MEPS 148:241-250 (1997) - doi:10.3354/meps148241
Investigations of the marine lysogenic bacterium H24. III. Growth of bacteria and production of phage under nutrient-limited conditions
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 H24wt 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
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