Fibrinal blood clots are essential defences that present
a barrier to pathogens in order to protect the organism’s internal environment.
The blood clot is also vital in activating many other innate immune responses
within an organism. A common bacterial species that infects marine organisms is
Vibrio harveyi, and is immediately
captured within clots. However, this bacterium has been shown by Chaikeeratisak
et al. (2014) to utilise serine- and metallo-proteases
in order to escape clots in just over two hours after entrapment.
The infection of the Pacific white shrimp, Litopenaeus vannamei, by V. harveyi
was studied, with the behaviour of the Vibrio
noted. As this species of shrimp is very susceptible to infection from Vibrios, and is an important part of the
aquacultural industry, it was deemed essential to record the performance of the
blood clot in the defence against infection.
Freshly drawn haemolymph from the shrimp was mixed
with washed, free swimming V. harveyi
cells and allowed to clot in order to estimate the blood clots’ efficiency. The
result showed that 84% of the Vibrio cells
were entrapped during the clotting process (30 mins, room temperature), with
cytolysis not playing a significant role in bacterial removal. This suggests
that the clot is the principal process for removing bacteria.
Microscopic analysis revealed that immediately after
entrapment, there was no flagellular movement of the Vibrio cells. However, after only one hour increasing numbers of
the bacteria were resuming the flagellar swimming by fibrinolysis; after two
hours the area of fibrinolysis achieved by the bacteria had risen from 0.015 –
1.448 mm2. When the bacteria had extended the fibrinolysis area past
the clot, the cells were able to escape completely.
In order to understand how the Vibrios may achieve such efficient fibrinolysis, a cocktail of
protease inhibitors was added to the bacterial blood clots to determine which
of the many enzymes secreted is vital for fibrinolysis. It was found that
inhibitors to the serine- and metallo-proteases suppressed the breakdown of the
clot, and prevented the bacteria from escaping. It was thought that as the
blood clots of arthropods are resistant to many proteolytic attacks, the
bacteria need both of these proteases in order to successfully lyse the clots.
This study has been able to show that the infection
of V. harveyi in an economically important
shrimp may be hindered by the host’s immune system for only two hours before
the parasite can overcome it. The clot itself is an effective defence within
the first moments of a wound occurring, however the suite of proteases released
by the bacteria may act as virulence factors, allowing the survival of the Vibrio and so successful infection of
the shrimp.
I believe this study has prompted many questions as
to what extent the proteases play a role in virulence, and if any other Vibrios may infect organisms in this
way. It would be economically important to extend this line of research so that
possible methods of resistance to parasites in aquaculture could be produced.
Reference: Chaikeeratisak, V., Tassanakajon, A., &
Armstrong, P. B. (2014). Interaction of pathogenic vibrio bacteria with the
blood clot of the Pacific white shrimp, Litopenaeus vannamei. The
Biological Bulletin, 226(2), 102-110.
Really interesting study - it's almost as if the Vibrios are using the protease inhibitors as a back up system to ensure infection does occur. Did the paper mention and previous studies done on other species of crustacean or whether it would be viable to look at protease inhibitors in the wild (considering this is aquaculture)? Looking at how to overcome this and produce a resistance against these would be great as said for aquacultural purposes but also if this was studied in the wild. With increased sea surface temperatures, Vibrios are said to be increasing and could infect other organisms just as bad as in-situ, therefore if this study leads to protecting organisms in the wild - could have positive outcomes...
ReplyDeleteThe paper does suggest in the discussion that comparing the infectivity and pathologic potential of wild type V. harveyi with strains lacking one or more of the enzymes tested (the metallo- and serine-proteases etc) would be extremely beneficial in investigating how the shrimp would cope, so this idea could be taken forward with other species. One example mentioned is a review by Armstrong, (2001, 2006) which had looked at bacterial parasites of mammals and how they rely strongly on the fibrinolytic proteases - perhaps if any one of the enzymes were knocked out then the Vibrios of other species/strains as well as this one would not be able to escape clots in the hosts. This could be a massive benefit to the farmers and perhaps in the long run be worth the money to inoculate all their produce.
DeleteArmstrong, P. B. 2001. The contribution of protienase inhibitors to immune defence. Trends Innunol. 22:47-52
DeleteArmstrong, P. B. 2006 Proteases and protease inhibitors: a balance of activities in host-pathogen interaction. Immunobiology 211:263-281
Hi Lucy, interesting-if not a little worrying- article.
ReplyDeleteI was wondering if the article investigated the innate haemolytic activity of V.harveyi? I quickly had a look myself at the haemolytic activity of the bacteria and apparently it does express haemolytic proteins. With this in mind, I don't think this result is surprising, that the Vibrios can escape the blood clots.
Hi Joss, the paper does mention the use of haemolysins and other proteases as dominant virulent factors, which was shown in a paper by Austin and Zhang, (2006), so they do touch on this innate ability, if only briefly. The use of metallo-proteases and serine-proteases by the bacteria was the main focus, but it isn't surprising that Vibrios are able to utilise so many enzymes to increase the chances of infection.
DeleteAustin, B., and X. H. Zhang. 2006. Vibrio harveyi: a significant pathogen of marine vertebrates and invertebrates. Lett. Appl. Microbiol. 43:119-124
Delete