Host-derived amino acids support the proliferation of symbiotic bacteria

Abstract

Animals are typically colonized by diverse bacterial symbionts, many of which are commensal and, in numerous cases, even essential for their host's proper development and growth. In exchange, the host must supply a sufficient array and quantity of nutrients to support the proliferation and persistence of its microbial community. In this investigation, we have examined such a nutritional environment by determining the symbiotic competence of auxotrophic mutants of the bioluminescent bacterium Vibrio fischeri, and have demonstrated that the host squid Euprymna scolopes provides at least 9 aa to the growing culture of symbiotic V. fischeri present in its light-emitting organ. We also collected and analyzed the extracellular fluid from this organ, in which the symbionts reside, and confirmed that it contained significant amounts of amino acids. The combined results suggested that host-derived free amino acids, as well as peptides or proteins, are a source of the amino acids that support the growth of the symbionts. This work describes a technique to sample the symbionts and their surrounding environment without contamination by host tissue components and, in combination with molecular genetic studies, allows the characterization of the nutritional conditions that support a cooperative animal-bacterial symbiosis.

Figure 1

The light organ environment of symbiotic V. fischeri cells. (A) Thin-section transmission electron micrograph showing symbionts (s) colonizing a portion of the light organ crypts of an adult E. scolopes. The crypts are bounded by microvillus epithelial cells (e), which are believed to supply nutrients to the symbiont population (12). A host cell nucleus (n) can also be seen. (Bar = 5 μm.) In an adult animal, all of these crypts join at and exit through two lateral pores, one on each of the lobes (12). (B) One lobe of the light organ of an adult E. scolopes squid viewed during the process of venting. The crypt contents can be seen exiting the pore (p) of the light organ, appearing as a whitish cylindrical stream. (Bar = 1 mm.)

Figure 2

Persistence of auxotrophic strains of V. fischeri in the juvenile E. scolopes light organ. The number of bacterial cells present at three times after inoculation was determined in light organs of animals infected with either a glycine auxotroph (hatched bar), a threonine auxotroph (open bar), or the parent strain (solid bar). The error bars (where appropriate) indicate 1 standard deviation.