EVIDENCE OF A DORMANT BUT INFECTIVE STATE OF THE FISH PATHOGEN PASTEURELLA-PISCICIDA IN SEAWATER AND SEDIMENT

The stability of Pasteurella piscicida strains in seawater and sedimen t microcosms at different temperatures (6 and 20-degrees-C) was invest igated during a 1-month period. Three strains of P. piscicida showed s imilar survival kinetics. By a standard plate count method they surviv ed in water and sediment for only 6 to 12 days, depending on the strai n and type of microcosm. During this starvation period, the metabolic activity of the cells was reduced by more than 80%. Culturable cells o f each P. piscicida strain persisted better in sediment than in water, as well as at 20-degrees-C compared to 6-degrees-C. However, in all t he microcosms, the acridine orange direct counts remained at about 10( 5) cells per ml during the experimental period, which demonstrated tha t P. piscicida possesses a capacity to enter a viable but not culturab le state. Moreover, dormant cells were always resuscitated by the addi tion of fresh medium to the microcosms, since we recovered numbers of culturable cells similar to the acridine orange direct counts. These r esuscitated cells exhibited the same respiration rate as that seen pri or to the start of the experiments. Although the biochemical, physiolo gical, and serological characteristics; lipopolysaccharides; membrane proteins; and plasmid content of P. piscicida strains were unaffected during the starvation conditions, the dormant cells were smaller (dwar f cells) and had increased surface hydrophobicity. The starved cells m aintained their infectivity and pathogenic potential for fish, with 50 % lethal doses similar to those of the original strains.