Aims: To develop a common serological system for rapid and routine identification of the fish pathogen Flavobacterium psychrophilum. Methods: Thirty‐four isolates of Fl. psychrophilum from different fish species and different geographical areas were typed using a slide agglutination test and an enzyme‐linked immunosorbent assay (ELISA). Results: Seven host‐dependent serovars (1: salmon; 2: trout; 3: trout; 4: eel; 5: carp; 6: tench; 7: ayu) were found. Serovar 2 was divided into two antigenic subgroups (2a and 2b). The results achieved by both slide agglutination and ELISA methods were totally consistent with each other. Although both techniques proved to be simple to carry out and useful, only the ELISA allowed identification of Fl. psychrophilum serovars using unabsorbed antiserum and whole‐cells as antigens. Significance and Impact of the Study: This paper proposes a harmonized scheme for serological identification of Fl. psychrophilum to be used for diagnostic and seroepidemiological studies of the diseases it causes.

The capacity of Pasteurella piscicida strains to survlve contact with macrophages obtained from rainbow trout Oncorhynchus mykiss, sea bass Dicentrarchus labrax and gilthead sea bream Sparus aurata was evaluated using an in vitro assay. Both virulent and avirulent isolates were killed by all the macrophages tested after 3 and 5 h incubation. The increased production of superoxide anion ( 0 2 -) by rainbow trout macrophages infected with P. pisclcida coinciding wlth the highest bactericidal activity (5 h incubation) suggests that the 0,-could be involved in the killing of P pjscicida. Bactericidal activity of cell-free generated 02-confirms this hypothesis, although this indicates that some other microbicidal factors must also contribute to the killing of P. pisclcida by fish macrophages.

An IncP plasmid probe (pUCV2), coding for Cm-r, Km-r and bearing Ap (am) and Tc (am) resistance determinants was constructed by transposition of Tn9 (Cm-r) onto plasmid pLM2 for an efficient selection of potential recipients among natural marine bacteria. Using a Dap-E. coli donor, transmission of pUCV2 to marine bacteria was tested. pUCV2 is transferred to about 4-8% of natural, marine bacterial cells capable of forming colonies on a low nutrient, marine agar medium. The following bacterial genera, commonly found in the marine environment, could be detected when twenty of the transconjugant colonies obtained were identified: Vibrio, Pseudomonas and Aeromonas.