Development of a Bacteriophage Cocktail against Pectobacterium carotovorum Subsp. carotovorum and Its Effects on Pectobacterium Virulence

Bacteriophage cocktail for biocontrol of soft rot disease caused by Pectobacterium species in Chinese cabbage

Pectobacterium spp. are necrotrophic plant pathogens that cause the soft rot disease in Chinese cabbage, resulting in severe yield loss. The use of conventional antimicrobial agents, copper-based bactericides, and antibiotics has encountered several limitations, such as bioaccumulation on plants and microbial resistance. Bacteriophages (phages) are considered promising alternative antimicrobial agents against diverse phytopathogens. In this study, we isolated and characterized two virulent phages (phiPccP-2 and phiPccP-3) to develop a phage cocktail. Morphological and genomic analyses revealed that two phages belonged to the Tevenvirinae and Mccorquodalevirinae subfamilies, respectively. The phiPccP-2 and phiPccP-3 phages, which have a broad host range, were stable at various environmental conditions, such as various pHs and temperatures and exposure to ultraviolet light. The phage cocktail developed using these two lytic phages inhibited the emergence of phage-resistant bacteria compared to single-phage treatments in in vitro challenge assays. The phage cocktail treatment effectively prevented the development of soft rot symptom in matured Chinese cabbage leaves. Additionally, the phage cocktail comprising three phages (phiPccP-1, phiPccP-2, and phiPccP-3) showed superior biocontrol efficacy against the mixture of Pectobacterium strains in Chinese cabbage seedlings. These results suggest that developing phage cocktails is an effective approach for biocontrol of soft rot disease caused by Pectobacterium strains in crops compared to single-phage treatments. KEY POINTS: •Two newly isolated Pectobacterium phages, phiPccP-2 and phiPccP-3, infected diverse Pectobacterium species and effectively inhibited the emergence of phage-resistant bacteria. •Genomic and physiological analyses suggested that both phiPccP-2 and phiPccP-3 are lytic phages and that their lytic activities are stable in the environmental conditions under which Chinese cabbage grows. •Treatment using a phage cocktail comprising phiPccP-2 and phiPccP-3 efficiently suppressed soft rot disease in detached mature leaves and seedlings of Chinese cabbage, indicating the applicability of the phage cocktail as an alternative antimicrobial agent.

Exploring the historical roots, advantages and efficacy of phage therapy in plant diseases management

In the drug-resistance era, phage therapy has received considerable attention from worldwide researchers. Phage therapy has been given much attention in public health but is rarely applied to control plant diseases. Herein, we discuss phage therapy as a biocontrol approach against several plant diseases. The emergence of antibiotic resistance in agriculturally important pathogenic bacteria and the toxic nature of different synthetic compounds used to control microbes has driven researchers to rethink the century-old strategy of phage therapy''. Compared to other treatment strategies, phage therapy offers remarkable advantages such as high specificity, less chances of drug resistance, non-harmful nature, and benefit to soil microbial flora. The optimizations and protective formulations of phages are significant accomplishments; however, steps towards a better understanding of the physiologic characteristics of phages need to be preceded to commercialize their use. The future of phage therapy in the context of plant disease management is promising and could play a significant role in sustainable agriculture. Ongoing research will likely affirm the safety of phage therapy, ensuring that it does not harm non-target organisms, including beneficial soil microbes. Phage therapy could become vital in addressing global food security challenges, particularly in regions heavily impacted by plant bacterial diseases. Efforts to create formulations that enhance the stability and shelf-life of phages will be crucial, especially for their use in varied environmental conditions.

Targeted metabolite profiling of Salvia rosmarinus Italian local ecotypes and cultivars and inhibitory activity against Pectobacterium carotovorum subsp. carotovorum

Introduction

The development of agriculture in terms of sustainability and low environmental impact is, at present, a great challenge, mainly in underdeveloped and marginal geographical areas. The Salvia rosmarinus "Eretto Liguria" ecotype is widespread in Liguria (Northwest Italy), and farmers commonly use it by for cuttings and for marketing. In the present study, this ecotype was characterized in comparison with other cultivars from the same geographical region and Campania (Southern Italy), with a view to application and registration processes for the designation of protected geographical indications. Moreover, the possibility of using the resulting biomass after removing cuttings or fronds as a source of extracts and pure compounds to be used as phytosanitary products in organic farming was evaluated. Specifically, the potential of rosemary extracts and pure compounds to prevent soft rot damage was then tested.

Methods

A targeted NMR metabolomic approach was employed, followed by multivariate analysis, to characterize the rosemary accessions. Bacterial soft rot assay and disk diffusion test were carried out to evaluate the activity of extracts and isolated compounds against Pectobacterium carotovorum subsp. carotovorum. Enzymatic assay was performed to measure the in vitro inhibition of the pectinase activity produced by the selected pathogen. Molecular docking simulations were used to explore the possible interaction of the selected compounds with the pectinase enzymes.

Results and Discussion

The targeted metabolomic analysis highlighted those different geographical locations can influence the composition and abundance of bioactive metabolites in rosemary extracts. At the same time, genetic factors are important when a single geographical area is considered. Self-organizing maps (SOMs) showed that the accessions of "Eretto Liguria" appeared well characterized when compared to the others and had a good content in specialized metabolites, particularly carnosic acid. Soft rotting Enterobacteriaceae belonging to the Pectobacterium genus represent a serious problem in potato culture. Even though rosemary methanolic extracts showed a low antibacterial activity against a strain of Pectobacterium carotovorum subsp. carotovorum in the disk diffusion test, they showed ability in reducing the soft rot damage induced by the bacterium on potato tissue. 7-O-methylrosmanol, carnosol and isorosmanol appeared to be the most active components. In silico studies indicated that these abietane diterpenoids may interact with P. carotovorum subsp. carotovorum pectate lyase 1 and endo-polygalacturonase, thus highlighting these rosemary components as starting points for the development of agents able to prevent soft rot progression.

Occurrence, Characteristics, and qPCR-Based Identification of Pectobacterium versatile Causing Soft Rot of Chinese Cabbage in China

Pectobacterium is one of the most important genera of phytopathogenic bacteria. It can cause soft-rot diseases on a wide range of plant species across the world. In this study, three Pectobacterium strains (KC01, KC02, and KC03) were isolated from soft-rotted Chinese cabbage in Beijing, China. These three strains were identified as Pectobacterium versatile based on phylogenetic analysis of Pectobacterium 16S ribosomal RNA, pmrA, and 504 Pectobacterium core genes, as well as a genomic average nucleotide identity analysis. Their biochemical characteristics were found to be similar to the P. versatile type strain ICMP9168T but differed in response to citric acid, stachyose, D-glucuronic acid, dextrin, and N-acetyl-β-D-mannosamine. All of the tested P. versatile strains showed different carbohydrate utilization abilities compared with P. carotovorum and P. odoriferum, particularly in their ability to utilize D-arabitol, L-rhamnose, and L-serine. Under laboratory conditions, the maceration ability of P. versatile on Chinese cabbage was the highest at 28°C, compared with those at 13, 28, 23, and 33°C. Additionally, P. versatile could infect all of the 17 known Pectobacterium host plants, except for Welsh onion (Allium fistulosum). A SYBR Green quantitative PCR (qPCR) detection system was developed to distinguish P. versatile from other soft-rot bacteria based on the combined performance of melting curve (with a single melting peak at around 85°C) and fluorescence curve (with cycle threshold <30) when the bacterial genomic DNA concentration was in the range of 10 pg/µl to 10 ng/µl. This study is the first to report the presence of P. versatile on Chinese cabbage in China, as well as a specific and sensitive qPCR assay that can be used to quickly identify P. versatile. The work contributes to a better understanding of P. versatile and will facilitate the effective diagnosis of soft-rot disease, ultimately benefitting commercial crop production.

Spontaneous mutations in hlyD and tuf genes result in resistance of Dickeya solani IPO 2222 to phage ϕD5 but cause decreased bacterial fitness and virulence in planta

Lytic bacteriophages able to infect and kill Dickeya spp. can be readily isolated from virtually all Dickeya spp. containing environments, yet little is known about the selective pressure those viruses exert on their hosts. Two spontaneous D. solani IPO 2222 mutants (0.8% of all obtained mutants), DsR34 and DsR207, resistant to infection caused by lytic phage vB_Dsol_D5 (ΦD5) were identified in this study that expressed a reduced ability to macerate potato tuber tissues compared to the wild-type, phage-susceptible D. solani IPO 2222 strain. Genome sequencing revealed that genes encoding: secretion protein HlyD (in mutant DsR34) and elongation factor Tu (EF-Tu) (in mutant DsR207) were altered in these strains. These mutations impacted the DsR34 and DsR207 proteomes. Features essential for the ecological success of these mutants in a plant environment, including their ability to use various carbon and nitrogen sources, production of plant cell wall degrading enzymes, ability to form biofilms, siderophore production, swimming and swarming motility and virulence in planta were assessed. Compared to the wild-type strain, D. solani IPO 2222, mutants DsR34 and DsR207 had a reduced ability to macerate chicory leaves and to colonize and cause symptoms in growing potato plants.

The application of adaptively evolved thermostable bacteriophage ΦYMFM0293 to control spp. in poultry skin

Bacteriophages, bacterial viruses, are now being re-highlighted as one of the promising alternative antimicrobial agents to control bacterial pathogens in various fields, including the food industry. However, wild-type (WT) phages isolated from nature are vulnerable to external stresses such as heat, limiting the usability of phages in thermal processing. Here, we applied an adaptive laboratory evolution approach to improving the heat stability of newly isolated Salmonella-infecting lytic phage ΦYMFM0293 and examined its application in the poultry scalding process. After 15 cycles of exposure to sub-lethal temperature, the obtained adaptively evolved (AE) phages maintained approximately 3-log more infectious particles at 73 or 74 °C than the WT and non-heat-treated control phages. Missense mutations mainly concentrated in the genes related to the phage tail module were identified from the independently obtained heat-challenged phages, regardless of host Salmonella's heat-shock protein chaperone induction. These results demonstrated the necessity and sufficiency of the phage exposures to heat for thermal adaptation and suggested the involvement of the phage tail in heat stability. No significant physiological or morphological changes except the mutually offsetting phage replication parameters were observed in the AE phages. Accordingly, hot water supplemented with the AE phages significantly reduced the number of artificially contaminated Salmonella cells on chicken and duck skin in the mimicked scalding process. The AE strategy used here could be applied to other WT phages to improve their usability as more feasible antimicrobials for food safety.

Disruption of the metC Gene Affects Methionine Biosynthesis in Pectobacterium carotovorum subsp. carotovorum Pcc21 and Reduces Soft-Rot Disease

Plant pathogenic Pectobacterium species cause severe soft rot/blackleg diseases in many economically important crops worldwide. Pectobacterium utilizes plant cell wall degrading enzymes (PCWDEs) as the main virulence determinants for its pathogenicity. In this study, we screened a random mutant, M29 is a transposon insertion mutation in the metC gene encoding cystathionine β-lyase that catalyzes cystathionine to homocysteine at the penultimate step in methionine biosynthesis. M29 became a methionine auxotroph and resulted in growth defects in methionine-limited conditions. Impaired growth was restored with exogenous methionine or homocysteine rather than cystathionine. The mutant exhibited reduced soft rot symptoms in Chinese cabbages and potato tubers, maintaining activities of PCWDEs and swimming motility. The mutant was unable to proliferate in both Chinese cabbages and potato tubers. The reduced virulence was partially restored by a complemented strain or 100 µM of methionine, whereas it was fully restored by the extremely high concentration (1 mM). Our transcriptomic analysis showed that genes involved in methionine biosynthesis or transporter were downregulated in the mutant. Our results demonstrate that MetC is important for methionine biosynthesis and transporter and influences its virulence through Pcc21 multiplication in plant hosts.