The lens of the hologenome concept does not see the host and associated microbiota as separate entities, but rather as a single evolutionary unit, the holobiont. The principles underlying this idea lie in how all multicellular life has symbiotes, transmission of symbiotes, benefits of symbioses and how microbiota changes enhance the plasticity of the holobiont against environmental stress.
Bivalves are an ideal model for how microbiota protect marine invertebrate holobionts by competing with pathogens and producing antimicrobial compounds, because filter feeding exposes bivalves to many microbes. The microbiota of healthy bivalve haemolymph has been found to produce antimicrobials and could play an important role in protecting bivalve holobionts from infection by pathogens. This study investigated the antimicrobial activity of culturable bacteria isolated from the haemolymph of an oyster, clam, mussel and scallop species; bacteria with the strongest antimicrobial activity were also tested for cytotoxicity towards bivalve immune system phagocyte cells (hemocytes) and resistance to antibiotics commonly used in aquaculture.
On marine agar, haemolymph bacteria (HB) isolated from the more mobile bivalves Pecten maximus and Tapes rhomboides were almost below detection levels more frequently than those from the fixed bivalves Crassostrea gigas and Mytilus edulis. On average M. edulis had the highest HB concentrations and P. maximus the lowest. The paper posits that HB densities are individual and species dependent, possibly influenced by environmental factors. 843 HB strains in total were isolated from the bivalves and 26 created inhibition zones for at least one pathogen strain; most affected were Vibrio. This apparently differs from most antibacterial spectra, which are never as narrow as this; the specificity of HB antimicrobial activity suggests that the holobionts containing them have been selected because they inhibit common pathogens of bivalves. Other findings for potential further research include the lack of antimicrobial activity in any clam isolates and the rapid loss of HB viability and activity.
Anti Vibrio activity has potential applications in aquaculture, since this genus is a prolific pathogen of fish, molluscs and crustaceans. Oyster and mussel isolates had the most potent antimicrobial activity and were identified to be either the Vibrionales or Alteromonadales orders within the Gammaproteobacteria; nine strains were from Pseudoalteromonas, a genus known to produce bioactive secondary metabolites and increase survival in bivalves. However the strains isolated in this study are phylogenetically distinct from those already used in probiotics and have unique properties.
Hemocytes are short lived and die quickly when incubated in sterile seawater, but the presence of HB strains, their mortality actually decreased and some did in a concentration dependent manner. Reduction of hemocyte mortality by microbes has never been shown before and it demonstrates the role of HB in stimulating the immune system of marine invertebrates. Many HB were found to be resistant to tetracycline, the most common antibiotic in aquaculture, which could apparently be useful for picking out individual strains in future experiments.
This is only a preliminary in vivo study, so it seems to me that the extrapolating HB abundance to be species or fixed/mobile dependent is flawed, given that only four, very phylogenetically distinct species were used, leaving the results open to confounded by evolutionary differences. Future work should also look at how bivalves acquire their symbionts, as it would be interesting if they were acquiring them by filter feeding, the same activity that exposes them to pathogens. I am not aware of how they administer probiotics to bivalves in aquaculture, but it seems questionable that HB will necessarily be able to survive in the guts of other aquaculture species which obtain probiotics orally. The signalling pathways occurring between the HB and hemocytes should be investigated further, because it could represent a good model for how microbes and immune systems communicate with each other; for example, how do hemocytes recognise HB from pathogens? Are there pathogens which imitate beneficial bacteria to avoid phagocytosis?
Desriac, F., Le Chevalier, P., Brillet, B., Leguerinel, I., Thuillier, B., Paillard, C., & Fleury, Y. (2013). Exploring the hologenome concept in marine bivalvia: haemolymph microbiota as a pertinent source of probiotics for aquaculture. FEMS Microbiology Letters.