Pseudomonas spp.

Bacteria Pseudomonas chlororaphis e Pseudomonas fluorescens are notable species within the genus Pseudomonas, widely recognized for their versatile metabolic characteristics, ecological importance and practical applications in various fields, such as biological pest control and plant growth promotion.

• Pseudomonas chlororaphis: The species name derives from the ability of this bacterium to produce green (chloroform) pigments under some cultivation conditions.

• Pseudomonas fluorescens: It receives this name due to the fluorescence it produces under ultraviolet light, resulting from the synthesis of compounds such as pyocyanin and fluorescein.

Taxonomy

Both species belong to the genus Pseudomonas, a group of Gram-negative, aerobic bacteria with polar flagella that give them mobility. The taxonomic classification of the two species is:

• Kingdom: Bacteria

• Phylum: Proteobacteria

• Class: Gammaproteobacteria

• Order: Pseudomonadales

• Family: Pseudomonadaceae

• Genus: Pseudomonas

The differentiation between P. chlororaphis e P. fluorescens is based on genetic, biochemical and metabolic analyses.

These species are closely related, but they present significant differences in their physiological and ecosystem profiles.

Etiology

The etiology of Pseudomonas chlororaphis e Pseudomonas fluorescens is closely linked to their relationship with the environment and the organisms with which they interact.

These bacteria are not pathogenic to humans or animals, but they have mechanisms that make them beneficial to plants and harmful to phytopathogenic microorganisms and insect pests.

Both species colonize mainly the rhizosphere (region around the roots of plants), where they establish mutualistic relationships. They promote plant growth through the production of hormones such as auxins and gibberellins, solubilization of insoluble phosphates and fixation of atmospheric nitrogen. This behavior is crucial for plant nutrition and for increasing their resistance to biotic and abiotic stresses.

A striking feature of these bacteria is the synthesis of secondary metabolites with antimicrobial properties. For example:

• Pseudomonas chlororaphis: produces phenazines, antimicrobial compounds that inhibit the growth of pathogenic fungi such as Fusarium oxysporum e Botrytis cinerea.

• Pseudomonas fluorescens: produces compounds such as 2,4-diacetylphloroglucinol (DAPG) and pyocyanin, which have antifungal and antibacterial activity. These metabolites help suppress diseases caused by soil-borne pathogens such as Rhizoctonia solani e pythium ultimum.

These bacteria can trigger defense responses in plants, a phenomenon known as Induction of Systemic Resistance (ISR). This occurs when bacteria release molecules that stimulate the plant to produce defense compounds, such as phytoalexins and pathogenesis-related proteins (PR-proteins). This response strengthens the plant against future attacks by pathogens and insect pests.

Bacteria compete with pathogenic microorganisms for essential resources such as iron and organic nutrients.

The production of siderophores (iron chelating compounds) by P. chlororaphis e P. fluorescens limits the availability of iron in the environment, making it difficult for pathogens that depend on this element to survive to grow.

Biology

the species Pseudomonas chlororaphis e Pseudomonas fluorescens are adapted to a wide variety of habitats, including soils, water, plant surfaces, and even the digestive tracts of insects.

They have a diversified metabolism, being able to use a wide range of organic substrates as a source of carbon and energy.

These bacteria also have high resistance to adverse environmental conditions, such as low temperatures and variable pH.

One of the most remarkable aspects of their biology is the production of secondary metabolites, such as siderophores, antibiotics, and extracellular enzymes. These compounds play crucial roles in microbial competition and interaction with plant hosts.

Features

Morphology: are Gram-negative bacilli, with dimensions ranging from 0,5–1,0 µm in width and 1,5–5,0 µm in length. They are mobile thanks to one or more polar flagella.

Growth: both grow well on standard culture media, such as nutrient agar, and can form colonies with distinctive characteristics (emerald green for P. chlororaphis and yellowish / fluorescent for P. fluorescens).

Metabolism: They are chemoorganotrophic, oxidase positive and catalase positive. They can fix nitrogen under certain conditions, contributing to soil fertility.

Genetics: have large and complex genomes, with the ability to adapt quickly to different environments.

Uses in agricultural pest control

The use of Pseudomonas chlororaphis e Pseudomonas fluorescens in agricultural pest control is based on its ability to modulate agricultural ecosystems, reducing populations of insect pests and associated pathogens.

These bacteria are used as biopesticides, biofertilizers and biological control agents in sustainable agricultural systems.

Direct control of insect pests: some strains of Pseudomonas fluorescens produce toxins that directly affect the digestive system of herbivorous insects, such as caterpillars (Spodoptera frugiperda), aphids (Myzus persicae ) and beetles (Diabrotica virgifera ). These toxins interfere with the absorption of nutrients and the intestinal microbiota of insects, leading to death or reducing their reproductive capacity.

Suppression of pest-associated pathogens: Many agricultural pests, such as aphids and leafhoppers, act as vectors for pathogenic viruses and fungi. By inhibiting the growth of these pathogens, Pseudomonas chlororaphis e Pseudomonas fluorescens indirectly reduce the damage caused by pests.

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• Fungal control such as Colletotrichum gloeosporioides (causing anthracnose) and Xanthomonas campestris (canker-causing bacteria) reduces the need for chemical insecticides.

• The suppression of nematodes, such as Meloidogyne incognita, also contributes to the reduction of secondary pests that exploit weakened plants.

Biofertilizers and biostimulants: By promoting plant growth, these bacteria indirectly strengthen plants against pest infestations. Healthy, well-nourished plants are less attractive to herbivorous insects and more resistant to stresses associated with infestations.

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• Pseudomonas chlororaphis improves the absorption of nutrients, especially nitrogen and phosphorus, in crops such as corn and wheat.

• Pseudomonas fluorescens stimulates the production of root biomass, increasing the plants' ability to access water and nutrients.

Integrated pest management (IPM): In the context of integrated pest management, these bacteria are used in combination with other strategies such as crop rotation, use of resistant varieties and selective chemical control. This reduces dependence on synthetic pesticides, minimizing environmental impacts and promoting agricultural sustainability.

Examples:

• Tomato cultivation: strains of Pseudomonas fluorescens were applied to control the red spider mite (Tetranychus urticae) and gray mold (Botrytis cinerea).

• Rice cultivation: Pseudomonas chlororaphis has been used to suppress fungal diseases and reduce populations of virus-vectoring leafhoppers.

• Potato cultivation: Both species have been used to control the potato beetle (Leptinotarsa ​​decemlineata) and pathogens such as Phytophthora infestans.
• Soybean cultivation: P. chlororaphis reduces disease severity by inhibiting fungal growth Phakopsora pachyrhizi; also acts against Fusarium solani e Rhizoctonia solani. P. fluorescens acts against Septoria glycones, pythium ultimum e Macrophomina phaseolina. And it helps to reduce populations of Meloidogyne incognita.

Some examples of agricultural pests against which pesticides containing Pseudomonas chlororaphis e Pseudomonas fluorescens: Aphis gossypii, Bemisia tabaci, Caliothrips brasiliensis, Dalbulus maidis, diaphorina citri, Dichelops melacanthus, Euschistus heros, Frankliniella schultzei, Leucoptera coffeella, Rhopalosiphum graminum, Tetranychus urticae.