Stewart et al. 2004. Studies on the virulence of Aerococcus
viridans (var.) homari, the causative agent of gaffkemia,
a fatal disease of homarid lobsters. Diseases of aquatic organisms. 60,
149-155.
Lobsters are sold live and therefore are stored for varying
periods in often-inadequate conditions. The increased disease risk has promoted
studies into how disease works. The free-living, gram positive, tetrad-forming coccus, Aerococcus
viridans (var.) homari (previously Gaffkya homari), causes systematic infections of gaffkemia in lobsters, resulting in heamorageing and
often death. A. viridans homari can be considered pathogenic because it multiplies in the environment of the host’s tissues, resists or
interferes with the host’s defense mechanisms and causes damage to the tissues
of the host (Smith 1984 pathogen requirements).
In previous
studies less than half the strains actually developed a fatal infection. In
this study the authors wanted to understand the virulence of this pathogen.
Serum was extracted from lobsters and strains were tested for agglutination.
Lobsters were also injected with virulent and avirulent strains of the
pathogen. Virulent strains possess polysaccharide capsules and are not
agglutinated by lobster hemolymph serum; avirulent strains do not have
capsules, are agglutinated by the lobster hemolymph serum, and most do not grow
well in lobster hemolymph serum.
The capsules did not prevent phagosytosis of the bacteria
but did prevent adsorption of the serum agglutinin and therefore the bacteria
were not killed by phagosytosis. The capsules could have been interfering with
immune serum killing as what occurs with Streptococcus iniae found by
Barnes et al. (2003). Avirulent strains grew slowly in lobster hemolymph serum
and therefore were unable to take hold and become a fatal infection. The
authors suggested this was an important factor in their avirulence. An
interesting finding was that in laboratory media most of the bacteria strains
lost virulence but if grown in sterile lobster heamolymph serum virulence was
restored by induction of virulent features. However the authors noted that
virulence was not readily or rapidly lost, it took years as far as I could make
out. Growth of the strains in heated serum did not allow virulence restoration
and this may be of some interest as to ways of minimizing virulence in lobster
populations.
Further research into the fact that Aerococcus
viridans (var.) homari
has unique components
compared to any other naturally occurring material in the capsular
polysaccharide, and the relevance of this to why the bacterium is specifically
pathogenic to lobsters is suggested. This may help to find ways of preventing
virulence of bacteria rather than having to use sometimes-drastic techniques to
rid livestock of the bacteria altogether. Studies such as this one are
advancing our knowledge of virulence and may find new strategies to disease
prevention. Interception of a disease at any stage of development can prevent
death and therefore understanding disease virulence is key. Disease prevention
in lobsters is of particular interest due to their economical importance.
3 comments:
Hi Alice,
An interesting post. It is good that they have furthered our understanding of the fatal disease and its virulence. I also found it interesting that only some lobsters are affected, as I read that it causes little harm to homarid lobsters in North America, whereas European lobsters will die in days. It would interesting to compare how the immune serum varies between the two and what factors are enabling it to phagocytose the bacteria, which could also perhaps lead to another method of prevention or treatment for the lobsters. You mentioned that heated serum prevented virulence restoration, I also read that similarly lower temperature reduce its virulence. It would be interesting to understand exactly how temperature affects the bacteria.
Thankyou for your comment. Yes it is something to do with the serum that causes the virulence and maybe the component can be identified and altered. The temperature effects is interesting but im not sure how that can be applied to prevent disease because the lobsters may not grow in different temperatures well.
Yes you're right, I've just read that they are incredibly sensitive to temperature and can detect changes as little as 0.15 degrees..crazy! Perhaps not the solution then!
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