Hamiltonella defensa

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Domain: Bacteria; Phylum: Proteobacteria; Class: Gammaproteobacteria; Order: Enterobacterales; family: Enterbacteriaceae; Genus: Hamiltonella


Hamiltonella defensa

Description and Significance

H. defensa is a host-restricted, mutualist symbiont which is conditionally beneficial. This means that a couple of factors have to be present in order for Hamiltonella defensa to serve its purpose. Its main purpose is protecting aphids from braconid wasp parasitoids. It is an endosymbiont of aphids, so it lives inside these sap-sucking insects. This bacterium is important because it has the unique ability to defend its aphid host from these invasive parasitoids.

Genome Structure

Hamiltonella defensa has an extremely dynamic genome. It is relatively small, only 2.1 Mb which consists of a 2,110,331-bp circular chromosome. It encodes 2,100 protein-coding genes, has a relatively large number of pseudogenes, and is littered with mobile DNA, insertion sequences, and phage remnants. Horizontal gene transfer plays a role in its dynamic genome.

Additionally, approximately half of H. defensa's DNA comes from toxin-encoding bacteriophages called APSEs. APSEs are similar to lamda-like phages, which are bacterial viruses that infect E. coli. It has other similarities to the E. coli bacterium, including a similar number of pseudogenes. Depending on which strain of APSE H. defensa obtains its DNA from, the development stage at which the parasitoid wasps are killed varies.

H. defensa is auxotrophic for 8 of its 10 essential amino acids. It relies on Buchnera, a primary endosymbiont of aphids, to synthesize these amino acids. However, despite H. defensa's limited biosynthetic capabilities, it has considerably more cell structural, DNA replication, recombination, and repair genes than do obligate endosymbionts. This is most likely due to its extremely varied sources of DNA. Additionally, its diverse genome allows for a wide variety of regulatory genes which help H. defensa to cope with changes to its environment, such as attack of hosts by parasitoids or an invasion of a new host species.

Cell Structure, Metabolism and Life Cycle

Hamiltonella defensa can be maternally transmitted of passed through horizontal gene transfer. It is found in approximately 34% of aphids. The reason it is not found in a higher percentage of aphids is because its presence can be costly to its host if there is no threat present. Specific costs to a host vary among aphid species, but generally, survivorship and life span were lower in aphids infected with H. defensa when no threat was present.

Additionally, Hamiltonella defensa can conduct glycolysis on its own, which is a very unique ability for endosymbionts. This is important because it allows for energy production, even if H. defensa is living in oxygen-limited environments.

Ecology and Pathogenesis

As previously mentioned, Hamiltonella defensahas the ability to kill braconid wasp parasitoids. The exact mechanism by which it does this is unknown. Some proposed theories are that the phages could be directly poisoning the wasps or that the phages could be splitting the Hamiltonella apart causing their toxins to spill over the wasps, or a combination of the two.

In order for this bacterium to be beneficial to its aphid host, a parasite threat has to be present and the bacteriophage APSE DNA has to be a part of H. defensa's genome. This is why it is conditionally beneficial.


Chevignon, Germain et al. “Culture-Facilitated Comparative Genomics of the Facultative Symbiont Hamiltonella defensa.” Genome Biology and Evolution vol. 10,3 (2018): 786-802. doi.:10.1093/gbe/evy036

Degnan, Patrick H et al. “Hamiltonella defensa, genome evolution of protective bacterial endosymbiont from pathogenic ancestors.” Proceedings of the National Academy of Sciences of the United States of America vol. 106,22 (2009): 9063-8. doi:10.1073/pnas.0900194106

Dykstra, Hannah R et al. “Factors Limiting the Spread of the Protective Symbiont Hamiltonella defensa in Aphis craccivora Aphids.” Applied and Environmental Microbiology, vol. 80,18 (2014): 5818-27. doi:10.1128/aem.01775-14.

Moran, Nancy A et al. “Evolutionary relationships of three new species of Enterobacteriaceae living as symbionts of aphids and other insects.” Applied and environmental microbiology vol. 71,6 (2005): 3302-10. doi:10.1128/AEM.71.6.3302-3310.2005

Oliver, K. M., et al. "Variation in Resistance to Parasitism in Aphids Is Due to Symbionts Not Host Genotype." Proceedings of the National Academy of Sciences, vol. 102,36 (2005): 12795-800. doi:10.1073/pnas.0506131102.

Yong, Ed. I Contain Multitudes: the Microbes Within Us and a Grander View of Life. Ecco, an Imprint of HarperCollinsPublishers, 2016.


Page authored by Isabella Valli, student of Prof. Jay Lennon at IndianaUniversity.