The main objective of this paper is to try to understand why certain species of pathogenic bacteria can be species specific. Filter feeding bivalves process a lot of water and because of this they are exposed to a lot of bacteria. Most of these bacteria are degraded (in Mytilus edulis rates of up to 10⁹ cells an hour) however certain Vibrio species have been shown to inhibit this degradation. Bivalves’ first line of defence against pathogens is through the use of haemocytes, which from what I gather are kind of their version of leucocytes. Previous studies had shown that pathogens induced greater rounding of haemocytes in bivalves of which they are known pathogens than species of which they are non-pathogenic. This study tries to see if the interaction between pathogens and haemocytes is possibly the reason for certain pathogens specificity.
In this experiment they wanted to see the degree of rounding nine species of vibrios had on seven species of bivalves. Haemocyte monolayers were prepared by adding haemolymph from the bivalve samples to 24-welled flat bottomed microtitre plates. Haemocytes were given time to adhere and then any surplus haemocytes were removed. Washed vibrios were then added to the monolayer at a concentration of 50 bacteria per haemocyte. They were then incubated for three hours at 20°C. The rounded cells were counted through the use of photographs.
Overall there was some degree of specificity. M.edulis was the most sensitive species with six of the nine vibrios rounding at least 80% of the haemocytes. Two species of Vibrio were highly specific for individual species of bivalves with another Vibrio species which only affected the clam species.
They conclude that the most likely reason for this specificity is probably due to the receptors present on the haemocytes. It’s the most exciting or inventive conclusion but makes sense as there have been many recent papers which have shown toxins to be specific to certain phagocytic cells. Overall I thought this was a very interesting and well written paper with the added bonus of only being four pages long. What I find particularly interesting is that the main bacteria which cause this rounding are vibrios which are also the main marine human pathogens. Papers have shown that many of the adaptations which vibrios possess allowing them to infect humans have evolved in their natural environment for other reasons. Possibly they use this adaptation to defend themselves from haemocytes when infecting humans. This could explain why immunocompromised individuals are more susceptible to vibrio-related illnesses.
A review of: Lane, E. and Birkbeck, T. (2000) Species specificity of some bacterial pathogens of bivalve molluscs is correlated with their interaction with bivalve haemocytes. Journal of Fish Diseases 23: 275–279.
8 comments:
what does rounding a haemocyte mean? is it that the haemocytes are specific to the vibrio or the other way around because that would be interesting in as far as who is changing who and why?
Basically a rounded haemocyte means there is no pseudopodium formation which would highly suggest that no further phagocytosis is occuring. So the vibrios cause this rounding to the haemocyte (probably by specific receptors or toxins) and stop phagocytosis. Interestingly only bacteria which are suspended in a haemolymph diluant cause this rounding. Haemolymph has been shown to contain opsonins which help in phagocytosis which could suggest that some form of intial phagocytosis is required for further inhibition. Maybe a lot of vibrios are sacrificed to inhibit the haemocytes individually which could explain why such high concentration of vibrios are needed and why there are low levels of disease in adult bivalves. This could also explain the higher mortality rates in larvae which may have much lower levels of lysozyme. To be honest there are still a lot of unanswered questions in this areas.
Hi Matt, this is a really interesting paper! Am I right in thinking that, as we've seen from lectures, its the mussels own immune system which ends up being the death of it? As if it didn't respond the vibrios wouldn't be able to take over? I wonder if other bacteria are capable of the same thing and whether vibrios infect other organisms via their haemocytes...?
well that seems to make sense thanks for the clarification :)
there are always so many questions!
Alice: Forgot to mention that the specificity does go both ways. I think it probably is the vibrios causing the haemocyte to round but haemocytes will target and moves towards specific bacteria aswell.
Natasha: The vibrios arent really infecting the bivalves via the haemocytes they re just stopping them from working or else they would just engulf all of the vibrios. I dont think it really is an overreaction of the immune system, mainly because the immune system actually is really good and there are low levels vibrios infection in wild. The reasons why there are higher infections in aquaculture and larva is possibly due to vibrios being concentrated to undefendable levels in farming areas and possibly that larva dont have enough haemocytes to control vibrio infection. However if the toxins they use to inhibit haemocytes are the same that damage the bivalves i suppose without haemocytes they would better off, to be honest I dont really know.
Hey Matt, good paper,I have a question regarding the specificity aspect. Surely if the vibrios are inhibiting the mussels ability to phagocytose them, wouldn't that mean that their immune system is repressed and that any bacteria would them be able to infect the mussel voiding the specificity aspect? or do the Vibrio species present prevent the invasion of other species?
Umm i dont really know. The bivalve immune system is really complex and phagocytosis isnt their only methods of defence from pathogens. Haemocytes can can produce many different cytotoxic reactions including production of anit-microbial peptides and lysosomal enzymes. The haemolymph itself also produces these products so their immune system wouldnt be completely suppressed and they could possibly deal with other bacteria. However there is evidence that some bacteria like vibrio cholerae (some of the nasty strains) can mess up communication signals of the immune system which i suppose could suppress the immune system allowing other bacteria to take advantage. The main problem is when human pathogens like vibrios can survive and multiply in bivalves which then act as resevoirs of these harmful bacteria (may have no effect on bivalves). This then would increase the risk of disease among the idiots who consume shellfish raw. I dont really know if they prevent invasion of other species. They have been shown to stop bivalves filter feeding but whether its for this purpose or a defence mechanism of the bivalve i really dont know.
Ok cool, its interesting that they can prevent them filter feeder and a lot of bacteria are probably acquired in this way! Thanks!
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