Diversity of pathogenic Pseudomonas isolated from citrus in Tunisia

Abstract

The damages observed in Tunisian citrus orchards have prompted studies on the Pseudomonas spp. responsible for blast and black pit. Prospective orchards between 2015 and 2017 showed that the diseases rapidly spread geographically and to new cultivars. A screening of Pseudomonas spp. isolated from symptomatic trees revealed their wide diversity according to phylogenetic analysis of their housekeeping rpoD and cts genes. The majority of strains were affiliated to Pseudomonas syringae pv. syringae (Phylogroup PG02b), previously described in Tunisia. However, they exhibited various BOX-PCR fingerprints and were not clonal. This work demonstrated, for the first time in Tunisia, the involvement of Pseudomonas cerasi (PG02a) and Pseudomonas congelans (PG02c). The latter did not show significant pathogenicity on citrus, but was pathogenic on cantaloupe and active for ice nucleation that could play a role in the disease. A comparative phylogenetic study of citrus pathogens from Iran, Montenegro and Tunisia revealed that P. syringae (PG02b) strains are closely related but again not clonal. Interestingly P. cerasi (PG02a) was isolated in two countries and seems to outspread. However, its role in the diseases is not fully understood and it should be monitored in future studies. The diversity of pathogenic Pseudomonas spp. and the extension of the diseases highlight that they have become complex and synergistic. It opens questions about which factors favor diseases and how to fight against them efficiently and with sustainable means.

Antimicrobial Multiresistant Phenotypes of Genetically Diverse Pseudomonas spp. Isolates Associated with Tomato Plants in Chilean Orchards

“…The analysis of the individual genes showed that the result obtained from the individual analysis of the rpoD gene was the one that best represented the clustering results obtained by MLSA. This agrees with what was seen by Oueslati et al, who used the rpoD, 16S, and gyrB genes to study the phylogenetic relationship of Pseudomonas syringae isolates [63]. Other authors have proposed that the cts gene alone can accurately predict the phylogenetic grouping for the P. syringae group [47].…”

“…Regarding gentamicin, the absence of resistance among the isolates studied agrees with earlier studies where it was seen that resistance to this antibiotic is not a widespread characteristic in the genus Pseudomonas [28,63]. This can be explained since, in contrast to streptomycin, gentamicin binds to multiple sites on ribosomes; thus, several mutations in bacterial chromosome would be needed to generate spontaneous mutants resistant to gentamicin [38].…”

“…However, apart from the high frequency of resistance to ampicillin, these "intrinsic" resistance characteristics have not been observed in environmental or agricultural crop-associated Pseudomonas isolates in this or in previous studies [28,63,69]. Although resistance to these antimicrobials cannot be considered an intrinsic characteristic, some Pseudomonas isolates with phenotypes resistant to ampicillin, chloramphenicol, and tetracycline have been found in this study and previously [28,37,63,69]. In addition to ampicillin resistance, a chloramphenicol resistance phenotype was also seen with high frequency among the Pseudomonas isolates.…”

“…In the context of antibiotic resistance, it has been determined that P. aeruginosa, which has been extensively studied due to its effect on human health, possesses intrinsic resistance to some of these compounds, including ampicillin, chloramphenicol, and tetracyclines [68]. However, apart from the high frequency of resistance to ampicillin, these "intrinsic" resistance characteristics have not been observed in environmental or agricultural crop-associated Pseudomonas isolates in this or in previous studies [28,63,69]. Although resistance to these antimicrobials cannot be considered an intrinsic characteristic, some Pseudomonas isolates with phenotypes resistant to ampicillin, chloramphenicol, and tetracycline have been found in this study and previously [28,37,63,69].…”

Biological and Molecular Characterization of the Lytic Bacteriophage SoKa against Pseudomonas syringae pv. syringae, Causal Agent of Citrus Blast and Black Pit in Tunisia

“…Although some studies have tested the efficacy of certain phages against the P. syringae complex, no study has so far reported on the isolation and characterization of phages with potential lytic activity against the citrus phytopathogenic Pseudomonas, and particularly against P. syringae pv. syringae PG02b, the most aggressive phylogroup in Tunisian citrus orchards [23,24]. In the present study, we report on the isolation and molecular characterization of the SoKa phage.…”