Winter is coming... and so is Cold Water Vibriosis (CWV)

Aliivibrio salmonicida causes Cold Water Vibriosis (CWV) and is the most important bacterial disease effecting salmon aquaculture. Standardized fish feeding in the late 1970’s and vaccinations against major bacterial pathogens in the 1980’s lead to global production (mostly from Norway, Scotland and Icelandic farms) of Atlantic salmon reaching 80,000 tonnes in 1986. Motivated by a CWV outbreak in 2011, this 2017 review gives a concise summary of our knowledge of the bacterial pathogen A. salmonicida from the past 30 years of study; including its genetics, virulence, pathogenesis and vaccinations.

Virulence factors

There are few studies describing the role of different virulence factors of A. salmonicida. Regulation of this pathogen appear to be related to the lux operon, and as with most vibrios, limited by iron availability. A. salmonicida is able to grow in iron-limiting conditions due to 1) iron acquisition systems that can operate in a wide range of temperatures; 2) major siderophore Bisucaberin induced at temperatures below 10°C. Proliferation in colder waters is likely attributed to the components of non-siderophore iron acquisition being suppressed at temperatures above 15°C. Cell division of A. salmonicida is highest on solid substrate at 15°C and highest in liquid media at 10°C. Salinity also effects expression of flagellins leading to impaired motility at low salinities. This motile pathogen is capable of sensing and moving towards fish mucus as well as bacterial quorum-sensing molecules.

Pathogenesis
It is assumed, but not known, that the lifecycle of A. salmonicida (like other vibro spp.) consists of free-living and facultative pathogenic phases, with the production of their own bio-film allowing their persistence in the environment for >1 year. Since motility is effected by salinity, salinity impacts their pathogenicity. 12 Norwegian fish farms were sampled from October – June, revealing the persistence of A. salmonicida in the environment, even when no outbreaks of CWV occurred. A further study expanded on this, revealing the pathogen to be consistently present within a 200-250 m radius from aquaculture sites. A. salmonicida enters the host’s blood stream via intact skin, avoiding detection by muting its gene expression. It can alter its microbe-associated molecular patterns (MAMPs), changing them to environmental MAMPs, which go undetected by the Toll-like receptors (TLRs) of Atlantic salmon.

Vaccination
Early vaccinations involved immersion of Atlantic salmon in formalin-treated bacterin (immersion immunisation), giving limited and short lived protection. All subsequent vaccination involved injection of formalin-treated cells, giving longer lasting protection. Physiological state of the fish and water temperature during immunisation is very important to its success, owing to antibody production being greatest in relatively warm waters.

In conclusion, this paper is useful for highlighting gaps in the knowledge of CWV. The full life cycle, as well as its environmental source remains unknown and avoidance of the hosts TLR system needs further investigation. I feel this review could be more detailed and assumes a lot of the knowledge of the reader without providing referral to supporting material or much explanation. Lastly, the economic impacts of A. salmonicida are not discussed which I think could be used to add to the importance of this study and motivation for further research.

Reference: 

Kashulin, A., Seredkina, N. and Sørum, H. (2016). Cold-water vibriosis. The current status of knowledge. Journal of Fish Diseases, 40(1), pp.119-126. 

TEP's: not just from plankton!

Transparent Exopolymer Particles (TEP) are found in both marine and freshwater systems and can have a huge impact on coagulation of suspended matter, resulting in marine snow that sink rapidly to the benthos (Alldredge and Silver, 1988). These sinking aggregates can act as a substrate for bacteria, or as a potential food source for benthic and pelagic organisms (Heinonen, Ward and Holohan, 2007).

TEP has been seen to be produced by both phytoplankton and bacteria, however its production by macro-organisms had previously not been studied. A variety of marine organisms secrete mucopolysaccharides in the form of mucus sheaths, webs, nets or mucus covered appendages (Heinonen, Ward and Holohan, 2007). Ward et al., 1993, stated that due to the widespread use of these mono-polysaccharides, some of this material is probably released in a dissolved form or is sheared off the organism’s appendages. These mucins are chemically similar to TEP precursors so may contribute to the TEP pool (McKee et al., 2005).

Heinonen, Ward and Holohan, 2007, hypothesised that mucopolysaccharides released by benthic suspension feeders enhance the ambient concentrations of TEP. DOC may also serve as an indicator of TEP production by suspension feeders, as TEP forms from dissolved precursors. For this study, individuals of the blue mussel (Mytilus edulis), bay scallop (Argopecten irradians), slipper snail (Crepidula fornicata) and two species of solitary tunicates (Ciona intestinalis and Styela clava) were collected to determine whether these organisms could produce detectable levels of TEP under controlled conditions.

The results from this study show that all benthic suspension feeders used in this study enhanced TEP concentrations above background levels after 5 hours of active feeding.  They measured low bacterial abundance and growth rates throughout the study, so can assume the heightened TEP levels were not greatly influenced by bacteria. The two bivalves studied (M. edulis and A. irradians) had the potential to produce the greatest amount of TEP per unit dry mass, followed by the two tunicate species (Ciona intestinalis and Styela clava). They suggest that C. fornicata has the lowest potential to produce TEP due to it’s feeding mechanisms and use of a high-velocity mucus which would have lower solubility – so fewer mono-polysaccharides would be released into the surrounding waters.

The results seen in this study did not support the hypothesis of DOC concentrations being a good predictor of TEP concentrations, as only S. clava significantly enhanced DOC concentrations over background levels after 5 hours. However, they have been able to show that mono-polysaccharide production by benthic suspension feeders does contribute to the TEP pool.

This study provided us with a very interesting insight into the production of TEP by organisms other than plankton and bacteria. In terms of taking this study further, it seems as though very few studies have looked into the consumption and degradation of TEP, as well as its distribution away from the source.  This could be important for learning more about TEP’s role in marine systems, and so may be a useful direction to take this field of research in.

Studied paper

Heinonen, K., Ward, J., and Holohan, B. (2007). Production of Transparent Exopolymer Particles (TEP) by Benthic Suspension Feeders in Coastal Systems. Journal of Experimental Marine Biology and Ecology, (341), 184-195.

References

Alldredge, A., Passow, U., Haddock, S. (1998). The Characteristics and Transparent Exopolymer Particle (TEP) Content of Marine Snow Formed from Thecate Dinoflagellates. Journal of Plankton Research, (20), 393–406.

McKee, M., Ward, J., MacDonald, B., Holohan, B. (2005). Production of Transparent Exopolymer Particles (TEP) by the Oyster (Crassostrea virginica). Marine Ecology Progress Series. (248), 141–149.

Ward, J., MacDonald, B., Thompson, R., Beninger, P. (1993). Mechanisms of Suspension Feeding in Bivalves: Resolution of Current Controversies by Means of Endoscopy. Limnology and Oceanography, (38), 265–272.

There she blows... investigating the microbiomes found within a whale’s blow.

“Whales” and “microbiology” aren’t two words you often hear in the same sentence, however, like most animals on earth, microbes play a key role in their existence. This study looks at the microbiome found within the blow of humpback whales (Megaptera novaeagiliae) and how this may provide an insight into the health of the individual whales as well as other species.

Numerous large whale populations worldwide are currently listed as critically endangered, to know how to best conserve these populations it is key to investigate how anthropogenic factors impact cetacean’s health. Due to the size and the nature of these animals, monitoring their health is no easy feat, therefore previous studies tend to focus on dead, beached or captive individuals. Like human breath, the blow is an exhalation from the pulmonary system, which could be home to a core microbiome as well as pathogens.

The experiment was done using a sterilized drone, this would hover 2-4 m above the whales blow and using mounted PCR plates collect samples from the blow. The previous method for this was using a boat and a long pole to collect the blow samples but drones provide a less intrusive and safer method for both the whale and the researcher. The samples were collected in 2 locations: Race point-cape cod, Massachusetts and around Vancouver Island, British Colombia. The samples from the blow were compared to air samples taken using the same method, as well as sea water samples all in the same area; this was to determine if the blow microbiome was significantly different to the surroundings.

After sequencing and comparing partial small subunits of the rRNA of bacteria and archaea it was shown that blow microbiomes were 50-90% similar between individuals and between the different sample sites, but these microbes were significantly different to the seawater. The humpback blow contained diverse assemblages of both abundance and phylogeny of archaea and bacteria (eukaryotic genes were not tested for). 26 OTU’s were present across all humpback whale blows with 25 of these considered to make up the core pulmonary microbiome. Most of these, when compared to genetic data bases, were like those found in other species of marine mammal; such as the mouth and blow hole of a Bottlenose dolphin (Turpsiops truncates).

This study offers a new less intrusive way of sampling, as well as finding evidence for a core pulmonary microbiome between healthy individuals and species. However, this method needs significant improvement as the drone can only provide low volume samples, increasing risk of contamination. Additionally, they only tested “healthy” whales, therefore an interesting further point of study would be to characterise the microbiome of an un-healthy whale. This study provides plentiful opportunities for future studies as well as a baseline understanding of the bacterial and archaeal composition of a whales blow which may also prove useful and comparable across numerous cetacean species. It’s also an exciting new insight into whale health and the use of technology to further our understanding of a fairly under researched area.

Bibliography

Apprill, A., Miller, C., Moore, M., Durban, J., Fearnbach, H., & Barrett-Lennard, L. (2017). Extensive Core Microbiome in Drone-Captured Whale Blow Supports a Framework for Health Monitoring. Msystems, 2(5), e00119-17. http://dx.doi.org/10.1128/msystems.00119-17

Bacteria like their Oil Hot

Microbial processes are known to be involved in the biodegradation of petroleum hydrocarbons. The occurrence of hydrocarbon-degrading bacteria in polluted sites, their role in degradation and their ecological significance have been well documented. However, generalised trends of microbial degradation potential are not often elucidated due to the difficulties in comparing communities across varied systems. Moreover, high impact cases such as the Deep Horizon Oil Spill have seemingly received more attention in comparison to more chronically polluted sites. Bargiela et al. (2015) therefore conducted a large scale study in the Mediterranean and Red Seas to investigate drivers of bacterial biodegradation potential. Understanding the microbial degradation capacity in the the Mediterranean Sea is crucial as it has a continual, high level of oil pollution and a water turnover time of 70-90 years!

Bargiela et al. (2015) used an integrative approach to compare the influence of environmental, geographic and anthropogenic factors on seven chronically exposed oil polluted sites across the Mediterranean and one in the Gulf of Aqaba, Red Sea. Further to this, they compared these sites with meta-sequences obtained from the Deepwater Horizon oil spill, a site that was otherwise pristine. Initially, they characterized community composition through taxonomic barcoding. They then used a metagenomic approach to assess whether microbial communities would be predicted to have different degradation capacities in relation to environmental conditions. They were able to reconstruct a meta-network that specified with a confidence of at least 90%, the total relative abundance of catabolic genes involved with degradation reactions, and the number and identity of substrate pollutants or intermediates likely degraded. Subsequently, they conducted a metabolome-wide scan on sediment samples, using a combination of mass spectrometry with liquid chromatography separation, to assess whether chemical diversity would also affect microbial distribution and diversity.

A striking finding of this study was that in the chronically polluted, high temperature sites, although total bacterial diversity was lower, the catabolic diversity (i.e. pollutant degradation) was higher, than those of bacterial communities residing in lower temperature sites. Notably, they also found that bacteria in chronically polluted sites had an increased degradation potential than that of microbial communities in pristine sites, as assessed by the metagenomic analysis. Further to this, they found an increased abundance of genes encoding alkane degrading enzymes in the chronically polluted sites. The authors state that collectively, these findings suggest that microbial communities in warmer, chronically polluted sites may be more catabolically versatile and thus better able to respond to accidental oil spills than microbial communities found in pristine sites.

Understanding the role of temperature in catabolic activities of bacteria is crucial as oil pollution is occurring and will occur in parallel to global warming. Bargiela et al. (2015) provide correlative evidence suggesting that temperature may be used as a predictor of marine bacterial catabolic diversity. Previously, it was well known that temperature affects the rate of biodegradation (Leahy and Colwell, 1990), however it has not been established prior to this study the extent to which temperature, and other environmental parameters, regulate hydrocarbon metabolism. Future studies should explore this in more detail, by performing mesocosm-type experiments to directly investigate the effect of temperature on microbial composition and biodegradation capacity. Clearly, in a multi-stressor world, the combined effects of ocean warming, acidification, and other stressors are likely to be different to those of individual stressors. Therefore, future studies should aim to include multiple stressors in their experimental design.

Reviewed paper:

Bargiela, R., Mapelli, F., Rojo, D., Chouaia, B., Tornés, J., & Borin, S. et al. (2015). Bacterial population and biodegradation potential in chronically crude oil-contaminated marine sediments are strongly linked to temperature. Scientific Reports, 5(1). http://dx.doi.org/10.1038/srep11651

References:

Leahy, J., & Colwell, R. (1990). Microbial degradation of hydrocarbons in the environment. Microbiology And Molecular Biology Reviews, 54(3), 305-315.