Identification of novel virulence factors in Erwinia amylovora through temporal transcriptomic analysis of infected apple flowers under field conditions

Efficacy of Erwinia amylovora and Xanthomonas campestris pv campestris phages to control fire blight and black rot in vivo

Phytopathogens, such as Erwinia amylovora and Xanthomonas campestris, pose significant threats to agriculture, leading to substantial economic losses. Traditional chemical pesticides can harm soil fertility, contaminate water, and impact non-target organisms such as natural predators and pollinators, highlighting the need for sustainable pest control methods. This study explores the use of bacteriophages as biocontrol agents against E. amylovora, which causes fire blight, and X. campestris pv. campestris, responsible for black rot in cruciferous vegetables. Bacteriophages were isolated from urban wastewater and tested for their lytic activity against these pathogens. Three virulent phages were identified: ɸEF1 and ɸEF2 against E. amylovora and ɸXF1 against X. campestris pv. campestris. Genetic analysis confirmed the absence of known lysogeny-related genes, indicating that these phages are ideal candidates for biocontrol applications. In vitro assays demonstrated significant bacterial population reductions. Specifically, ɸEF1 killed 92.1% of the E. amylovora population at a multiplicity of infection (MOI) of 1 after 3 h, while ɸEF2 reduced the population by 98.1%. When combined in a 1:1 ratio, the two phages reduced E. amylovora populations by 99.7%, and no regrowth of resistant cells was observed, which was not the case when the phages were applied individually. ɸXF1 killed 99.9% of X. campestris pv. campestris populations at an MOI of 1 after 5 h. In vivo experiments using pears and kohlrabi as infection models further validated the phage effectiveness. Treated pears showed reduced fire blight symptoms, and kohlrabi plants exhibited markedly less necrosis from black rot compared to untreated controls.IMPORTANCEThree new virulent phages have been isolated: two targeting Erwinia amylovora and one targeting Xanthomonas campestris pv. campestris. All phages were able to rapidly reduce the population of their corresponding phytopathogens and alleviate disease symptoms in in vivo plant models. These findings highlight the potential of these phages as biocontrol agents for managing bacterial plant diseases, offering an alternative to traditional chemical treatments.

Characterization of novel Erwinia amylovora-specific phiEaSP1 phage and its application as phage cocktail for managing fire blight in apples

Erwinia amylovora (Ea) is a devastating bacterial pathogen that causes fire blight disease in Rosaceae family plants, including apples and pears. The use of bacteriophages is an alternative strategy to antibiotics for managing bacterial pathogens. In this study, the Ea-specific virulent phiEaSP1 phage was characterized, and its biocontrol efficacy against Ea was evaluated in apple seedlings. Genomic analyses revealed that phiEaSP1 belongs to the family Chaseviridae, subfamily Cleopatravirinae, and genus Loessnervirus. Most phiEaSP1 particles bound to the host cell surface within 5 min, and one virion made 68 progenies within 20 min of infection. The phage rapidly lysed Ea cells in vitro and maintained its lytic activity after incubation under different environmental conditions, including temperature, pH, and UV-A, as well as in the soil, with surfactants, and on apple seedlings. Receptor analysis using the Tn5 random mutant library of Ea TS3128 demonstrated that phiEaSP1 recognizes lipopolysaccharide as a receptor, whereas phiEaP-8 and phiEaP-21 recognize cellulose as a receptor. Protective efficacy against fire blight was tested on apple seedlings pretreated with the single phiEaSP1 or a phage cocktail containing phiEaSP1, phiEaP-8, and phiEaP-21. No or only weak symptoms were observed in the phage-treated seedlings. The application of a phage cocktail showed better control efficacy, indicating the potential of the phage cocktail, including phiEaSP1, as a preventive agent. Taken together, these results suggest that the use of a phage cocktail containing phiEaSP1 could be a potential strategy for the biocontrol of fire blight disease in apples.

Evaluation of pathogenicity variation between two Erwinia species in apples and their population using a duplex real-time PCR method

Fire blight and black shoot blight diseases, caused by Erwinia amylovora and Erwinia pyrifoliae, respectively, continue to spread several areas in Korea, despite intensive efforts by the government to control diseases. The distribution pattern of fire blight and black shoot blight is different from each other in Korea. Consequently, it is required to investigate the pathogenicity of E. amylovora and E. pyrifoliae in apple trees. The disease severity of fire blight and black shoot blight was compared in this study by an artificial inoculation of E. amylovora and E. pyrifoliae suspensions into the abaxial veins of apple leaves and measuring their pathogenicity at varying temperatures. Furthermore, disease severity was assessed by inoculating E. amylovora and E. pyrifoliae in apple flowers and assessing their pathogenicity at various temperatures. The E. amylovora-inoculated flowers displayed greater disease index than E. pyrifoliae-inoculated flowers at temperatures ranging from 18°C to 25°C. Upon examining the population sizes of E amylovora and E. pyrifoliae in flowers using a real-time polymerase chain reaction (PCR), the Ct value of E. amylovora was found to be lower in the style including stigma and hypanthium than the Ct value of E. pyrifoliae, except at 18°C. Hypanthium contained E. amylovora TS3128 and E. pyrifoliae YKB12327 at >107 and 105 CFU/mL, respectively at 15°C. Furthermore, in this study, we investigated the population size of E. amylovora and E. pyrifoliae in apple flowers in relation to temperature in order to clarify the differences in their pathogenicity.

The Role of Heat Shock Protein (Hsp) Chaperones in Environmental Stress Adaptation and Virulence of Plant Pathogenic Bacteria

Plant pathogenic bacteria are responsible for a substantial number of plant diseases worldwide, resulting in significant economic losses. Bacteria are exposed to numerous stress factors during their epiphytic life and within the host. Their ability to survive in the host and cause symptomatic infections depends on their capacity to overcome stressors. Bacteria have evolved a range of defensive and adaptive mechanisms to thrive under varying environmental conditions. One such mechanism involves the induction of chaperone proteins that belong to the heat shock protein (Hsp) family. Together with proteases, these proteins are integral components of the protein quality control system (PQCS), which is essential for maintaining cellular proteostasis. However, knowledge of their action is considerably less extensive than that of human and animal pathogens. This study discusses the modulation of Hsp levels by phytopathogenic bacteria in response to stress conditions, including elevated temperature, oxidative stress, changes in pH or osmolarity of the environment, and variable host conditions during infection. All these factors influence bacterial virulence. Finally, the secretion of GroEL and DnaK proteins outside the bacterial cell is considered a potentially important virulence trait.

Erwinia amylovora Type III Secretion System Inhibitors Reduce Fire Blight Infection Under Field Conditions

Fire blight, caused by Erwinia amylovora, is an economically important disease in apples and pears worldwide. This pathogen relies on the type III secretion system (T3SS) to cause disease. Compounds that inhibit the function of the T3SS (T3SS inhibitors) have emerged as alternative strategies for bacterial plant disease management, as they block bacterial virulence without affecting growth, unlike traditional antibiotics. In this study, we investigated the mode of action of a T3SS inhibitor named TS108, a plant phenolic acid derivative, in E. amylovora. We showed that adding TS108 to an in vitro culture of E. amylovora repressed the expression of several T3SS regulon genes, including the master regulator gene hrpL. Further studies demonstrated that TS108 negatively regulates CsrB, a global regulatory small RNA, at the posttranscriptional level, resulting in a repression of hrpS, which encodes a key activator of hrpL. Additionally, TS108 has no impact on the expression of T3SS in Dickeya dadantii or Pseudomonas aeruginosa, suggesting that its inhibition of the E. amylovora T3SS is likely species specific. To better evaluate the performance of T3SS inhibitors in fire blight management, we conducted five independent field experiments in four states (Michigan, New York, Oregon, and Connecticut) from 2015 to 2022 and observed reductions in blossom blight incidence as high as 96.7% compared with untreated trees. In summary, the T3SS inhibitors exhibited good efficacy against fire blight.

Single-Cell Detection of Erwinia amylovora Using Bio-Functionalized SIS Sensor

Herein, we developed a bio-functionalized solution-immersed silicon (SIS) sensor at the single-cell level to identify Erwinia amylovora (E. amylovora), a highly infectious bacterial pathogen responsible for fire blight, which is notorious for its rapid spread and destructive impact on apple and pear orchards. This method allows for ultra-sensitive measurements without pre-amplification or labeling compared to conventional methods. To detect a single cell of E. amylovora, we used Lipopolysaccharide Transporter E (LptE), which is involved in the assembly of lipopolysaccharide (LPS) at the surface of the outer membrane of E. amylovora, as a capture agent. We confirmed that LptE interacts with E. amylovora via LPS through in-house ELISA analysis, then used it to construct the sensor chip by immobilizing the capture molecule on the sensor surface modified with 3'-Aminopropyl triethoxysilane (APTES) and glutaraldehyde (GA). The LptE-based SIS sensor exhibited the sensitive and specific detection of the target bacterial cell in real time. The dose-response curve shows a linearity (R2 > 0.992) with wide dynamic ranges from 1 to 107 cells/mL for the target bacterial pathogen. The sensor showed the value change (dΨ) of approximately 0.008° for growing overlayer thickness induced from a single-cell E. amylovora, while no change in the control bacterial cell (Bacillus subtilis) was observed, or negligible change, if any. Furthermore, the bacterial sensor demonstrated a potential for the continuous detection of E. amylovora through simple surface regeneration, enabling its reusability. Taken together, our system has the potential to be applied in fields where early symptoms are not observed and where single-cell or ultra-sensitive detection is required, such as plant bacterial pathogen detection, foodborne pathogen monitoring and analysis, and pathogenic microbial diagnosis.