Hamiltonella defensa

Hamiltonella defensa (H. defensa) is a species of bacteria. It is maternally or sexually transmitted and lives as an endosymbiont of whiteflies and aphids,[1] meaning that it lives within a host, protecting its host from attack. It does this through bypassing the host's immune responses by protecting its host against parasitoid wasps.[2] However, H. defensa is only defensive if infected by a virus. H. defensa shows a relationship with Photorhabdus species, together with Regiella insecticola. Together with other endosymbionts, it provides aphids protection against parasitoids.[2] It is known to habitate Bemisia tabaci.[3]

H. defensa is a member of the family Enterobacteriaceae.[4] It can be found both extracellularly and intracellulary in H. defensa itself, and also the bacteriocytes.[4] It is a gram-negative bacteria and has been found to have six distinct secretion systems that mediate the export of protein through the inner and outer membranes.[4][5] In general, "Candidatus Hamiltonella defensa"'s functions are relatively unknown,[3] and the H. defensa's discovery was not specified by a particular person.

H. defensa's draft genome sequence was first discovered in the B. tabaci complex of the "Candidatus Hamiltonella defensa".[3] This is only found in two invasive cryptic species: Mediterranean and Middle East-Asia.[3]

The H. defensa is significantly smaller (at 1.84-Mpb) than its bacteria relatives; Yersinia and Serratia species. It is also dependent on the essential amino acids produced by Buchnera.[2] It is autotrophic for eight out of the ten essential amino acids that Buchnera produces.[6] Although dependent, the H. defensa genome preserves more genes and pathways for cell structures and processes, than that of obligate symbionts. It also has several abundances: toxin homologs, encoding type-3 secretion systems, and putative pathogenicity loci.[2] Additionally, H. defensa holds mobile DNA, like phage-derived genes, plasmids, and insertion-sequence elements, that feature H. defensa's dynamicness, and also show the role horizontal gene transfer has on shaping it.[2] The H. defensa genome holds a 2,110,331-bp circular chromosome and a 59,034-bp conjugative plasmid. The chromosome carries a canonical origin of replication.[6] It has notably more cell structure, DNA replication, recombination, and repair of genes than obligate endosymbionts, despite its limited biosynthetic abilities.[6] The proteins present in H. defensa vary in length significantly. They have a double cycled nonapeptide sequence involved in binding calcium.[6]