Pathogenicity and virulence of bacterial strains associated with summer mortality in marine mussels (Perna canaliculus)

Abstract

The occurrence of pathogenic bacteria has emerged as a plausible key component of summer mortalities in mussels. In the current research, four bacterial isolates retrieved from moribund Greenshell࣪ mussels, Perna canaliculus, from a previous summer mortality event, were tentatively identified as Vibrio and Photobacterium species using morpho-biochemical characterisation and MALDI-TOF MS and confirmed as V. celticus, P. swingsii, P. rosenbergii and P. proteolyticum using whole genome sequencing. These isolates were utilised in a laboratory challenge where mussels were injected with cell concentrations ranging from 105 to 109 CFU/mussel. Of the investigated isolates, P. swingsii induced the highest mortality. Additionally, results from quantitative polymerase chain reaction analysis, focusing on known virulence genes were detected in all isolates grown under laboratory conditions. P. rosenbergii and P. swingsii showed the highest expression levels of these virulence determinants. These results indicate that Photobacterium spp. could be a significant pathogen of P. canaliculus, with possible importance during summer mortality events. By implementing screening methods to detect and monitor Photobacterium concentrations in farmed mussel populations, a better understanding of the host-pathogen relationship can be obtained, aiding the development of a resilient industry in a changing environment.

Investigating the effect of bacterial coinfections on juvenile and adult green‐lipped mussels (Perna canaliculus)

“…This is interesting as P. swingsii caused 100% mortality in adults and 50% in juvenile mussels. A similar outcome of adult mussel mortalities due to P. swingsii has been previously reported by Azizan et al (2022), attributed to virulence factors produced by the bacterium. Mortality often occurs in bivalves not as a direct effect of pathogenic assault and only restricted to the host's immune system, but as a consequence of changes in other traits that might have an impact on pathogens' infection processes (Labaude et al, 2017).…”

“…Additionally, no mortalities were recorded within these groups either. In a previous study on P. canaliculus , mortalities were seen within the marine broth injection (control) group within 2–5 days of experimental time, attributed to the act of injection causing disruption to the tissue (Azizan et al, 2022). Typically, marine broth contains all the nutrients (such as minerals, peptone—a source of nitrogen, vitamins and amino acids, yeast extracts—the source of B‐vitamins, and inorganic substances) necessary for the growth of marine bacteria (ZoBell, 1941).…”

“…The cultures were then quantified, and two single‐species ( V. mediterranei or P. swingsii ) and a mixture ( V. mediterranei and P. swingsii ) solution were prepared with a concentration target of 10 7 CFU/mL to be used in the mussel challenge experiment. This bacterial concentration was selected as per previous mussel bacterial infection studies conducted in the same laboratory (Azizan et al, 2022).…”

“…P. swingsii is known to be pathogenic in oysters (Fichi et al, 2015), abalone (Shi et al, 2017), and octopus (Gomez‐Gil et al, 2011). P. swingsii and V. meditteranei have both been isolated and deemed as highly pathogenic strains in a previous study on Greenshell™ mussels (Azizan et al, 2022). As an opportunistic pathogen, V. mediterranei appears to lay dormant in its host, but it can cause disease when protective barriers are breached or when immunosuppression occurs (Prado et al, 2020).…”

“…piscicida , have been found to exhibit dormancy or enter resting states under unfavorable conditions (Magarinos et al, 1994). During favorable conditions, P. swingsii has exhibited the expression of virulence genes ( hsp60 , zm , vcpA , toxR , ompU , mshA , chi , lip , and plp ), attributing to mussel mortalities in a challenge experiment, demonstrating the pathogenic qualities of this bacterium (Azizan et al, 2022). Likewise, V. mediterranei has been reported to possess virulence mechanisms, including adhesion, superoxide dismutase production, and toxin production (e.g., zona occludins toxin ( zot )) (Reshef et al, 2008).…”