Abstract
Six consecutively hatched cohorts and one cohort of pre-hatch eggs of farmed barramundi (Lates calcarifer) from south Australia were examined for Chlamydia-like organisms associated with epitheliocystis. To identify and characterise the bacteria, 59 gill samples and three pre-hatch egg samples were processed for histology, in situ hybridisation and 16S rRNA amplification, sequencing and comprehensive phylogenetic analysis. Cases of epitheliocystis were observed microscopically and characterised by membrane-enclosed basophilic cysts filled with a granular material that caused hypertrophy of the epithelial cells. In situ hybridisation with a Chlamydiales-specific probe lead to specific labelling of the epitheliocystis inclusions within the gill epithelium. Two distinct but closely related 16S rRNA chlamydial sequences were amplified from gill DNA across the seven cohorts, including from pre-hatch eggs. These genotype sequences were found to be novel, sharing 97.1 - 97.5% similarity to the next closest 16S rRNA sequence, Ca. Similichlamydia latridicola, from Australian striped trumpeter. Comprehensive phylogenetic analysis of these genotype sequences against representative members of the Chlamydiales order and against other epitheliocystis agents revealed these Chlamydia-like organisms to be novel and taxonomically placed them within the recently proposed genus Ca. Similichlamydia. Following Fredricks and Relman’s molecular postulates and based on these observations, we propose the epitheliocystis agents of barramundi to be known as “Candidatus Similichlamydia laticola” (sp. nov.).
Highlights
Aquaculture is the fastest growing primary industry in Australia and, in 2011, a total of 75,188 tonnes of fish were produced with a value of almost one billion dollars to the Australian economy [1]
Epitheliocystis was seen in all cohorts of fish except for cohort A, which were the oldest fish sampled at 514 dph
We have used several molecular methods to confirm that the agent of epitheliocystis found in barramundi gills farmed in South Australia is a novel member of the Chlamydiales
Summary
Aquaculture is the fastest growing primary industry in Australia and, in 2011, a total of 75,188 tonnes of fish were produced with a value of almost one billion dollars to the Australian economy [1] Established species such as the barramundi (Lates calcarifer) (Bloch, 1790), which was first developed in the mid-1980s and is farmed in every state of Australia except for Tasmania, are strong contributors to this output. Epitheliocystis is a condition of the gills and skin of finfish and is caused by intracellular Gram-negative bacteria It occurs in both wild and farmed fish populations and is currently known to affect over 80 different species of marine and freshwater fish [3,4], including barramundi [5,6]. Similichlamydia latridicola from striped trumpeter (Latris lineata) [11]
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