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Chapter 14 : The Vibrio fischeri–Euprymna scolopes Light Organ Symbiosis
Category: Environmental Microbiology
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The light organ symbiosis between the bioluminescent bacterium Vibrio fischeri and the Hawaiian bobtail squid Euprymna scolopes has received increasing interest from researchers representing a range of disciplines, including microbiology, zoology, oceanography, immunology, and genetics. An improved understanding of the factors underlying the natural symbioses can help face problems such as the (re)emergence of diseases with environmental hosts, the effective application of probiotics, overreactions to commensal bacteria by the immune system, and the negative effects of broad-spectrum antibiotics. The light organ is also accessible to experimentally added solutes. Pores connect the outside environment with the site of infection in the light organ, so reagents can be added to cure an infection or monitor biochemical processes at the site of infection. One of the most intriguing colonization factors is bioluminescence. Long considered an energetic drag on cultured cells, luminescence is critical for full colonization of the E. scolopes light organ by V. fischeri. The observation that lipopolysaccharide (LPS) and peptidoglycan monomer (PGM) stimulate developmental processes in a mutualistic animal-bacteria association is notable for at least two reasons. First, the molecules are also recognized by the innate immune systems of animals and often trigger antimicrobial responses. This suggests conserved mechanisms for mutualist and pathogen detection by animals, with host responses being context dependent. Second, the particular PGM molecule shed by V. fischeri is identical to "cytotoxins" of pathogens Bordetella pertussis and Neisseria gonorrhoeae, revealing an interesting and unanticipated similarity between mutualistic and pathogenic bacteria-animal associations.
Euprymna scolopes. An adult female Hawaiian bobtail squid sits on coralline sand near eggs (indicated by white arrows) that she laid on a PVC halfpipe and on the side of her tank. While housed in a 50-liter tank at the University of Georgia, this female produced 1,616 juveniles over a 4.5-month span.
Proposed counterillumination behavior of E. scolopes. Based on light organ architecture, it is thought that an adult E. scolopes with bioluminescent symbiotic V. fischeri can obscure its silhouette with a controllable, ventrally directed luminescence, whereas an aposymbiotic (lacking symbionts) squid would cast a shadow.
Varied roles for peptidoglycan monomer