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 $H24 when propagated in nutrient-rich medlum, was investigated by cultivation in synthet~c seawater enriched to vanous levels of yeast extract plus peptone (YEP, In a relation of 1:5). The incidence of phage mutants was found to depend on a balanced equilibrium of nutnent concentration (tested between 0.6 and 600 mg I-') 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 toolow 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 ~nfluence the outcome of the experiments. However, at nutnent concentrations of 6 and 0 6 mg 1-' release of particles of wild-type phage OH24 was found to be initiated by nutrient addition lnducing sufficiently rigorous bacterial multiplicat~on The latter findings are discussed as an ecologically promising explanation of how phagehost systems are maintained in nature. The number of free vinons of @H24, 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,,, cells which due to their immunity remained unharmed. Pseudolysogenization was indicated a t nutrient levels of 600 and 60 rng YEP 1-'. but not at lower YEP concentrations.