Sensitivity of Erwinia amylovora in Illinois Apple Orchards to Streptomycin, Oxytetracyline, Kasugamycin, and Copper

Isolation and Characterization of a Lytic Bacteriophage RH-42-1 of Erwinia amylovora from Orchard Soil in China

Fire blight, caused by the bacterium Erwinia amylovora, is a major threat to pear production worldwide. Bacteriophages, viruses that infect bacteria, are a promising alternative to antibiotics for controlling fire blight. In this study, we isolated a novel bacteriophage, RH-42-1, from Xinjiang, China. We characterized its biological properties, including host range, plaque morphology, infection dynamics, stability, and sensitivity to various chemicals. RH-42-1 infected several E. amylovora strains but not all. It produced clear, uniform plaques and exhibited optimal infectivity at a multiplicity of infection (MOI) of 1, reaching a high titer of 9.6 × 109 plaque-forming units (PFU)/mL. The bacteriophage had a short latent period (10 min), a burst size of 207 PFU/cell, and followed a sigmoidal one-step growth curve. It was stable at temperatures up to 60 °C but declined rapidly at higher temperatures. RH-42-1 remained viable within a pH range of 5 to 9 and was sensitive to extreme pH values. The bacteriophage demonstrates sustained activity upon exposure to ultraviolet radiation for 60 min, albeit with a marginal reduction. In our assays, it exhibited a certain level of resistance to 5% chloroform (CHCl3), 5% isopropanol (C3H8O), and 3% hydrogen peroxide (H2O2), which had little effect on its activity, whereas it showed sensitivity to 75% ethanol (C2H5OH). Electron microscopy revealed that RH-42-1 has a tadpole-shaped morphology. Its genome size is 14,942 bp with a GC content of 48.19%. Based on these characteristics, RH-42-1 was identified as a member of the Tectiviridae family, Alphatectivirus genus. This is the first report of a bacteriophage in this genus with activity against E. amylovora.

Biological and Genetic Characterizations of a Novel Lytic ΦFifi106 against Indigenous Erwinia amylovora and Evaluation of the Control of Fire Blight in Apple Plants

Erwinia amylovora is a devastating phytobacterium causing fire blight in the Rosaceae family. In this study, ΦFifi106, isolated from pear orchard soil, was further purified and characterized, and its efficacy for the control of fire blight in apple plants was evaluated. Its genomic analysis revealed that it consisted of 84,405 bp and forty-six functional ORFs, without any genes encoding antibiotic resistance, virulence, and lysogenicity. The phage was classified into the genus Kolesnikvirus of the subfamily Ounavirinae. ΦFifi106 specifically infected indigenous E. amylovora and E. pyrifoliae. The lytic activity of ΦFifi106 was stable under temperature and pH ranges of 4-50 °C and 4-10, as well as the exposure to ultraviolet irradiation for 6 h. ΦFifi106 had a latent period of 20 min and a burst size of 310 ± 30 PFU/infected cell. ΦFifi106 efficiently inhibited E. amylovora YKB 14808 at a multiplicity of infection (MOI) of 0.1 for 16 h. Finally, the pretreatment of ΦFifi106 at an MOI of 1000 efficiently reduced disease incidence to 37.0% and disease severity to 0.4 in M9 apple plants. This study addressed the use of ΦFifi106 as a novel, safe, efficient, and effective alternative to control fire blight in apple plants.

Phage Cocktail in Combination with Kasugamycin as a Potential Treatment for Fire Blight Caused by Erwinia amylovora

Recently, there has been an increasing number of blight disease reports associated with Erwinia amylovora and Erwinia pyrifoliae in South Korea. Current management protocols that have been conducted with antibiotics have faced resistance problems and the outbreak has not decreased. Because of this concern, the present study aimed to provide an alternative method to control the invasive fire blight outbreak in the nation using bacteriophages (phages) in combination with an antibiotic agent (kasugamycin). Among 54 phage isolates, we selected five phages, pEa_SNUABM_27, 31, 32, 47, and 48, based on their bacteriolytic efficacy. Although only phage pEa_SNUABM_27 showed host specificity for E. amylovora, all five phages presented complementary lytic potential that improved the host infectivity coverage of each phage All the phages in the cocktail solution could lyse phage-resistant strains. These strains had a decreased tolerance to the antibiotic kasugamycin, and a synergistic effect of phages and antibiotics was demonstrated both in vitro and on immature wound-infected apples. It is noteworthy that the antibacterial effect of the phage cocktail or phage cocktail-sub-minimal inhibitory concentration (MIC) of kasugamycin was significantly higher than the kasugamycin at the MIC. The selected phages were experimentally stable under environmental factors such as thermal or pH stress. Genomic analysis revealed these are novel Erwinia-infecting phages, and did not encode antibiotic-, virulence-, or lysogenic phage-related genes. In conclusion, we suggest the potential of the phage cocktail and kasugamycin combination as an effective strategy that would minimize the use of antibiotics, which are being excessively used in order to control fire blight pathogens.

Rapid identification of Erwinia amylovora and Pseudomonas syringae species and characterization of E. amylovora streptomycin resistance using quantitative PCR assays

Erwinia amylovora and Pseudomonas syringae are bacterial phytopathogens responsible for considerable yield losses in commercial pome fruit production. The pathogens, if left untreated, can compromise tree health and economically impact entire commercial fruit productions. Historically, the choice of effective control methods has been limited. The use of antibiotics was proposed as an effective control method. The identification of these pathogens and screening for the presence of antibiotic resistance is paramount in the adoption and implementation of disease control methods. Molecular tests have been developed and accepted for identification and characterization of these disease-causing organisms. We improved existing molecular tests by developing methods that are equal or superior in robustness for identifying E. amylovora or P. syringae while being faster to execute. In addition, the real-time PCR-based detection method for E. amylovora provided complementary information on the susceptibility or resistance to streptomycin of individual isolates. Finally, we describe a methodology and results that compare the aggressiveness of the different bacterial isolates on four apple cultivars. We show that bacterial isolates exhibit different behaviors when brought into contact with various apple varieties and that the hierarchical clustering of symptom severity indicates a population structure, suggesting a genetic basis for host cultivar specificity.

Two types of genetic carrier, the IncP genomic island and the novel IncP-1β plasmid, for the aac(2')-IIa gene that confers kasugamycin resistance in Acidovorax avenae ssp. avenae

A unique aminoglycoside antibiotic, kasugamycin (KSM), has been used to control many plant bacterial and fungal diseases in several countries. The emergence of KSM-resistant Acidovorax avenae ssp. avenae and Burkholderia glumae, which cause rice bacterial brown stripe and rice bacterial grain and seedling rot, respectively, is a serious threat for the effective control of these diseases. Previously, we have identified the aac(2')-IIa gene, encoding a KSM 2'-N-acetyltransferase, from both KSM-resistant pathogens. Although all KSM-resistant isolates from both species possess the aac(2')-IIa gene, only A. avenae strain 83 showed higher resistance than other strains. In this research, kinetic analysis indicates that an amino acid substitution from serine to threonine at position 146 of AAC(2')-IIa in strain 83 is not involved in this increased resistance. Whole draft genome analysis of A. avenae 83 shows that the aac(2')-IIa gene is carried by the novel IncP-1β plasmid pAAA83, whereas the genetic carrier of other strains, the IncP genomic island, is inserted into their chromosomes. The difference in the nucleotides of the promoter region of aac(2')-IIa between strain 83 and other strains indicates an additional transcription start site and results in the increased transcription of aac(2')-IIa in strain 83. Moreover, biological characterization of pAAA83 demonstrates that it can be transferred by conjugation and maintained in the host cells. These results demonstrate that acquisition of the aac(2')-IIa gene takes place in at least two ways and that the gene module, which includes aac(2')-IIa and the downstream gene, may be an important unit for the dissemination of antibiotic resistance.