Abstract
Piscirickettsia salmonis is the causative agent of piscirickettsiosis, a disease with high socio-economic impacts for Chilean salmonid aquaculture. The identification of major environmental reservoirs for P. salmonis has long been ignored. Most microbial life occurs in biofilms, with possible implications in disease outbreaks as pathogen seed banks. Herein, we report on an in vitro analysis of biofilm formation by P. salmonis Psal-103 (LF-89-like genotype) and Psal-104 (EM-90-like genotype), the aim of which was to gain new insights into the ecological role of biofilms using multiple approaches. The cytotoxic response of the salmon head kidney cell line to P. salmonis showed interisolate differences, depending on the source of the bacterial inoculum (biofilm or planktonic). Biofilm formation showed a variable-length lag-phase, which was associated with wider fluctuations in biofilm viability. Interisolate differences in the lag phase emerged regardless of the nutritional content of the medium, but both isolates formed mature biofilms from 288 h onwards. Psal-103 biofilms were sensitive to Atlantic salmon skin mucus during early formation, whereas Psal-104 biofilms were more tolerant. The ability of P. salmonis to form viable and mucus-tolerant biofilms on plastic surfaces in seawater represents a potentially important environmental risk for the persistence and dissemination of piscirickettsiosis.
Highlights
P. salmonis can adhere to living surfaces, such as salmonid macrophages and ova[13,14], as well as form biofilms on abiotic substrates, such as glass, polystyrene plastic, and mussel s hells[7,15,16]
Cytotoxic trials using SHK-1 cells and P. salmonis derived from 288-h old biofilms did not support the attenuation hypothesis
Interactions of the isolate with growth state (F-ratio = 12.37, ***P < 0.001) or with time (F-ratio = 71.20, ***P = 0.001) significantly affected the general patterns of cytotoxicity induced by P. salmonis on SHK-1 cells (Table 1)
Summary
P. salmonis can adhere to living surfaces, such as salmonid macrophages and ova[13,14], as well as form biofilms on abiotic substrates, such as glass, polystyrene plastic, and mussel s hells[7,15,16]. P. salmonis Psal-103 and Psal-104, collected from biofilms as individual cells and/or as aggregates, were cytotoxic for the salmon head kidney (SHK-1) cell line. Both isolates formed biofilms tolerant to Atlantic salmon (Salmo salar) skin mucus in seawater. The formation of P. salmonis biofilms on plastic surfaces indicates the existence of abiotic reservoirs (here defined as any inert surface able to harbor bacteria using a biofilm lifestyle) Such reservoirs in aquaculture settings likely favor the persistence and dissemination of virulent varieties of P. salmonis – varieties able to maintain cell viability and that are tolerant to salmon skin mucus under conditions of severe nutrient starvation in seawater
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