Effect of strain origin on siderophore production in Vibrio harveyi isolates

A total of 92 isolates of Vibrio harveyi (as identified by ribotyping) were tested for siderophore activity on a modified chrome azurol sulphate agar. Whilst 4 isolates failed to grow, of the remaining 88 strains, 78.4% were positive for the production of siderophores. There was no correlation between siderophore production and length of time since isolation. There were significant differences between the host groups from which the bacteria were isolated (F = 3.23, df = 3,85; p < 0.05). The colony halos produced by environmental isolates (halo to colony ratio = 2.7) and fish isolates (halo ratio = 2.4) were significantly larger than the halos from invertebrate isolates (halo ratio = 1.6). Pathogenicity in an Atlantic salmon bioassay was barely significantly related to siderophore production (t = -1.87, df = 64; p < 0.1). The maximal non-inhibitory levels of 2,2'-dipyridyl showed no association with siderophores (f = -1.65, df = 30; p > 0.05) nor with pathogenicity (t = 0.16, df = 87; p > 0 05). Siderophores must be considered a virulence factor in piscine infecting V. harveyi but not in strains infecting invertebrates.