Evolution of Pseudomonas aeruginosa cells towards a filterable stage in seawater

In sterile nutrient-free seawater, the number of Pseudomonas aeruginosa culturable cells decreased progressively over time and the bacteria developed cells capable of passing through a 0.45 μm pore membrane. This development was more rapid in non-autoclaved, stirred seawater and the recovery of filterable cells varied depending on the membrane type used. Minicells were observed under an electron microscope. They yielded normal cells in bacteriological media with analogous colonies and an unchanged antibiotic resistance profile. Some biochemical characters, such as gelatinase or urease activity, were however modified in the filterable cells.