Buchnera aphidicola

A Microbial Biorealm page on the genus Buchnera aphidicola

Classification Higher order taxa: Kingdom: Bacteria Phylum: Proteobacteria Class: Gamma Proteobacteria Order: Enterobacteriales Family: Enterobacteriaceae Genus: Buchnera Species: B. aphidicola Species: Buchnera aphidicola (4) Description and Significance Buchnera aphidicola are prokaryotes within the phylum gamma-Proteobacteria, where great diversity of bacteria can be found (1). Proteobacteria are all Gram-negative bacteria, and contain various pathogenic bacteria and various nitrogen fixing bacteria (1). Buchnera aphidicola is a primary endosymbiont of aphids (A. pisum), a type of insects which digest plants (2). Researchers argue that the ancestors of Buchnera aphidicola were free living Gram-negative bacteria similar to a modern Enterobacteriaceae such as Escherichia coli (2). Various evidences support that Buchnera aphidicola are closly related to Enterobacteriaceae family, such as the studies using 16S rRNA sequence comparisons, which two families share a closely related 16S rRNA sturucture (4). The size of Buchnera aphidicola is 3 µm in diameter and they share the most of the key components of their Gram-negative cell wall with related Enterobacteriaceae (3). However, unlike most other Gram-negative bacteria, Buchnera aphidicola does not have the genes to produce lipopolysaccharides (LPS) for their outer cell membrane, which is different than Enterobacteriaceae (3). Since Buchnera aphidicola are closly related to some well known organisms such as Escherichia coli and Haemophilus influenzae, the study of Buchnera aphidicolan can reveal the evolutionary mechanisms as well as molecular basis of intracellular endosymbiosis and the parasitic behavior of symbionts (4). Genome Structure Buchnera aphidicola has one of the smallest known genomes of any living bacteria (3). As most Proteobacteria do, Buchnera aphidicola has one circular chromsome with few numbers of plasmids, two in most cases (1). Verification of the phylogeny of Buchnera aphidicola was done by comparing the genome sequence with various species in the family called Enterobacteriaceae, the bacteria researchers believe to be most closely related to Buchnera, and thought to be diverged from each other most recently (3). By comparing the genomes between Buchnera and general species of Enterobacteriaceae, one can find the genomic difference between symbiotic bacteria and free-living bacteria (4). There are several distint features which Buchnera aphidicola hold. The ratio of G+C to A+T in their chromosome is fairly high due to the higher GC nucleotide substitution rates that has been occuring since the divergence of the species, which is estimated from 0.622 to 19.51 million years (5). Furthermore, it is also one of the most genetically stable organism due to the lack of many enzymes and proteins involved in metabolisms due to the symbiotic relationship with Aphids (3). Besides a nutritional co-dependence with Aphids, because of a co-existence of millions of years, Buchnera have lost the ability to produce cell surface components such as lipopolysaccharides, which is a common key characteristics of Gram-negative bacteria (4). Lastly, Buchnera aphidicola has lost many genes which are involved in nutrient synthesis by deletions in their chromosome (3). Cell Structure and Metabolism Buchnera aphidicola does not have many metabolic enzymes which normal Gram-negative bacteria have. The symbiotic relationship with aphids and the lack of chromosome crossover events due to vertical transmission of genetic materials within its species has caused the deletion of genes required for various metabolism (3). Researchers have identified those enzymes, and they are found to be essential for anaerobic respiration, the synthesis of amino-sugars, fatty acids, phospholipids, and complex carbohydrates (3). Buchnera aphidicola also have lost regulatory factors allowing continuous overproduction of tryptophan and other amino acids (3). All these loss of genes are thought to be the result of symbiotic relationship with Aphids, as they share key nutrients together. The symbiosis not only reduced the genome involved in metabolism but also reduced the genome which codes lipopolysaccharides (LPS), the key component of the cell walls of Gram-negative bacteria (4). The lack of lipopolysaccharides resulted in non-pathogenicity of Buchnera aphidicola, which is quite rare for normal Gram-negative bacteria (1). Ecology The symbiotic relationship of Buchnera aphidicola with aphids began between 200 million and 150 million years ago, according to the study of genome sequences and phylogeny within closely related species and Aphids (2). Buchnera aphidicola has persisted through maternal transmission and co-speciation, with very few lateral gene transfer, which supressed the diversity of their genome (2). Aphids have developed bacteriocyte cells to host Buchnera aphidicola (2). It is estimated that a fully grown aphid may host 5.6 × 10^6 Buchnera aphidicola cells (2). Each bacteriocyte contains many vesicles called symbiosomes, which are derived from the plasma membrane. (2) Most of the aphids contain maternally transmitted bacteriocyte cells, which themselves contain Buchnera aphidicola, which explains the nature of vertical gene transfer within Buchnera themselves. The aphids live on a restricted diet (plant sap), rich in carbohydrates, but poor in nitrogenous or other essential compounds (4). It is believed that the Buchnera provide the essential nutrients that the host lacks, by keeping the enzymes necessary to synthesize those essential nutrients (4). In return, Buchnera lost other key metabolic enzymes because they also are dependent on Aphids to provide those nutreints (4). Furthermore, Buchnera have lost the ability to produce cell surface components such as lipopolysaccharides, which forces them to keep the endosymbiont relationship with Aphids (4). Pathogenicity Proteobacteria contain various pathogenic bacteria such as Escherichia, Salmonella, Vibrio, and Helicobacter (1). But Buchnera aphidicola lacks lipopolysaccharides in their cell walls, which leads to a reduced-pathogenicity (4). There are no known pathogenic events found due to Buchnera aphidicola yet. Application to Biotechnology There aren’t any known application to biotechnology involving Buchnera aphidicola, but the study of Buchnera aphidicola can result in various progressions in the field of biotechnology. For example, the study of Buchnera aphidicola and other Enterobacteriales can reveal key enzymes and proteins that are required for symbiosis (4). One can then artificially produce symbiotic bacteria and study various metabolic enzymes with knock-ins and knock-outs of specific genes. Current Research Buchnera aphidicola was first named by Paul Baumann and his graduate student, as well as the first molecular characterization of a symbiotic bacterium using Buchnera aphidicola (2). The studies on Buchnera aphidicola later contributed to many studies on symbionts of many types of insects, pursued by numerous researchers like Paul and Linda Baumann, Nancy Moran, Serap Aksoy, Roy Gross, those who studied symbionts of aphids, tsetse flies, ants, leafhoppers, mealybugs, whiteflies, psyllids, etc. (2) References 1. Madigan M; Martinko J (editors). (2005). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0131443291 2. Douglas, A E (1998). "Nutritional interactions in insect-microbial symbioses: Aphids and their symbiotic bacteria Buchnera". Annual Review of Entomology 43: 17-38. ISSN 00664170. 3. Pérez-Brocal V, Gil R, Ramos S, Lamelas A, Postigo M, Michelena J, Silva F, Moya A, Latorre A (2006). "A small microbial genome: the end of a long symbiotic relationship?". Science 314 (5797): 312-3. PMID 17038625. 4. MUNSON (M.A.), BAUMANN (P.) and KINSEY (M.G.): Buchnera gen. nov. and Buchnera aphidicola sp. nov., a taxon consisting of the mycetocyte-associated, primary endosymbionts of aphids. Int. J. Syst. Bacteriol., 1991, 41, 566-568. 5. Gómez-Valero L, Silva FJ, Christophe Simon J, Latorre A. “Genome reduction of the aphid endosymbiont Buchnera aphidicola in a recent evolutionary time scale”. Institut Cavanilles de Biodiversitat i Biologia Evolutiva and Departament de Genètica, Universitat de València, Apartat 22085, 46071 Valencia, Spain

Edited by Kyu Hahn student of Rachel Larsen and Kit Pogliano