DORMANCY IN STATIONARY-PHASE CULTURES OF MICROCOCCUS-LUTEUS - FLOW CYTOMETRIC ANALYSIS OF STARVATION AND RESUSCITATION

Cultures of the copiotrophic bacterium Micrococcus luteus were stored in spent growth medium for an extended period of time following batch culture. After an initial decrease, the total cell counts remained con stant at approximately 60 to 70% of the counts at the beginning of sto rage. The level of viability, as judged by plate counts, decreased to less than 0.05%, while respiration and the ability to accumulate the l ipophilic cation rhodamine 123 decreased to undetectable levels. Howev er, using penicillin pretreatment (to remove viable cells) and flow cy tometry and by monitoring both the total and viable counts, we found t hat at least 50% of the cells in populations of 75-day-old cultures we re not dead but were dormant. Resuscitation in liquid medium was accom panied by the appearance of a population of larger cells, which could accumulate rhodamine 123 and reduce the dye 5-cyano-2,3-ditolyl tetraz olium chloride to a fluorescent formazan, while a similar fraction of the population was converted to colony-forming, viable cells. We surmi se that dormancy may be far more common than death in starving microbi al cultures.