First report of Pectobacterium brasiliense causing blackleg and soft rot of potato in Argentina.

Blackleg and soft rot diseases represent a major threat to the health of potato (Solanum tuberosum) and other vegetable, ornamental and fruit crops worldwide; their main causal agents are species of Pectobacterium and Dickeya. In May 2022, 60% of potato plants (cv. Spunta) in a production field in Córdoba, Argentina (31°32'36''S 64°09'46''W) showed soft rot, blackleg and wilt. To isolate the causal agent, decayed plant tissues were disinfected in 2% NaClO, macerated in sterile water and streaked on crystal violet pectate (CVP) medium. Plates were incubated at 28°C for 48 h. Colonies that produced a pit on CVP medium were purified on nutrient agar. Two of the isolates, called 1Aia and 1B, were characterized by tests commonly employed for the identification of pectinolytic bacteria (Schaad et al. 2001). Both produced Gram-negative rods that were facultatively anaerobic, oxidase negative, nonfluorescent on King´s B, resistant to erythromycin and caused soft rot of potato slices. In addition, these isolates did not produce the blue pigment indigoidine and grew on nutrient glucose agar containing 5% NaCl. Phenotypic characteristics of the isolates 1Aia and 1B were compatible with Pectobacterium spp. Genomic DNA was extracted using the commercially available Wizard® Genomic DNA Purification Kit (Promega) according to the manufacturer's instructions for the purification of DNA from Gram-negative bacteria. The isolates were positive in a PCR assay for Pectobacterium brasiliense (Duarte et al. 2004). The purified DNA of isolate 1Aia was used to construct a pooled Illumina library, which was sequenced at the Genomics Unit from the National Institute of Agricultural Technology (INTA, Argentina), by using high-throughput Illumina sequencing technology. Average nucleotide identity (ANI) calculation performed by FastANI v0.1.3 (Jain et al. 2018) showed 96.11% identity between the genome of the type strain LMG 21371 of P. brasiliense (Acc. no. JQOE00000000) and our strain 1Aia (Acc. no. JAYGXQ000000000). For pathogenicity test, 3-weeks-old potato plants (cv. Spunta) planted in pots were infiltrated with 10 µl of a bacterial suspension (1x107 CFU/ml) 5 cm above the base of the stem using a sterile syringe. Negative controls were infiltrated with sterile water. Plants were kept under greenhouse conditions and regularly watered. The experiment was performed twice with six plants per treatment. Two days after inoculation, plants treated with P. brasiliense strain 1Aia or 1B showed necrotic lesions on the stems and tubers soft rot symptoms while control plants remained asymptomatic. To fulfill Koch´s postulates, bacteria were re-isolated from symptomatic plants. Re-isolated bacteria, called 1Aia d and 1B d, were confirmed as P. brasiliense according to biochemical and PCR results, as outlined above. Also, the % ANI value between P. brasiliense isolates 1Aia and 1Aia d was 99.99% (Acc. no. JAYGXR000000000). To our knowledge, this is the first report of the occurrence of P. brasiliense in Argentina. This pathogen has been observed causing blackleg and tuber soft rot on potato in Brazil (Duarte et al. 2004), Netherlands (Nunes Leite et al. 2014), Switzerland (de Werra et al. 2015), Russia (Voronina et al. 2019), Serbia (Loc et al. 2022) and USA (Zhang et al. 2023), among other countries worldwide. Due to the important economic and nutritional value of the crop, the distribution of P. brasiliense needs to be investigated and monitored in order to develop effective control strategies.

Phylogenetic determinants of toxin gene distribution in genomes of Brevibacillus laterosporus.

Brevibacillus laterosporus is a globally ubiquitous, spore forming bacterium, strains of which have shown toxic activity against invertebrates and microbes and several have been patented due to their commercial potential. Relatively little is known about this bacterium. Here, we examined the genomes of six published and five newly determined genomes of B. laterosporus, with an emphasis on the relationships between known and putative toxin encoding genes, as well as the phylogenetic relationships between strains. Phylogenetically, strain relationships are similar using average nucleotide identity (ANI) values and multi-gene approaches, although PacBio sequencing revealed multiple copies of the 16S rDNA gene which lessened utility at the strain level. Based on ANI values, the New Zealand isolates were distant from other isolates and may represent a new species. While all of the genomes examined shared some putative toxicity or virulence related proteins, many specific genes were only present in a subset of strains.

Insecticidal spectrum and mode of action of the Bacillus thuringiensis Vip3Ca insecticidal protein.

The Vip3Ca protein, discovered in a screening of Spanish collections of Bacillus thuringiensis, was known to be toxic to Chrysodeixis chalcites, Mamestra brassicae and Trichoplusia ni. In the present study, its activity has been tested with additional insect species and we found that Cydia pomonella is moderately susceptible to this protein. Vip3Ca (of approximately 90kDa) was processed to an approximately 70kDa protein when incubated with midgut juice in all tested species. The kinetics of proteolysis correlated with the susceptibility of the insect species to Vip3Ca. The activation was faster to slower in the following order: M. brassicae (susceptible), Spodoptera littoralis (moderately susceptible), Agrotis ipsilon and Ostrinia nubilalis (slightly susceptible). Processing Vip3Ca by O. nubilalis or M. brassicae midgut juice did not significantly changed its toxicity to either insect species, indicating that the low susceptibility of O. nubilalis is not due to a problem in the midgut processing of the toxin. M. brassicae larvae fed with Vip3Ca showed binding of this toxin to the apical membrane of the midgut epithelial cells. Histopathological inspection showed sloughing of the epithelial cells with further disruption, which suggests that the mode of action of Vip3Ca is similar to that described for Vip3Aa. Biotin-labeled Vip3Ca and Vip3Aa bound specifically to M. brassicae brush border membrane vesicles and both toxins competed for binding sites. This result suggests that insects resistant to Vip3A may also be cross-resistant to Vip3C, which has implications for Insect Resistance Management (IRM).

Genomic diversity in European Spodoptera exigua multiple nucleopolyhedrovirus isolates.

Key virus traits such as virulence and transmission strategies rely on genetic variation that results in functional changes in the interactions between hosts and viruses. Here, comparative genomic analyses of seven isolates of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) with differing phenotypes were employed to pinpoint candidate genes that may be involved in host-virus interactions. These isolates obtained after vertical or horizontal transmission of infection in insects differed in virulence. Apart from one genome containing a piggyBac transposon, all European SeMNPV isolates had a similar genome size and content. Complete genome analyses of single nucleotide polymorphisms and insertions/deletions identified mutations in 48 ORFs that could result in functional changes. Among these, 13 ORFs could be correlated with particular phenotypic characteristics of SeMNPV isolates. Mutations were found in all gene functional classes and most of the changes we highlighted could potentially be associated with differences in transmission. The regulation of DNA replication (helicase, lef-7) and transcription (lef-9, p47) might be important for the establishment of sublethal infection prior to and following vertical transmission. Virus-host cell interactions also appear instrumental in the modulation of viral transmission as significant mutations were detected in virion proteins involved in primary (AC150) or secondary infections (ME35) and in apoptosis inhibition (IAP2, AC134). Baculovirus populations naturally harbour high genomic variation located in genes involved at different levels of the complex interactions between virus and host during the course of an infection. The comparative analyses performed here suggest that the differences in baculovirus virulence and transmission phenotypes involve multiple molecular pathways.

Draft genome sequence of Bacillus thuringiensis strain V-AB8.18, a novel isolate with potential nematicidal activity.

We report the draft genome of Bacillus thuringiensis strain V-AB8.18, comprising 308 contigs totaling 6,182,614 bp, with 35% G + C content. It contains 6,151 putative protein-coding genes, including App6 and Cry5-like crystal proteins, exhibiting 99% pairwise identity to nematicidal proteins App6Aa2 and Cry5Ba2, active against Meloidogyne incognita and Meloidogyne hapla.

Bacillus thuringiensis: A Broader View of Its Biocidal Activity.

Bacillus thuringiensis (Bt) is a Gram-positive bacterium that forms spores and produces parasporal crystalline inclusions containing Cry and Cyt proteins [...].

Positive selection analysis of Cyt proteins from Bacillus thuringiensis: A conservative trend driven by negative (purifying) selection.

Bacillus thuringiensis is a Gram-positive entomopathogenic bacterium that produces different pesticidal proteins: vegetative insecticidal proteins (Vpb1/Vpa2, Vip3, and Vpb4) during vegetative growth, which are secreted to the culture medium, and δ-endotoxins (Cry and Cyt) during sporulation, which accumulate into parasporal crystals. Cyt proteins are the smaller subset of δ-endotoxins targeting Diptera species. While Cry and Vip3 proteins undergo positive selection, our analysis suggests that Cyt proteins evolve following a conservative trend driven negative (purifying) selection.

Genome Sequence Analysis of Native Xenorhabdus Strains Isolated from Entomopathogenic Nematodes in Argentina.

Entomopathogenic nematodes from the genus Steinernema (Nematoda: Steinernematidae) are capable of causing the rapid killing of insect hosts, facilitated by their association with symbiotic Gram-negative bacteria in the genus Xenorhabdus (Enterobacterales: Morganellaceae), positioning them as interesting candidate tools for the control of insect pests. In spite of this, only a limited number of species from this bacterial genus have been identified from their nematode hosts and their insecticidal properties documented. This study aimed to perform the genome sequence analysis of fourteen Xenorhabdus strains that were isolated from Steinernema nematodes in Argentina. All of the strains were found to be able of killing 7th instar larvae of Galleria mellonella (L.) (Lepidoptera: Pyralidae). Their sequenced genomes harbour 110 putative insecticidal proteins including Tc, Txp, Mcf, Pra/Prb and App homologs, plus other virulence factors such as putative nematocidal proteins, chitinases and secondary metabolite gene clusters for the synthesis of different bioactive compounds. Maximum-likelihood phylogenetic analysis plus average nucleotide identity calculations strongly suggested that three strains should be considered novel species. The species name for strains PSL and Reich (same species according to % ANI) is proposed as Xenorhabdus littoralis sp. nov., whereas strain 12 is proposed as Xenorhabdus santafensis sp. nov. In this work, we present a dual insight into the biocidal potential and diversity of the Xenorhabdus genus, demonstrated by different numbers of putative insecticidal genes and biosynthetic gene clusters, along with a fresh exploration of the species within this genus.

UV protection and insecticidal activity of microencapsulated Vip3Ag4 protein in Bacillus megaterium.

In this study, secretable Vip3Ag4 protein was encapsulated in Bacillus megaterium and used for quantitative bioassays, in order to determine the UV photoprotective capacity of the cell, for preventing inactivation of the insecticidal activity of the protein. The non-encapsulated and purified protein was exposed to the UV light showing a LC50 of 518 ng/cm2 against Spodoptera littoralis larvae, whereas the exposed encapsulated protein exhibited 479 ng/cm2. In addition to the capability to accumulate Vip3 proteins for the development of novel insecticidal formulates, the B. megaterium cell has demonstrated to provide moderate protection against the deleterious action of UV light.

Bacillus thuringiensis Bt_UNVM-84, a Novel Strain Showing Insecticidal Activity against Anthonomus grandis Boheman (Coleoptera: Curculionidae).

Bacillus thuringiensis is a Gram-positive bacterium known for its insecticidal proteins effective against various insect pests. However, limited strains and proteins target coleopteran pests like Anthonomous grandis Boheman, causing substantial economic losses in the cotton industry. This study focuses on characterizing a Bacillus sp. strain, isolated from Oncativo (Argentina), which exhibits ovoid to amorphous parasporal crystals and was designated Bt_UNVM-84. Its genome encodes genes for the production of two pairs of binary Vpb1/Vpa2 proteins and three Cry-like proteins showing similarity with different Cry8 proteins. Interestingly, this gene content was found to be conserved in a previously characterized Argentine isolate of B. thuringiensis designated INTA Fr7-4. SDS-PAGE analysis revealed a major band of 130 kDa that is proteolytically processed to an approximately 66-kDa protein fragment by trypsin. Bioassays performed with spore-crystal mixtures demonstrated an interesting insecticidal activity against the cotton boll weevil A. grandis neonate larvae, resulting in 91% mortality. Strain Bt_UNVM-84 is, therefore, an interesting candidate for the efficient biological control of this species, causing significant economic losses in the cotton industry in the Americas.

Vip3C, a novel class of vegetative insecticidal proteins from Bacillus thuringiensis.

Three vip3 genes were identified in two Bacillus thuringiensis Spanish collections. Sequence analysis revealed a novel Vip3 protein class (Vip3C). Preliminary bioassays of larvae from 10 different lepidopteran species indicated that Vip3Ca3 caused more than 70% mortality in four species after 10 days at 4 µg/cm(2).

Analysis of a naturally-occurring deletion mutant of Spodoptera frugiperda multiple nucleopolyhedrovirus reveals sf58 as a new per os infectivity factor of lepidopteran-infecting baculoviruses.

The Nicaraguan population of Spodoptera frugiperda multiple nucleopolyhedrovirus, SfMNPV-NIC, is structured as a mixture of nine genotypes (A-I). Occlusion bodies (OBs) of SfMNPV-C, -D and -G pure genotypes are incapable of oral transmission; a phenotype which in SfMNPV-C and -D is due to the absence of pif1 and pif2 genes. The complete sequence of the SfMNPV-G genome was determined to identify possible factors involved in this phenotype. Deletions of 4860 bp (22,366-27,225) and 60 bp (119,759-119,818) were observed in SfMNPV-G genome compared with that of the predominant complete genotype SfMNPV-B (132,954 bp). However no genes homologous to previously described per os infectivity factors were located within the deleted sequences. Significant differences were detected in the nucleotide sequence in sf58 gene (unknown function) that produced changes in the amino acid sequence and the predicted secondary structure of the corresponding protein. This gene is conserved only in lepidopteran baculoviruses (alpha- and betabaculoviruses). To determine the role of sf58 in peroral infectivity a deletion mutant was constructed using bacmid technology. OBs of the deletion mutant (Sf58null) were not orally infectious for S. frugiperda larvae, whereas Sf58null rescue virus OBs recovered oral infectivity. Sf58null DNA and occlusion derived virions (ODVs) were as infective as SfMNPV bacmid DNA and ODVs in intrahemocelically infected larvae or cell culture, indicating that defects in ODV or OB morphogenesis were not involved in the loss of peroral infectivity. Addition of optical brightener or the presence of the orally infectious SfMNPV-B OBs in mixtures with SfMNPV-G OBs did not recover Sf58null OB infectivity. According to these results sf58 is a new per os infectivity factor present only in lepidopteran baculoviruses.

Sequence comparison between three geographically distinct Spodoptera frugiperda multiple nucleopolyhedrovirus isolates: Detecting positively selected genes.

The complete genomic sequence of a Nicaraguan plaque purified Spodoptera frugiperda nucleopolyhedrovirus (SfMNPV) genotype SfMNPV-B was determined and compared to previously sequenced isolates from United States (SfMNPV-3AP2) and Brazil (SfMNPV-19). The genome of SfMNPV-B (132,954bp) was 1623bp and 389bp larger than that of SfMNPV-3AP2 and SfMNPV-19, respectively. Genome size differences were mainly due to a deletion located in the SfMNPV-3AP2 egt region and small deletions and point mutations in SfMNPV-19. Nucleotide sequences were strongly conserved (99.35% identity) and a high degree of predicted amino acid sequence identity was observed. A total of 145 open reading frames (ORFs) were identified in SfMNPV-B, two of them (sf39a and sf110a) had not been previously identified in the SfMNPV-3AP2 and SfMNPV-19 genomes and one (sf57a) was absent in both these genomes. In addition, sf6 was not previously identified in the SfMNPV-19 genome. In contrast, SfMNPV-B and SfMNPV-19 both lacked sf129 that had been reported in SfMNPV-3AP2. In an effort to identify genes potentially involved in virulence or in determining population adaptations, selection pressure analysis was performed. Three ORFs were identified undergoing positive selection: sf49 (pif-3), sf57 (odv-e66b) and sf122 (unknown function). Strong selection for ODV envelope protein genes indicates that the initial infection process in the insect midgut is one critical point at which adaptation acts during the transmission of these viruses in geographically distant populations. The function of ORF sf122 is being examined.

Baculovirus Expression and Functional Analysis of Vpa2 Proteins from Bacillus thuringiensis.

The mode of action underlying the insecticidal activity of the Bacillus thuringiensis (Bt) binary pesticidal protein Vpb/Vpa (formerly Vip1/Vip2) is uncertain. In this study, three recombinant baculoviruses were constructed using Bac-to-Bac technology to express Vpa2Ac1 and two novel Vpa2-like genes, Vpa2-like1 and Vpa2-like2, under the baculovirus p10 promoter in transfected Sf9 cells. Pairwise amino acid analyses revealed a higher percentage of identity and a lower number of gaps between Vpa2Ac1 and Vpa2-like2 than to Vpa2-like1. Moreover, Vpa2-like1 lacked the conserved Ser-Thr-Ser motif, involved in NAD binding, and the (F/Y)xx(Q/E)xE consensus sequence, characteristic of the ARTT toxin family involved in actin polymerization. Vpa2Ac1, Vpa2-like1 and Vpa2-like2 transcripts and proteins were detected in Sf9 culture cells, but the signals of Vpa2Ac1 and Vpa2-like2 were weak and decreased over time. Sf9 cells infected by a recombinant bacmid expressing Vpa2-like1 showed typical circular morphology and produced viral occlusion bodies (OBs) at the same level as the control virus. However, expression of Vpa2Ac1 and Vpa2-like2 induced cell polarization, similar to that produced by the microfilament-destabilizing agent cytochalasin D and OBs were not produced. The presence of filament disrupting agents, such as nicotinamide and nocodazole, during transfection prevented cell polarization and OB production was observed. We conclude that Vpa2Ac1 and Vpa2-like2 proteins likely possess ADP-ribosyltransferase activity that modulated actin polarization, whereas Vpa2-like1 is not a typical Vpa2 protein. Vpa2-like2 has now been designated Vpa2Ca1 (accession number AAO86513) by the Bacillus thuringiensis delta-endotoxin nomenclature committee.

Draft Genome Sequence of Bacillus cereus CITVM-11.1, a Strain Exhibiting Interesting Antifungal Activities.

Bacillus cereus is a gram-positive, spore-forming bacterium possessing an important and historical record as a human-pathogenic bacterium. However, several strains of this species exhibit interesting potential to be used as plant growth-promoting rhizobacteria. Here, we report the draft genome sequence of B. cereus strain CITVM-11.1, which consists of 37 contig sequences, accounting for 5,746,486 bp (with a GC content of 34.8%) and 5,752 predicted protein-coding sequences. Several of them could potentially be involved in plant-bacterium interactions and may contribute to the strong antagonistic activity shown by this strain against the charcoal root rot fungus, Macrophomina phaseolina. This genomic sequence also showed a number of genes that may confer this strain resistance against several polluting heavy metals and for the bioconversion of mycotoxins.

Is the Insect World Overcoming the Efficacy of Bacillus thuringiensis?

The use of chemical pesticides revolutionized agriculture with the introduction of DDT (Dichlorodiphenyltrichloroethane) as the first modern chemical insecticide. However, the effectiveness of DDT and other synthetic pesticides, together with their low cost and ease of use, have led to the generation of undesirable side effects, such as pollution of water and food sources, harm to non-target organisms and the generation of insect resistance. The alternative comes from biological control agents, which have taken an expanding share in the pesticide market over the last decades mainly promoted by the necessity to move towards more sustainable agriculture. Among such biological control agents, the bacterium Bacillus thuringiensis (Bt) and its insecticidal toxins have been the most studied and commercially used biological control agents over the last 40 years. However, some insect pests have acquired field-evolved resistance to the most commonly used Bt-based pesticides, threatening their efficacy, which necessitates the immediate search for novel strains and toxins exhibiting different modes of action and specificities in order to perpetuate the insecticidal potential of this bacterium.