Antimicrobial susceptibility testing of Serpulina hyodysenteriae

Summary The macrobroth dilution technique was used to test the in-vitro effectiveness of 4 commonly used antimicrobial agents against 23 Australian isolates and 7 overseas strains of Serpulina hyodysenteriae. Minimum inhibitory concentrations and minimum bactericidal concentrations were determined. The growth of 90% of isolates was inhibited by dimetridazole at a concentration of 4 μg/mL, and by tiamulin at 8 μg/mL Australian isolates resistant to both antimicrobial agents were identified. Lincomycin was less effective than these antimicrobial agents, with 90% of isolates requiring a concentration of 128 μg/mL for inhibition of growth, and 54% being susceptible at 64 μg/mL. Tylosin did not prevent the growth of the majority of S hyodysenteriae isolates tested, and 90% were resistant to concentrations of 128 μg/mL. Resistant isolates came from different geographical areas. Resistance was not related to overall genetic background of the spirochaetes, and was not correlated with the presence of plasmids or the serogroup of the isolates

Corrigendum to “Genetic diversity of Australian isolates of Photobacterium damselae subsp. damselae is associated with virulence to yellowtail kingfish (Seriola lalandi)” [Aquaculture 538 (2021) 736552]

The authors regret that the printed version of the above article contained an error in our documentation of ethics approval for challenge infection of fish. The correct and final version follows. The authors would like to apologise for any inconvenience caused. Our experiment was conducted using methodology approved by the Animal Ethics Committee of the Department of Primary Industries and Regional Development (Permit number: 19-4-14), which follows the principles and guidelines of Australian Animal Care

Genetic diversity of Australian isolates of Photobacterium damselae subsp. damselae is associated with virulence to yellowtail kingfish (Seriola lalandi)

Photobacterium damselae subsp. damselae is an emerging pathogen which recently has been found with increasing frequency globally, in both wild and cultured fishes. The aim of this study was to characterize 16 isolates of Photobacterium damselae damselae from cultured fish in Australia and to investigate their pathogenicity to yellowtail kingfish. Considerable differences in phenotypic and genotypic traits occurred between bacterial isolates, indicating that different clonal lineages can co-exist in the same geographical area. Phylogenetic analysis did not support a monophyletic origin for P. damselae damselae, distinct from the other recognized subspecies, P. damselae piscicida. Of note was the significant relationship between the toxR genetic distance among isolates of P. damselae damselae and differences in their phenotypic profiles, enzyme activities and strength of haemolysis on blood agar plates. Isolates containing the pPHDD1 plasmid, and its associated haemolysin genes, showed a significantly larger haemolytic zone than did plasmid-free isolates containing only chromosome-I encoded virulence genes. An infection trial demonstrated that presence of the pPHDD1 plasmid is not essential for pathogenicity, although the plasmid does increase virulence towards Seriola lalandi. Understanding the intraspecific variability in virulence of Australian isolates of P. damselae damselae will assist the aquaculture industry, in particular the yellowtail kingfish industry, to develop more targeted treatment and control methods for infections with this pathogen

Streptococcus iniae associated mass marine fish kill off Western Australia

Streptococcus iniae causes high mortality in cultured and wild fish stocks globally. Since the first report in captive Amazon river dolphins Inia geoffrensis in 1976, it has emerged in finfish across all continents except Antarctica. In March 2016, an estimated 17000 fish were observed dead and dying along a remote 70 km stretch of the Kimberley coastline north of Broome, Western Australia. Affected species included finfish (lionfish Pterois volitans, angelfish Pomacanthus sp., stripey snapper Lutjanus carponotatus, sand bass Psammoperca waigiensis, yellowtail grunter Amniataba caudavittata, damselfish Pomacentridae sp.), flatback sea turtles Natator depressus, and olive (Aipysurus laevis) and black-ringed (Hydrelaps darwiniensis) sea snakes. Moribund fish collected during the event exhibited exophthalmia and abnormal behaviour, such as spiralling on the surface or within the water column. Subsequent histopathological examination of 2 fish species revealed bacterial septicaemia with chains of Gram-positive cocci seen in multiple organs and within brain tissue. S. iniae was isolated and identified by bacterial culture, species-specific PCR, Matrix-Assisted Laser Desorption Ionisation Time-Of-Flight (MALDI-TOF) and biochemical testing. This is the first report of S. iniae associated with a major multi-species wild marine fish kill in Australia. Extreme weather events in the region including a marked decrease in water temperatures, followed by an extended period of above-average coastal water temperatures, were implicated as stressors potentially contributing to this outbreak