The kinetics of adhesion of Vibrio strains isolated from diseased fish to skin mucus of gilt-head sea bream was studied. A modified Langmuir adsorption isotherm was calculated, and the results obtained indicate that the strains tested (Vibrio alginolyticus DP1HE4 and Vibrio anguillarum-like DC12R8 and DC12R9) showed a saturation kinetics except for V. alginolyticus (CAN), which showed a proportional adsorption kinetics. The adhesive capability for skin mucus does not seem to be an essential virulence factor of pathogenic strains of Vibrio, since this specific interaction depended on several environmental factors, temperature and salinity being the most important. However, the absence of an inhibitory effect of mucus on the pathogenic microorganisms, and the capability of the Vibrio strains to utilize mucus as a carbon source, could favor their settlement on the skin with a potential for infection of cultured, stressed fish.

Several characteristics, including biochemical, serological, drug resistance and plasmid profiles, of strains isolated from clams in southwestern Spain affected with brown ring disease have been comparatively studied. On the basis of 36 standard physiological and biochemical tests, all strains isolated were included in the genus Vibno and further divided into 6 groups. The groups were differentiated on the basis of only 8 phenotypic traits: growth at 35 "C, arginine dihydrolase, gelatinase production, Voges-Proskauer and ONPG tests, and acid production from sucrose, amygdalin and mannitol. Applying these characteristics, the strains resembled Vibno pelagius and V. splendidus species. However, the isolates of each group showed no cross-reactions with the antisera raised against several reference strains of different species of Vibrio, including V. angu~llarum, V. tubiashii, V. damsela, V. pelagius, V. splendidus and the unclassified Vibno PI. Although only 57.7 % of the strains tested harbored one or more plasmids, the majority of the plasmid-containing strains (93.3 %) carried a large plasmid band of 34.4 MDa. A high number of isolates, regardless of their taxonomic group, were resistant to ampicillin and erythromycin. However, all the Vibrio strains were sensitive to chloramphenicol, tetracycline, gentamicin, nitrofurantoin, nalidixic acid and trimethoprim sulphamethoxazole. No correlation between plasmid content and drug resistance was observed.

Summary. The plasmid profiles, and their association with antimicrobial resistance, of 60 strains of Aeromonas hydrophila isolated from fish, shellfish and water were investigated. Only two strains were susceptible to all the antimicrobial agents tested ; the highest incidences of resistance were to tetracycline (93.7%), prystanamycin (93.379, ampicillin (9 1 -7%) and cephalothin (91.7%). Forty strains harboured one or more plasmids and the plasmid profile most frequently detected (15%) was the association of three small plasmids of 4-2, 3.2 and 2.8 Mda. Curing experiments indicated that the plasmid-free derivative strains simultaneously lost their resistance determinants to tobramycin, neomycin, gentamicin and kanamycin. More than 90% of the strains tested produced siderophores and displayed haemolytic activity. However, the relationship between these virulence characters and the presence of plasmids was different; in 74.5% of the strains there was siderophore production and plasmids were detectable, whereas only 60% of the strains simultaneously possessed plasmids and haemolytic activity.

The aetiological agents of the abdominal swelling affecting farmed larvae of gilt-head seabream, Sparus aurata L,, were studied. Four Vibrio strains were isolated from larvae of S. aurata affected by this disease, and all strains reproduced the disease in healthy larvae under controlled infection experiments, producing a significant increase of the mortality rates compared to the control (noninoculated larvae). Several enzymatic properties, which can act as .virulence factors, were demonstrated both in the extracellular products (ECPs) and in live cells of the strains tested, Histopathological examinations of the infected fish larvae revealed important changes of the anterior intestine and liver characterized by a marked hyperthrophy of the intestinal epithelium and hepatocytes, and by a separation of the mucosal and submucosal layers in the digestive tube. These histological alterations were associated with the constant presence of cocobacillar bacteria in the anterior intestine and in the liver. However, the precise pathogenic mechanisms of the strains tested have not been completely elucidated yet.