectivity of ten phage isolates was investigated at different pH levels (pH 4 – 12). At pH 8 all phage isolates (ten isolates) formed lysed spots wider than those formed at any other pH tested. This may indicate that pH 8 is the optimum pH for the ten phage isolates under study. The host range of each phage isolate was studied. Each of the ten phage isolates of B. megaterium was tested against four different bacillus species. All phage isolates were infectious to B. megaterium (the main host). In addition, phage No. 3, 5, 7, 9 and 10 were found to be infectious to B. subtilis. Moreover, B. licheniformis infected by phage isolates No. 4 and 8. Finally, B. thuringiensis was found to be resistant to all ten phage isolates. Such result may indicate that the phage isolates which exhibited the same hosts range may be belonging to one phage type. To confirm these hypotheses, further characterizations of the phage isolates are needed to be carried out. The phage isolates of B. megaterium were divided into four groups (groups A, B, C, and D) according to thermal inactivation point. Phage isolates from each group were found to have the same thermal inactivation point. The sensitivity of the isolated phages of B. megaterium to UV radiation (at a wavelength of 254) was studied. According to the sensitivity to UV radiation, the ten phage isolates were divided into four groups. Interestingly, the four phage groups of B. megaterium, which were divided on the basis of the the thermal inactivation point, were found to be the same as those classified on the basis of the sensitivity to UV radiation. These results could indicate that phages of each group are belonging to a single phage type. The longevity in vitro of each phage isolate of B. megaterium was studied. The ten phage isolates were divided into four groups according to similarity of lasting infectivity at room temperature. The four groups of the isolated phages, which were divided on the basis of longevity in vitro results, were found to be the same as those classified according to thermal inactivation point and sensitivity to UV radiation. B. megaterium bacteriophage of each group were negatively stained and examined by electron microscope. All phage isolates were found to be of head and tail type, but the dimensions of heads and tails differed from one group to another. Interestingly, phages of each group (divided on the basis of the thermal inactivation point, sensitivity to UV radiation and longevity in vitro were found to be morphologically similar. The differences in head and tail dimensions of the phages of each group are within the standard deviation and are not statistically significant. Therefore, on the basis of the above-mentioned information, it can be concluded that the phages of each group represent one phage type. i.e., the ten isolated phages of B. megaterium were found to be belonging to four phage types. No single method for characterizing phages is in itself sufficient for complete identification or classification, but these features (thermal stability, longevity in vitro, sensitivity to U.V. radiation, host range and electron micrographs) must be studied all together to give clear differences between the phage isolates tested. In the protection of B. megaterium as phosphate dissolving bacterium against bacteriophages, two techniques were used as an attempt to avoid the depressive effect of bacteriophages on such economically important bacteria. The phage resistant mutants of the tested bacteria exhibited high resistant to their phages, it was of a particular interest to study their efficiencies in dissolving insoluble phosphate in soil and their effect on plant growth in presence and absence of bacteriophage. The tested phosphate dissolving bacterium was used as inoculum for wheat plants in the presence of their phages to determine how the immobilization method may protect the immobilized bacteria against their phages. Generally, on the basis of the obtained results it can be concluded that, the presence of phages specific to P-dissolving bacteria in the soil is one of the most important environmental factors affecting the activity and maintenance of such desired bacteria. Presence of bacteriophages reduced the densities of the applied bacterial inocula and consequently the desired biological activities of these bacteria. Therefore, presence of bacteriophages in the soil may be one of the most important factors behind the failure of bacterial inocula in certain soils. According to the obtained results in this study, the depressive effect of the bacteriophages can be avoided by application of the bacterial inocula in form of alginate immobilized cells. Moreover, isolation of phage resistant mutants of such desired bacterium can be used as well to avoid the phage attack. Therefore, application of immobilized cells or phage resistant mutants of this desired bacterium as a biofertilizer is highly recommended to avoid the phage attack and to promote the efficiency and maintenance of this microorganism in the soil.