GROWTH AND PHAGE RESISTANCE OF ANABAENA SP. STRAIN PCC-7120 IN THE PRESENCE OF CYANOPHAGE AN-15

The cyanophage AN-15 was found to have a requirement for either 1 mM c alcium or 1 mM magnesium ions to maintain viral stability, whereas 1 m M calcium ions alone were essential for the infection process to proce ed in Anabaena sp. strain PCC 7120. Following prolonged incubation, ph age-resistant cells were detected at a high frequency (approximately 1 0(-5)) in lysates, as either renewed growth in liquid cultures, or as colonies in confluently lysed lawns. Southern hybridisation failed to detect AN-15 DNA in any of the resistant strains, implying that resist ance is unlikely to be due to the presence of temperate phages. A high rate of spontaneous mutation is therefore likely to be the cause of r esistance. Two classes of resistant cells were identified; those in wh ich AN-15 failed to attach to host cells, and those in which attachmen t occurred, but subsequent replication was defective. However, it was possible to overcome phage resistance by the isolation of spontaneous mutants of AN-15, capable of infecting phage-resistant cells. These ob servations imply that if cyanophages are to be assessed as a means of controlling cyanobacterial blooms in freshwater bodies, the ionic (not ably calcium) concentration of the water must be considered, together with the possible need to employ alternative cyanophage strains if res istance to the original one arises.