Azotobacter paspali

Azotobacter paspali: A Comprehensive Reference

1. Identity, Taxonomy, and Nomenclature

Azotobacter paspali is a Gram-negative, free-living, aerobic, nitrogen-fixing bacterium first described formally in 1966 by Brazilian microbiologist Johanna Döbereiner. It was formally named Azotobacter paspali sp. nov. by Döbereiner in 1966, in a publication titled "uma bactéria fixadora de nitrogênio na rizosfera de Paspalum," published in Pesquisa Agropecuária Brasileira 1:357–365.

The organism has undergone taxonomic revision. The current formally accepted name is Azorhizophilus paspali (Döbereiner 1966) Thompson and Skerman 1981, though the earlier designation Azotobacter paspali Döbereiner 1966 remains widely used in the scientific and commercial literature. Per the NCBI Taxonomy Browser, the type strain is deposited under ATCC 23833, DSM 2283, JCM 20726, BCCM/LMG 3864, NBRC 102228, and NRRL B-14628.

Taxonomically, Azorhizophilus paspali is a species of Gammaproteobacteria in the family Pseudomonadaceae. More specifically, the genus Azotobacter is an adjunct to the Pseudomonadaceae family, a subclass of γ-Proteobacteria, which comprises seven recognized species: A. vinelandii, A. chroococcum, A. salinestris, A. nigricans, A. beijerinckii, A. paspali, and A. armeniacus.

Morphologically, Azotobacter are Gram-negative, blunt to oval short rods, 1.5–2 μm or more in diameter, found in soil, aquatic, plant rhizospheric, and phyllospheric habitats. In extreme environments, Azotobacter can form small circular thick-walled cysts, although cysts are unable to fix nitrogen. They are polymorphic, possess peritrichous flagella, produce polysaccharides, are sensitive to acidic pH, high salts, and temperatures above 35 °C, and grow on nitrogen-free medium, fixing atmospheric nitrogen.

In the supplement industry, this organism is listed under both its original name (Azotobacter paspali) and its currently accepted name (Azorhizophilus paspali). Commercially and in culture collections, Azorhizophilus paspali is identified as a bacterium isolated from rhizosphere soil of the plant Paspalum notatum in Brazil.

2. Natural Source and Ecological Niche

Only Azotobacter paspali was described to specifically associate with plant roots of Paspalum notatum cv Batatais, a tetraploid subtropical grass. This host-plant specificity is an ecologically remarkable property. This root-association bacterium shows remarkable signs of host-plant specificity to one ecotype of this grass — a specificity that is rare in non-symbiotic plant–microbe interactions.

First described in detail by Döbereiner and colleagues, A. paspali has been isolated from the rhizosphere of Paspalum notatum, a tetraploid subtropical grass, and is highly host specific.

The broader genus Azotobacter is commonly found in soil, water, sediments, and plant roots, and its species are generally found in slightly acidic to alkaline soils. The number of Azotobacter strains in soils is generally low (fewer than 10⁴ CFU per gram of soil), but they are found throughout the world, typically in 30 to 80 percent of sampled soils.

The bacterium's intimate association with Paspalum notatum is a form of associative symbiosis — distinct from the classic nodule-forming symbiosis of legume–rhizobium partnerships. This symbiotic association without nodule formation in grasses is called associative nitrogen fixation, and Azotobacter paspali was reported specifically from the rhizosphere of Paspalum notatum, a tropical grass.

3. Discovery and Historical Context

The genus Azotobacter was discovered in 1901 by Dutch microbiologist Martinus Willem Beijerinck, who isolated the type species Azotobacter chroococcum from Dutch soil samples — marking it as the first identified free-living, aerobic bacterium capable of fixing atmospheric nitrogen.

In subsequent decades, numerous additional Azotobacter types were isolated from rhizosphere and soil and categorized under the family Azotobacteraceae; among these, Döbereiner (1966) isolated and characterized a unique Azotobacter having a specific association with wild grassroots (Paspalum notatum), namely Azotobacter paspali.

Due to the fact that the new organism was found exclusively on the roots of two species of Paspalum, the name Azotobacter paspali sp. n. was proposed. The possibility that this Azotobacter represents an intermediate step in the evolution of legume symbiosis from nonsymbiotic nitrogen fixers was also discussed by Döbereiner.

Since the discovery of Azotobacter in 1901, it has attracted microbiologists' attention for its interesting potential in agriculture for nitrogen fixation as well as the synthesis of biologically active substances. It has a distinctly enhancing effect on crop production.

There is no documented tradition of deliberate human use of A. paspali as a dietary or medicinal supplement in any pre-modern culture. Its context prior to the late twentieth century is entirely agricultural — specifically as a component of biofertilizer preparations in tropical and subtropical farming systems. Its appearance in human dietary supplements is a relatively recent commercial development, as described below.

4. Commercial Forms and Preparations

Azotobacter paspali enters the supplement marketplace almost exclusively as one constituent within multi-strain soil-based organism (SBO) probiotic preparations, rather than as a standalone product. The most prominent commercial vehicle is the Prescript-Assist line of SBO probiotic–prebiotic supplements.

In its original Prescript-Assist formulation, Azotobacter paspali appears alongside other soil-based organisms including Arthrobacter agilis, Arthrobacter citreus, Azotobacter chroococcum, Azospirillum brasiliense, multiple Bacillus species, and over a dozen additional bacterial strains.

The Prescript-Assist formulation is described as a combination of 28 probiotic microflora "Soil-Based-Organisms (SBOs)" uniquely combined with a humic/fulvic acid prebiotic that enhances SBO proliferation. Other ingredients in the encapsulated form include cellulose (vegetarian capsule), L-leucine, and bamboo (Bambusa vulgaris) extract.

The prebiotic carrier used alongside the bacterial strains is leonardite — a form of oxidized lignite containing humic and fulvic acids. The prebiotic component, largely leonardite, is a nutritional medium that enhances SBO proliferation.

Unlike most probiotics that incorporate easily degradable lactic acid-based microorganisms, the utilization of Soil Based Organisms in Prescript-Assist provides for a pH-resistant (able to survive transit through the stomach) and shelf-stable (no refrigeration needed) product.

In agriculture, A. paspali is also formulated as a liquid or powder biofertilizer inoculant for application to seeds or soil; these agricultural preparations are distinct from human dietary supplements.

5. Key Constituents and Active Compounds

As a living microorganism, A. paspali does not have discrete "active compounds" in the conventional phytochemical sense. Rather, its biological activity is attributed to products it synthesizes and secretes in its natural environment and, by extrapolation in supplement marketing, within the gastrointestinal tract.

5.1 Nitrogen Fixation and Nitrogenase

Biological nitrogen fixation (BNF) refers to a microbial-mediated process based on enzymatic "Nitrogenase" conversion of atmospheric nitrogen (N₂) into ammonium readily absorbable by roots. Nitrogenase (nif) genes required for nitrogen fixation include structural genes, genes involved in activation of the Fe protein, iron–molybdenum cofactor biosynthesis, electron donation, and regulatory genes. In diazotrophic (nitrogen-fixing) bacteria, nif genes are typically found in a cluster of around 20–24 kb with seven operons encoding 20 different proteins.

Nitrogenase activity in the rhizosphere of Paspalum notatum was measured by the reduction of acetylene. Roots of the cultivar 'batatais' colonized by Azotobacter paspali, when taken from the soil, produced 1 to 32 nmol C₂H₄/g dry wt/h, whereas the cultivar 'pensacola', which is not colonized by A. paspali, produced less than 0.5 nmol/g/h. Activity was almost completely inhibited in air or in the absence of O₂ and was greatest at around pO₂ of 0.04 atm.

5.2 Phytohormones and Biologically Active Substances

Azotobacter synthesizes and secretes considerable amounts of biologically active substances including B vitamins, nicotinic acid, pantothenic acid, biotin, heteroxins, and gibberellin, which enhance root growth of plants.

Azotobacter has the ability to produce vitamins such as thiamine and riboflavin, and plant hormones including indole acetic acid (IAA), gibberellins, siderophores, and cytokinins. These plant growth-promoting substances are exogenously released by Azotobacter, improving the growth and productivity of the plant.

Like many plant-growth-promoting bacteria, azotobacteria have the capacity to excrete auxins into culture medium. Auxins and indole-3 acetic acid (IAA) as the most common member of the auxin family were the first plant hormones discovered and are implicated in virtually every aspect of plant growth and development.

Azotobacter paspali grows in the rhizospheres of Paspalum notatum, where it is thought to fix nitrogen and improve pasture growth. In experimental work, A. paspali improved growth primarily by producing growth-regulating substances.

5.3 Siderophores and Antifungal Substances

Besides nitrogen fixation, Azotobacter species produce siderophores, antifungal substances, and plant growth regulators. Some strains of Azotobacter, including Azorhizophilus paspali and A. vinelandii, have been characterized by their capacity to synthesize antifungal substances that inhibit the development of phytopathogenic species such as Helminthosporium sp., Macrophomina sp., and Fusarium sp. Hydrogen cyanide (HCN) and siderophore production have also been characterized for Azotobacter species.

5.4 Phosphate Solubilization

Inorganic and organic phosphate (P) solubilization by Azotobacter strains is another growth-promoting trait characterized for free-living N₂-fixing bacteria. It is well established that non-symbiotic fixation can improve plant growth only indirectly, by increasing soil nitrogen after mineralization of N₂-fixers' biomass. More likely, additional abilities of azotobacteria — such as phosphate solubilization and phytohormone and siderophore synthesis — contribute more directly to plant growth and crop yield.

6. Traditional and Historical Use

There is no documented traditional medicinal, culinary, or ethnobotanical use of Azotobacter paspali by any human population. The organism was unknown prior to its scientific description in 1966. Its agricultural utility as a component of biofertilizer preparations was recognized almost immediately after its discovery, particularly in tropical and subtropical farming contexts where Paspalum notatum-type grasslands are common.

The importance of Azotobacter species has been highlighted as both important free-living N₂-fixing bacteria and potential bacterial biofertilizer with proven efficacy for plant nutrition and biological soil fertility. Because of these properties, Azotobacter sp. is used as a beneficial biological agent, extensively sought for crops including wheat, rice, sorghum, sugarcane, and maize.

The organism's transition from an agricultural biofertilizer to a human dietary supplement ingredient represents a recent commercial development, dating to the late twentieth and early twenty-first century, when manufacturers of SBO-type probiotic products began formulating multi-strain products containing soil microflora as a means of introducing ecologically representative bacteria into the human gut.

7. Scientific Evidence by Area of Use

7.1 Agricultural Biofertilizer Use (Well-Established, Non-Human)

Yield increases from Azotobacter inoculation have been reported in the range of 2 to 45 percent in vegetables, 9 to 24 percent in sugarcane, and 0 to 31 percent in maize, sorghum, and mustard. These findings are from agricultural field trials and do not apply to human health outcomes.

The beneficial effects of Azotobacter on plant growth are attributed to improvement in root development, increases in the rate of mineral uptake by roots, and antagonism against fungi and plant pathogenic bacteria.

The plant growth promotion exerted by A. paspali specifically has been studied. A. paspali grows in the rhizospheres of Paspalum notatum, where it improves growth primarily by producing growth-regulating substances. In experimental work, nitrogenase activity was not detected in rhizospheres of young plants, and its occurrence in older plants was not necessarily associated with the presence of A. paspali.

7.2 Human Gastrointestinal Use as a Soil-Based Organism (Probiotic) — Evidence is Very Weak and Indirect

The only human clinical evidence pertaining to Azotobacter paspali as a dietary supplement ingredient comes from studies on the multi-strain Prescript-Assist SBO probiotic formulation, in which A. paspali is one of approximately 28–29 constituents. There are no clinical studies examining A. paspali as a single agent in humans.

Prescript-Assist is described as a probiotic–prebiotic complex; the probiotic component includes 29 soil-based microorganisms (SBOs). A methodologically oriented double-blind study in patients with IBS identified three subsyndromic factors of IBS — general ill feelings/nausea, indigestion/flatulence, and colitis — and combined probiotic–prebiotic treatment with Prescript-Assist was associated with significant reductions in these factors.

The primary published study is: Bittner AC, Croffut RM, and Stranahan MC. "Prescript-Assist probiotic–prebiotic treatment for irritable bowel syndrome: a methodologically oriented, 2-week, randomized, placebo-controlled, double-blind clinical study." Clin Ther 27, 755–761 (2005). A subsequent open-label, partially controlled, 1-year extension of that trial was also published: Bittner AC, Croffut RM, Stranahan MC, and Yokelson TN. "Prescript-Assist probiotic–prebiotic treatment for irritable bowel syndrome." Clin Ther 29, 1153–1160 (2007).

Critical limitations of this evidence base are significant. The trials tested a complex multi-strain formulation; no individual strain — including A. paspali — can be credited with any observed outcome. The trials are small, short in duration, and were funded by or conducted in association with the product's commercial development. No replicated independent clinical trials of equivalent design have been identified in the peer-reviewed literature specifically attributing benefit to A. paspali as a constituent. The evidence for human gastrointestinal benefit from A. paspali specifically must therefore be characterized as absent — no strain-specific human clinical evidence exists.

7.3 Plant Growth Promotion by Phytohormone Production (Preclinical/Agricultural, Not Human Health)

Various investigations have reported that inoculation with Azotobacter sp. has beneficial effects on plant yields, due both to the increase in fixed nitrogen content in soil and to microbial secretion of stimulating hormones such as gibberellins, auxins, and cytokinins. These mechanisms are documented in soil and plant studies; their relevance to human physiology when A. paspali is ingested is not established.

7.4 Antifungal Activity (Laboratory Evidence Only)

Some strains of Azotobacter, including Azorhizophilus paspali, have been characterized for their capacity to synthesize antifungal substances that inhibit certain phytopathogenic species. It was demonstrated that a mixture of Azotobacter, Azospirillum, and Klebsiella significantly reduced mycelial growth of pathogenic fungi such as Macrophomina phaseolina, Rhizoctonia solani, and Fusarium solani. This evidence comes from laboratory and agricultural experiments; no human-relevant antifungal clinical evidence exists for A. paspali.

8. Body Systems and Health Areas Claimed in Supplement Contexts

Within the context of SBO probiotic supplements containing A. paspali, the following health areas are associated with the multi-strain formulation (not with A. paspali individually):

• Gastrointestinal health: The combined formulation was associated with reduced subsyndromic factors of IBS including general ill feelings/nausea, indigestion/flatulence, and colitis in a small double-blind study.
• Gut microbiome balance: The SBO microflora are classified as micro-ecological units typical of those progressively found resident along the healthy human GI tract, with the formulation positioned for restoring gastrointestinal microflora compromised by antibiotics, medication, poor diet, stress, and digestive abnormalities.
• Immune support and nutrient absorption: These claims are made at the formulation level and are not substantiated by strain-specific evidence for A. paspali.

It bears repeating that none of the above associations are supported by human clinical evidence specific to A. paspali as an isolated ingredient. All human-context evidence pertains to the multi-strain mixture.

9. Dosage Forms and Reported Dosages

Azotobacter paspali is available to human consumers exclusively within multi-strain encapsulated SBO probiotic products. No standalone dosage form or single-agent preparation is commercially marketed or clinically evaluated for human use.

Within multi-strain products such as Prescript-Assist, the following dosages have been stated in sources:

• The suggested use of the Prescript-Assist multi-strain SBO product is 2 capsules per day for the first 30 days, followed by 1 to 2 capsules per week thereafter, or as recommended by a healthcare professional.
• Other products in this category instruct adults to take 1–2 capsules per day.

The proportion of A. paspali within these formulations' proprietary blends is not disclosed by manufacturers, and no strain-specific Colony Forming Unit (CFU) count for A. paspali in human SBO products has been identified in the peer-reviewed literature or publicly available product documentation.

In agricultural biofertilizer contexts, Azotobacter preparations are applied to seeds or soil; these dosage contexts are not relevant to dietary supplementation.

10. Safety Considerations

10.1 ATCC Biosafety Classification

A notable safety concern has been identified from authoritative microbiological sources. The American Type Culture Collection (ATCC) determines the biosafety level of materials based on risk assessment guided by the current edition of Biosafety in Microbiological and Biomedical Laboratories (BMBL), U.S. Department of Health and Human Services. ATCC assigns this organism a Biosafety Level 3 product designation, and provides the product sheet only to customers who have purchased this biosafety level 3 product.

This BSL-3 designation from ATCC is significant in the context of dietary supplementation. The U.S. CDC/NIH BMBL framework defines Biosafety Level 3 as appropriate for "agents that may cause serious or potentially lethal disease" and that pose a risk of transmission by inhalation. The presence of Azorhizophilus paspali (the same organism) at ATCC under BSL-3 classification has not been explained, clarified, or addressed in the published literature examining its use in human SBO probiotic formulations, and no published safety studies in human populations specific to A. paspali ingestion have been identified.

10.2 GRAS Status and Regulatory Standing

Azotobacter paspali does not appear on the U.S. Food and Drug Administration's database of microorganisms with Generally Recognized As Safe (GRAS) status. The GRAS status determined by the FDA is only awarded to chemicals or specific strains of microorganisms that are proven to be safe for ingestion. No GRAS notice or published FDA GRAS determination for Azotobacter paspali or Azorhizophilus paspali has been identified in the authoritative regulatory record.

No monograph for Azotobacter paspali exists in the European Pharmacopoeia, WHO monographs, ESCOP, German Commission E, or USP. No EFSA safety opinion specific to this organism for use in food supplements has been identified.

10.3 Absence of Independent Human Safety Studies

No peer-reviewed clinical trials, toxicological reports, or post-market surveillance data specifically evaluating the safety of A. paspali in human subjects have been identified in the published literature. Safety inferences in commercial contexts are drawn by analogy from the broader category of soil-based organisms and from the multi-strain Prescript-Assist clinical studies, which did not separately characterize adverse events attributable to individual bacterial constituents.

10.4 Sensitivity and Environmental Limitations

Azotobacter are sensitive to acidic pH, high salts, and temperatures above 35 °C. These physiological constraints may affect viability of the organism in both storage conditions and the human gastrointestinal tract. Azotobacter survives in soil for longer durations due to cyst formation. However, whether the cyst-forming capacity confers comparable survival in the human gut under physiological conditions has not been demonstrated in published human studies.

10.5 Interactions

No documented pharmacological or supplement interactions for Azotobacter paspali have been identified in the peer-reviewed clinical or pharmacological literature. Claims regarding antibiotic use and SBO probiotic supplementation appear at the formulation marketing level only and are not based on published clinical interaction data specifically involving A. paspali.

11. Summary Assessment of Evidence Quality

The scientific literature on Azotobacter paspali is extensive in the context of soil microbiology, biological nitrogen fixation, and agricultural biofertilizer science. The importance of Azotobacter species as free-living N₂-fixing bacteria and potential bacterial biofertilizers with proven efficacy for plant nutrition and biological soil fertility is well established in this literature.

However, for human dietary supplementation purposes, the evidence base is extremely limited:

• There are no human clinical studies examining Azotobacter paspali as a standalone dietary ingredient.
• All human clinical data originates from multi-strain SBO formulation studies where A. paspali is one of 28–29 constituents and cannot be individually attributed any observed outcome.
• No regulatory body (FDA, EMA, EFSA, Health Canada) has issued a safety approval, positive opinion, or GRAS determination for A. paspali as a human food or supplement ingredient.
• The organism's BSL-3 classification at ATCC represents a factual safety concern that has not been addressed in published clinical or regulatory literature specific to human dietary use.
• Proposed mechanisms for human benefit (gut microbiome modulation, immune support) are entirely extrapolated from the organism's known agricultural activities and are not supported by species-specific mechanistic human data.

References

• ScienceDirect Topics: Azotobacter Paspali — an overview
• PMC / Frontiers in Microbiology: Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers for Crop Nutrition and Yield Stability
• Frontiers in Microbiology: Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers (full text)
• Springer Nature: Isolation and Identification of Nonsymbiotic Azotobacter and Symbiotic Azotobacter Paspali–Paspalum notatum
• Microbiology Society: Nitrogenase Activity and Oxygen Sensitivity of the Paspalum notatum–Azotobacter paspali Association
• Pesquisa Agropecuária Brasileira: Azotobacter paspali sp. n. — A nitrogen fixing bacteria in the rhizosphere of Paspalum (Döbereiner, 1966)
• NamesforLife: Azotobacter paspali Döbereiner 1966 nomenclature record
• NCBI Taxonomy Browser: Azorhizophilus paspali (Dobereiner 1966) Thompson and Skerman 1981
• NCBI Datasets: Azorhizophilus paspali taxonomy
• BacDive DSMZ: Azorhizophilus paspali Ax-18, DSM 88, ATCC 23368
• ATCC Product 23367: Azorhizophilus paspali (Dobereiner) Thompson and Skerman — Biosafety Level information
• PMC: Genotypic Characterization of Azotobacteria — Plant-Growth-Promoting Traits and Prospects for Biofertilizer Production
• PMC: Mechanisms of Action of Plant Growth Promoting Bacteria
• PMC: Plant Growth-Promoting Bacteria: Mechanisms and Applications
• ResearchGate: Role of Azotobacter paspali in Association with Paspalum notatum
• OSTI.GOV: Final Report — The Rhizosphere Association of Azotobacter paspali with Paspalum notatum
• ResearchGate / Clinical Therapeutics (2005): Prescript-Assist probiotic–prebiotic treatment for IBS — Bittner et al., Clin Ther 27:755–761
• Journal of Pure and Applied Microbiology: Phenetic Characterization of Nitrogen Fixing Azotobacter from Rhizospheric Soil
• PMC: Pseudomonas putida KT2440 is HV1 certified, not GRAS — discussion of GRAS status for non-food microorganisms
• PMC: Transcriptional Profiling of Nitrogen Fixation in Azotobacter vinelandii
• Wikipedia: Azorhizophilus paspali