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Factors affecting the virulence of Photobacterium damselae subsp. damselae to yellowtail kingfish (Seriola lalandi)

Gupta, Nipa (2021) Factors affecting the virulence of Photobacterium damselae subsp. damselae to yellowtail kingfish (Seriola lalandi). PhD thesis, Murdoch University.

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PDF - Whole Thesis
Embargoed until January 2023.

Abstract

Infectious diseases are major threats to the aquaculture industry throughout the world. Yellowtail kingfish (Seriola lalandi, YTK) is a strong candidate for aquaculture in Australia, with good economic potential in domestic and international markets, however, the expansion of this industry has been hindered by invasive bacterial diseases. Photobacterium damselae subsp. damselae (Pdd) is an emerging bacterial pathogen which has been recently found with increasing frequency in both wild and cultured fishes, including YTK, and has already caused millions of dollars in lost aquaculture production. Until now, there have been no studies of the phenotypic and genetic diversity of Pdd isolates in Australia, their virulence to YTK, how this is affected by environmental conditions, and the potential for vaccine development. My project aimed to address these knowledge gaps.

In Chapter 1, I present a critical review of the farming status of YTK and the significant obstacles to developing this industry in Australia and globally. One of the major obstacles is mortality and production losses caused by infectious disease agents, including Pdd. A number of virulence factors have been identified in Pdd, and pathogenicity also varies with environmental conditions, fish species and fish size.

In Chapter 2, a collection of 16 Pdd isolates from Western Australia and South Australia were characterised. This collection comprised five different biotypes depending on their phenotypic and biochemical characteristics and substantial genetic variation was found, with differences among lineages in their haemolytic activity and in-vivo virulence. Photobacterium damselae subsp. damselae isolates (5 out of 16) with the pPHDD1 plasmid and associated virulence genes showed stronger haemolytic activity in-vitro than plasmid-free isolates, which contained only chromosome-I encoded virulence genes. An in-vivo challenge test was performed to determine the pathogenicity of selected Pdd isolates to YTK and revealed that both plasmid-bearing and plasmid-free Pdd isolates were pathogenic to YTK, though the plasmid-bearing isolate was more virulent.

As bacterial virulence is often influenced by environmental conditions, Chapter 3 investigated the impacts of different temperatures (17°C, 24°C, 28°C and 37°C) and salinities (0.5%, 1.0%, 1.5% and 3.5%) on the growth dynamics and expression of virulence genes of Pdd. There were significant differences in haemolytic activity at different temperatures and salinities, with haemolysis being greatest at 24°C and 28°C and at 0.5% salinity. The increased haemolysis at 24°C and 28°C was associated with growth rate and maximum cell density of Pdd, while the increased haemolysis at 0.5% salinity was associated with increased expression of the major virulence gene, dly. Thus, Pdd growth rate, maximum cell density and virulence gene expression all appear to be involved in determining haemolytic activity.

Analysis of host-pathogen interaction was studied in Chapter 4 by using an in-vivo challenge design where individual fish were followed for 8 days of challenge. The LD50 value for infection of YTK with a plasmid-bearing isolate of Pdd was 8.0×104 CFU/fish. A comparison of fish which either survived or succumbed to infection at this challenge dose showed that fish which subsequently died had greater expression of pro-inflammatory and anti-inflammatory cytokine genes than fish which survived.

Preliminary studies on vaccine efficacy were conducted in Chapter 5 and established that homologous and heterologous vaccines provided protection against Pdd infection, with greater production of specific IgM and higher survival rates with homologous vaccines. While this is encouraging, further experiments are required to optimise vaccine production.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Environmental and Conservation Sciences
Centre for Sustainable Aquatic Ecosystems
Harry Butler Institute
Supervisor(s): Lymbery, Alan, Currie, Andrew, Partridge, Gavin and Buller, Nicky
URI: http://researchrepository.murdoch.edu.au/id/eprint/63530
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