Agrobacterium radiobacter

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A Microbial Biorealm page on the genus Agrobacterium radiobacter

Classification

Higher order taxa

Bacteria; Proteobacteria; Alphaproteobacteria; Rhizobiales; Rhizobiaceae; Rhizobium/Agrobacterium group

Species

Agrobacterium radiobacter

Description and significance

Agrobacterium radiobacter is a gram negative bacillus found in soil containing organic material (rhizosphere). It is a saprophytic organism, meaning it uses dead plant material for nutrients.(1) A. radiobacter strain k84 is utilized for its competitive behavior with the closely related A. tumefaciens; the cause of crown gall disease in plants. A. radiobacter synthesizes and secretes a substance called agrocin 84, which blocks the ability of A tumefaciens to replicate its DNA, effectively halting its ability to reproduce.(2) The strain A. radiobacter J14 is useful in its ability to degrade Atrazine, an agricultural herbicide.(4)

It has been recognized as an opportunistic pathogen in humans, though is of relatively low virulence.(1) The bacteria’s presence in humans is thought to be nosocomial, meaning hospital-aquired. Cases of Agrobacterium infection occurred most frequently in conjunction with the presence of a catheter or lens implant. A. radiobacter has also been the cause of one case of psuedobacteremia, in which 15 blood samples were contaminated with water inhabited by A. radiobacter, further implicating that the bacteria is transmitted from the hospital to the host.(10)

Genome structure

The genome of A. radiobacter has been completely sequenced via the shotgun sequencing method. It contains 7.2 million base pairs with a G+C composition of 59.9%.(12) The genetic material is organized within one circular chromosome containing 4,005,130 base pairs, as well as four plasmids.

The five species of Agrobacterium are divided based on their pathogenicity towards plants. A. radiobacter is distinguished by being the only member of the genus that does not have pathogenic tendencies towards plants. This is a problematic method of categorization because the plasmids responsible for pathogenicity are easily transferrable amongst members of the genus.(8) An alternative classification system divides Agrobacterium into three biovarients based on the organization of their genetic material. Biovar I contains two chromosomes and two plasmids. Biovar III contain two chromosomes and five plasmids. Biovar II, to which A. radiobacter belongs, contains only one chromosome and four plasmids.(8)

The largest of A. radiobacter’s plasmids contains 2,650,913 base pairs; more than 33% of the organism’s total genetic material and more than the either of the other bivarient’s second chromosomes. However, because it does not contain genes that code for vital metabolic functions, it is still classified as a plasmid. It does share a significant number of genes with the other biovarients that are important, if not vital, to cell function. To distinguish its significance, the large plasmid is commonly referred to as a replicon. The replicon is known to contain at least one gene essential to cell survival. (8)

Cell and colony structure

A. radiobacter was first isolated from soil in Australia (5). It is a rod-shaped, gram-negative organism that senses nutrients through chemotaxis, and moves towards them propelled by use of peritrichous flagella. A. radiobacter forms organized biofilms in the tree roots it colonizes by secreting a sticky polysaccharide cell envelope. (9)

Metabolism

Rahnella aquatilis is a facultative anaerobe (it can live in the absence or presence of oxygen) that fixes Nitrogen [2]. R. aquatilis metabolizing whey lactose produces high levels of organic acids (except for lactic acid) [7].

Ecology

Rahnella aquatilis is named so because of its prevalence in fresh water. It has been found around the globe in places like the United States, Korea, Japan, Russia, the Ukraine, and Egypt. R. aquatilis has also been found in humans, soil, and snails [5]. One of the most unusual places for the the microbe to have been found was inside the gut of certain speicies of longicorn beetles in Korea [4].

Pathology

Rahnella aquatilis is pathogenic in humans. The organism can be diagnosed in patients via blood cultures, respiratory washings, and in wound cultures. Various infections, such as bacteremia (from renal infection), sepsis, respiratory infection, and urinary tract infection can be the result. One case involved an 11-month-old girl with congenital heart disease who developed infective endocarditis [8]. Another case involved a 76-year-old male who had prostatic hyperplasia presenting with acute pyelonephritis [9]. It is noted that R. aquatilis can potentially cause life-threatening infections in humans, infants and adults alike, especially the immunocompromised and organ transplant recipients. Treatments have included intravenous and oral levofloxacin therapy (and other members of the quinolone family).

References

[1] J Chemother. 2000 Feb;12(1):30-9. <http://www.ncbi.nlm.nih.gov/pubmed/10768513>

[2] R.J. Martinez. J Bacteriol. 2012 Apr;194(8):2113-4. <http://www.ncbi.nlm.nih.gov/genome/?term=Rahnella%20aquatilis>

[3] Robert Martinez, University of Alabama. <http://genome.jgi-psf.org/rahac/rahac.info.html>

[4] Park, Doo-Sang, Hyun-Woo Oh, Won-Jin Jeong, et al. "A Culture-Based Study of the Bacterial Communities within the Guts of Nine Longicorn Beetle Species and their Exo-enzyme Producing Properties for Degrading Xylan and Pectin." The Journal of Microbiology, October 2007, p. 394-401.

[5] Brenner, Don J., Hans E. Muller, Arnold G. Steigerwalt, et al. "Two new Rahnella genomospecies that cannot be phenotypically differentiated from Rahnella aquatilis." lnternstional Journal of Systematic Bacteriology (1 998), 48, 141 -149.

[6] Kim, Kil Yong, Diann Jordan, and Hari B. Krishnan. "Rahnella aquatilis, a bacterium isolated from soybean rhizosphere, can solubilize hydroxyapatite." FEMS Microbiology Letters Volume 153, Issue 2, 15 August 1997, Pages 273–277.

[7] Pintado, Manuela E., Ana I.E. Pintado, and F. Xavier Malcata. "Fate of Nitrogen During Metabolism of Whey Lactose by Rahnella aquatilis." Journal of Dairy Science, Volume 82, Issue 11, November 1999, Pages 2315-2326.

[8] Matsukura H., Katayama K., Kitano N., et al. "Infective endocarditis caused by an unusual gram-negative rod, Rahnella aquatilis." Pediatric Cardiology, 1996 Mar-Apr; 17(2): 108-11.

[9] Tash, Kaley. "Rahnella aquatilis Bacteremia from a Suspected Urinary Source." Journal of Clinical Microbiology. May 2005, vol. 43 no. 5, 2526-2528.

Edited by Jay Huber, student of Dr. Lisa R. Moore, University of Southern Maine, Department of Biological Sciences, http://www.usm.maine.edu/bio