MEPS 148:241-250 (1997)  -  doi:10.3354/meps148241

The marine lysogenic bacterium H24, known for the genetic instability of the resident wild-type phage phiH24 when propagated in nutrient-rich medium, was investigated by cultivation in synthetic seawater enriched to various levels of yeast extract plus peptone (YEP, in a relation of 1:5). The incidence of phage mutants was found to depend on a weighed equilibrium of nutrient concentration (tested between 0.6 and 600 mg l^-1) and the bacterial yield allowed by it on the one hand, and on the duration of incubation and the dilution between successive subcultures on the other. With too-low dilutions used in combination with high nutrient levels, non-virulent mutants, followed by virulent ones, increased in concentration until breakdown of the bacterial population occurred. Conversely, extinction of populations was caused by combinations of low nutrient levels with too-high dilution after too-short incubation periods. At low nutrient levels phage mutants did not influence the outcome of the experiments. However, at nutrient concentrations of 6 and 0.6 mg l^-1 release of particles of wild-type phage phiH24 was found to be initiated by nutrient addition inducing sufficiently rigorous bacterial multiplication. The latter findings are discussed as an ecologically promising explanation of how phage-host systems are maintained in nature. The number of free virions of phiH24, which always remained low, rapidly decreased in the cultures but survived in cell-free filtrates stored in the refrigerator for the same period of time. The loss of virions in cultures is attributed to infection of H24_wt cells which due to their immunity remained unharmed. Pseudolysogenization was indicated at nutrient levels of 600 and 60 mg YEP l^-1, but not at lower YEP concentrations.

Marine · Bacteria · Lysogeny · Pseudolysogeny · Phage · Mutation · Nutrients