Coral associated microbes interfere with the communication of coral pathogens
Corals are not only in symbiosis with the dinoflagellate algae but also with the numerous species of bacteria and other types of microbes. This consortium of organisms with a coral has been termed as a coral holobiont. Coral surface mucus layer plays a key role in structuring associated microbial community. Associated microbial community significantly influences health of the coral reef and its resilience to stresses and diseases. Coral mucus-associated microbes produce antibiotics in order to avoid pathogens and hence confer immunity to their hosts against these invasive microbes. Coral-associated bacterial community have antagonistic interactions with other microbes, particularly pathogens. Similarly, there may be other microbe-microbe co-operative interactions occurring on the mucus layer. Very little is known about the mechanisms that govern such interactions.
Bacteria uses signals made up of small molecules (e.g. N-acyl homoserine lactones abbreviated as AHLs) to talk to each other. They detect number of individuals of their species present around them, using these species-specific signalling molecules that bind to the specific cell receptors on the bacteria cell. Similarly, they also have signalling molecules that sense number of bacteria of other species present around them; which are used in inter-species communication. Bacteria use such chemical communication to switch on or off specific genes. They switch on certain genes only after realizing that there is higher number of individuals within the population. This social behaviour is exhibited by many bacteria and is known as quorum sensing (QS). For example, in many bacteria, genes of virulence are switched on through such communication, only after reaching certain bacterial cell density in order to evade host’s immunity. Many microbial interactions on coral reefs are thought to occur via QS which also involves manipulating QS signals of other bacteria. This study investigated if the invertebrate-dinoflagellate -associated bacteria can produce signals that manipulate QS of pathogens.
Serratia marcescens is a coral pathogen responsible for white pox disease. Swarming behaviour in S. marcescens occurring through QS was inhibited by molecules secreted by zooxanthellae-associated bacteria. Remarkably, some of the tested strains of bacteria inhibited biofilm (sedentary microbial community) formation but promoted swarming in S. marcescens, whereas others inhibited both swarming and biofilm formation of it. Authors suggested that associated bacteria may produce molecules that interfere with a universal regulatory switch which is concerned with controlling behaviours of swarming and biofilm formation in S. marcescens, without affecting its growth.
These findings from studies on bacterial cultures were applied on a cnidarian, Aiptasia pallida by inoculating its polyps with a white pox pathogenic strain of S. marcescens along with the antagonistic marine bacteria in one treatment and without those bacteria in another treatment. The polyps without those bacteria got degraded in a short period of time whereas the polyps with the antagonistic bacteria exhibited reduced manifestations of the disease although, being infected with the pathogen.
In summary, this study highlights importance of native associated bacterial community in the health of their coral hosts and in turn whole coral reef ecosystem. Quorum sensing is a fascinating phenomenon that characterizes how do these microscopic living entities, insignificant on their own, can impact significantly on their outer world when come together and behave like a multicellular organism. Bonnie Bassler (2009) in a talk even claims that “multicellularity” might have occurred in such a way with the basics of it being in QS activity of bacteria. This study critically demonstrates QS activity as a mechanism of microbe-microbe interactions among coral associated bacteria, in turn protecting their hosts against opportunistic pathogens.
Alagely A., Krediet C. J., Ritchie K. B., & Teplitski M. (2011) Signaling-mediated cross-talk modulates swarming and biofilm formation in a coral pathogen Serratia marcescens. The ISME journal, 5 (10):1609-1620.
To supplement the blog-post with some more background information on “Quorum Sensing”, I have used two more citations –
Munn C.B. (November 2012) MBIO208, Lecture 6, Quorum Sensing, Plymouth University, Plymouth UK.