Growing Ungrowable Bacteria: Overview and Perspectives on Insect Symbiont Culturability

Florent Masson; Bruno Lemaitre

doi:10.1128/MMBR.00089-20

DOI: 10.1128/MMBR.00089-20

SUMMARY

Insects are often involved in endosymbiosis, that is, the housing of symbiotic microbes within their tissues or within their cells. Endosymbionts are a major driving force in insects’ evolution, because they dramatically affect their host physiology and allow them to adapt to new niches, for example, by complementing their diet or by protecting them against pathogens. Endosymbiotic bacteria are, however, fastidious and therefore difficult to manipulate outside of their hosts, especially intracellular species. The coevolution between hosts and endosymbionts leads to alterations in the genomes of endosymbionts, limiting their ability to cope with changing environments. Consequently, few insect endosymbionts are culturable in vitro and genetically tractable, making functional genetics studies impracticable on most endosymbiotic bacteria. However, recently, major progress has been made in manipulating several intracellular endosymbiont species in vitro, leading to astonishing discoveries on their physiology and the way they interact with their host. This review establishes a comprehensive picture of the in vitro tractability of insect endosymbiotic bacteria and addresses the reason why most species are not culturable. By compiling and discussing the latest developments in the design of custom media and genetic manipulation protocols, it aims at providing new leads to expand the range of tractable endosymbionts and foster genetic research on these models.

GLOSSARY

anaplerotic reaction: Metabolic reaction that forms intermediate products of a metabolic pathway.
aposymbiotic: Devoid of symbionts.
axenic medium: Culture medium in which the cultured species is the only living organism.
bacteriocyte: Specialized cells dedicated to the intracellular housing of endosymbionts.
bacteriome: In most insect species, bacteriocytes group together to form a distinct organ called the bacteriome. In some species, bacteriocytes remain scattered throughout tissues and do not form a proper bacteriome structure.
endosymbiosis: Symbiotic interaction in which the symbiont lives within host tissues (hemolymph) or cells (bacteriocytes or nonspecialized cells). It is opposed to ectosymbiosis, in which the symbiont lives outside the host organism (e.g., on the insect cuticle or in the close environment).
facultative: A symbiont is facultative if it is not required for the host development or survival.
M9 minimal medium: Minimal growth medium commonly used for bacterial cultures, composed of inorganics salts that can be complemented with select carbon sources.
microbiota: All microorganism found in and on a host organism. The microbiota includes endosymbionts but also other species that can be mutualistic, commensal, or pathogenic.
obligate: Obligate symbionts are required by their host to develop and survive under wild conditions. Some hosts can survive under laboratory conditions without their obligate symbionts, but their fitness is severely impaired.
shuttle vector: Vectors (plasmids) that can propagate in at least two different species. Most shuttle vectors are designed to propagate in the species of interest and in Escherichia coli for easy cloning and amplification.
transmission (horizontal/vertical): A vertical transmission occurs when symbionts are transmitted from parents to offspring (most often by females, sometimes by males), as opposed to horizontal transmission, where the symbiont can be transmitted between individuals of the same generation.