Interaction of Alteromonas sp., a Nonobligate Bacterial Predator, and Bacterial Prey in Artificial Seawater

Changes in cell number and cellular morphology of Alteromonas sp., a nonobligate bacterial predator, and two types of bacterial prey, Moraxella sp. and Micrococcus sp., were investigated in a mixed incubation prepared with sterilized artificial seawater (105-106 viable cells/ml). A marked decrease was observed in the number of bacterial predator when the predator was incubated in the absence of prey cells for 60 days, while a relatively small decrease in the number of predator was found when the prey cells were present. Thus, the prey cells allowed the predator to maintain its number, but did not allow an increase in number.Microscopic observation showed that the predator attached to the prey cells and lysed them. The lysate from the prey cells was available as a nutrient to the predator and to the prey species examined. The lysate nutrients present in the vicinity of the predator probably contribute to the recovery of the prey number. The predator was able to produce plaques during growth on double-layer agar plates containing a high density of prey cells (108-109 viable cells/ml). Furthermore, the turbidity of the mixture of predator and prey (107-108 viable cells/ml) decreased drastically. These results suggested that a high density of prey cells in the vicinity of the predator was necessary for appreciable lysis of the prey cells.In the environmental condition, accumulated particulate organic matter and biofilms of solid matter in seawater would be colonized by potential prey bacteria, and the prey bacteria in turn would be lysed during attack by the predator bacterium.