A Microbial Biorealm page on the genus Erwinia amylovora
Higher order taxa
Bacteria; Proteobacteria; Gammaproteobacteria; Enterobacteriales; Enterobacteriaceae; Erwinia
Description and significance
Erwinia amylovora is a gram negative, facultative anaerobic, rod shaped bacteria. This bacterium is motile by peritrichous flagella at 37°C; it is not motile at 28°C. It is negative for the Voges-Proskauer test and positive for gelatin hydrolysis and it releases gas when it undergoes glucose fermentation (1). Fire blight, a disease that affects and can cause extensive damage to apple and pear trees, is caused Erwinia amylovora. It received its name from the appearance of the infected leaves and branches, which often appears blackened as if scorched by fire (4). It can destroy apple and pear blossoms, shoots, limbs, and even whole trees (2). It can also affect other plants in sub-family Pomoideae of the family Rosaceae3. It is native to North America but spread to northern Europe in the 1950s and 1960s. It has continued to spread throughout the eastern Mediterranean since the 1980s and throughout Europe (3). The bacteria can enter the plant through its blossoms, stomata, or wounds on various parts of the plant and is often carried by rain or insects (3). Some of the common symptoms include the blossom blight, shoot blight, canker blight, trauma blight, and rootstock blight (2). In some cases the bacteria can be seen as a white or golden ooze seeing from the infected plant tissue (3). Once it infects the tree it spreads rapidly and there is often no way to save the infected tree (3).
Cell structure and metabolism
Erwinia amylovora is a casual pathogen that causes the contagious disease fireblight. Fireblight mainly affects pears, apples, and ornamental plants of the Roseaceae family. Infections typically begin in spring due to optimal moisture and temperature conditions. The first sign of infection is a blossom with a water-soaked appearance. If the infection is not controlled and the infection progresses the blossoms, shoots and branches will wilt, ooze, and die. Erwinia amylovora can survive over winter in cankers and become an active infection again in spring.
The affected areas of the plants appear shriveled and blackened as if they were scorched by fire; hence the term “fireblight.” A primary infection occurs when the bacterium enters the plant via open stomata. Consequently, necrotic lesions and bacteria-laden exudate engulf the plant. The death of the plant often occurs once the roots have been invaded. Streptomycin antibiotic sprays have been shown to prevent the spread of infection. Other techniques, such as pruning—the removal of infected areas—have also proved to be an adequate measure to save the plant.
Erwinia amylovora is found mostly in the Northern Hemisphere in places like North America, Canada, Europe, but also in New Zealand. Erwinia amylovora can cause severe outbreaks leading to loss of flower and subsequently, loss of fruit. This bacterium more harshly affects young trees.
The two main agents used as antimicrobial agents against Erwinia amylovora are copper and streptomycin. However, strains of Erwinia amylovora resistant to streptomycin have been reported in various geographic areas. Methods of prevention include but are not limited to controlling how lush the trees are especially during youth, sterilizing pruning equipment, and removal of formed cankers.
Erwinia amylovora is profoundly challenging to control. The normal manner used to treat infected plants is to destroy the entire crop, which is very costly, or to spray with antibiotics; however, this is no longer legal in most of Europe due to the increase in antibiotic resistance displayed by bacteria repeatedly exposed to antibiotics. It has been questioned whether E. amylovora uses quorum sensing to regulate the virulence of its phenotype (7).
Many Gram negative cells use small N-acyl-homoserine lactone (AHL) molecules to trigger quorum sensing activity, and it has been demonstrated that E. amylovora does indeed use quorum sensing (7). This aspect of E. amylovora has not been discovered earlier because it takes much longer than usual for the bacteria to begin synthesizing AHL (7).
Evidence for this is that the supernatant of high density, older cultures of E. amylovora stimulates quorum sensing responses in younger E. amylovora cultures and a standard autoinducer biosensor Vibrio harveyi, suggesting that E. amylovora uses the same AHL mechanism to trigger quorum sensing as the standard does (7). The resultant traits, the production of two extracellular polysaccharides, amylovoran and levan (detected by measuring levansucrase), and the cell’s increased tolerance of oxidative stress may be measured (7).
Additionally, there are genes present in the genome of E. amylovora (involved with AHL) homologous to those in other organisms that use quorum sensing (7). Finally, comparing the degree of virulence in a wild type strain versus a strain of the bacteria transformed with a gene that destroys AHL, the mutant strain whose AHL is immediately destroyed shows significantly less severe symptoms as compared to the wild type(7).
As possible future treatment of the disease, E. amylovora could be combated by either creating transgenic crops containing the gene for the protein which degrades AHL, or by exposing the crops to large amounts of bacteria which naturally produce this protein(7).
The recent increase in drug resistance for Erwinia amylovora, specifically streptomycin, is a danger for many of the pear orchards in the western United States. Streptomycin is a drug commonly used in the prevention of growth of E. amylovora, or fire blight. The increased fear of the newly streptomycin resistant fire blight is that pear orchards that have little to no exposure to the antibiotic are showing streptomycin resistant strains (6). In a prolonged absence of streptomycin, E. amylovora has not shown a decrease in resistance (6). E. amylovora furthermore does not show a significant difference in mutation rate in accordance with a high or low exposure to streptomycin.. Previous studies have indicated that the streptomycin resistance in E. amylovora is caused by a chromosomal mutation (6). Many new antibiotics have been used to treat fire blight with positive results and showing little to no resistance. Further studies will need to be conducted in the resistance to other antibiotics against E. amylovora.
1. J. G. Holt et. al. Bergey’s Manual of Determinative Bacteriology.
2. Wilcox, Wayne. “Fire Blight” http://www.nysaes.cornell.edu/pp/extension/tfabp/firepm.shtml
3. EPPO. “Data Sheets on Quarantine Pests: Erwinia amylovora” http://www.eppo.org/QUARANTINE/bacteria/Erwinia_amylovora/ERWIAM_ds.pdf
4. “Fire Blight” http://en.wikipedia.org/wiki/Erwinia_amylovora
5. Percy, H. "HortFACT - Fire blight (Erwinia amylovora) in apples and pears - An Introduction to the Disease." 22 Apr. 2009 <http://www.hortnet.co.nz/publications/hortfacts/hf205017.htm>.
6. Schroth, M. N., Segun V. Thomson, and W. J. Moller. "Streptomycin Resistance in Erwiniaamylovora." Ecology and Epidemiology 69.6 (1979): 565-68. Web. <http://www.apsnet.org/phyto/PDFS/1979/Phyto69n06_565.pdf>.
Created by students of M Glogowski, Jonathan Everson, Erin Koller, Adam Kennedy, Amanda Bowler, Jennifer Cox, John Cealey and Billy Zhao
7. Molina, L., Rezzonico, F., Défago, G., and Duffy, B. “Autoinduction in Erwinia amylovora: Evidence of an Acyl-Homoserine Lactone Signal in the Fire Blight Pathogen” Journal of Bacteriology, May 2005. Vol. 187. p. 3206-3213,