Erwinia amylovora

A Microbial Biorealm page on the genus Erwinia amylovora

Classification

Higher order taxa

Bacteria; Proteobacteria; Gammaproteobacteria; Enterobacteriales; Enterobacteriaceae; Erwinia

Genus species

Erwinia amylovora

Common Name

Fire Blight

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).

Pathology

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.

Antibiotic Resistance

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 bight. The increased fear of the newly streptomycin resistant fire bight 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 bight 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.

References

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