Pseudonocardia sp.

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Classification

Members of the Pseudonocardia genus are classified as such:

Domain Bacteria

Phylum Actinomycetota

Class Actinomycetia

Order Pseudonocardiales

Family Pseudonocardiaceae

Genus Pseudonocardia

Species

This page covers details about the general genus of Pseudonocardia sp. There are currently 53 members of the Pseudonocardia genus.

Description and Significance

Pseudonocardia are Gram-positive bacteria, meaning they have only one cell membrane and a very thick peptidoglycan cell wall. They are non-motile bacteria, meaning they are unable to move independently in their environments. They are also aerobic bacteria, meaning they use oxygen as a terminal electron acceptor in metabolic pathways. They are rod-shaped and often grow in a branching pattern.

Many specific Pseudonocardia species have been isolated from soils, trees, and plant roots from China and Australia, highlighting their importance as environmental bacteria. However, they can also be found in aquatic ecosystems. Pseudonardia are known as "free-living," meaning they are not dependent on another organism for survival. That being said, Pseudonocardia species often are part of very significant symbiotic relationships

Attine ants are fungus-growing, leaf-cutting ants. Such ants have their own symbiotic relationships with fungi. These ants grow fungi underground, which they can then eat. In turn, the fungus relies on the ants for its vertical transmission. These fungi are incapable of reproducing sexually using spores, and so ant queens take a piece of the fungal colony with them when they go to establish a new colony, thus propagating the fungus. However, this symbiotic relationship is susceptible to disruption via infection by other fungal species like Escovopsis. This fungus can be extremely pathogenic toward the fungus the ants grow. At the same time that this fungus was discovered, a bacterial symbiont was discovered in the ant that produces compounds that fight infection by organisms like Escovopsis. This bacterium is Pseudonocardia. Compounds naturally produced by Pseudonocardia are used to protect both the ants and their fungi from infection. These compounds have both antibiotic (bacteria-killing) and antifungal (fungus-killing) properties. Thus, ants having a symbiotic relationship with Pseudonocardia protects their own symbiotic relationship with the fungi from disruption by infection.

Genome Structure

Pseudonocardia sp. have one circular chromosome about six million base-pairs (~6,135 kbp, ~6.1 mbp) in length. Pseudonocardia sp. also often have two extrachromosomal plasmids, named pFRP1-1 and pFRP1-2. Both of these plasmids are circular as well. Both are significantly smaller than the genome; pFRP1-1 is about 297 kbp in length while pFRP1-2 is about half of that size, at 121 kbp in length. All in all, the genetic material housed by Pseudonocardia sp. cells code for 5,109 proteins and 63 RNAs. However, because these bacteria live in the soil, they are difficult to isolate and thus there are only about 20 genome sequences currently available for the entire genus, which houses 53 species. DNA G+C content within this genus runs between 68-79%.

pFRP1-1 specifically codes for biosynthetic enzymes to make the antifungal compound gerumycin, which selectively inhibits Escovopsis, the fungal pathogen of the fungal species symbiotic with Attine ants.

Cell Structure

Pseudonocardia are Gram-positive bacteria, possessing a thick outer layer of peptidoglycan separated from the inner cell membrane by a thin periplasm. Cell membranes are mainly composed of isobranched hexadecanoic acid. Pseudonocardia are non-acid-fast, meaning they can be decolorized by acid during staining procedures, as they lack mycolic acids in their cell walls. In most species, membrane electron transport is primarily carried out by MK-8(H4), a tetrahydrogenated menaquinone with eight polymerized isoprene units. Determining the molecular composition of cell-envelope components of these species can be helpful in distinguishing them, a process referred to as chemotaxonomy.

Pseudonocardia are bacilliform, or rod-shaped, bacteria, which grow extensive networks of bacterial mycelium. This mycelium is composed of long, thin filaments called hyphae, which can be divided into "substrate" hyphae that grow throughout a nutrient source and take in nutrients from their surroundings, and "aerial" hyphae that grow upward and facilitate spore dispersal. Within Pseudonocardia, hyphae vary by species in terms of degree of branching, primary mode of growth, and diameter (0.3-2μm). These hyphae often fragment into rod- or oval-shaped elements as they grow, though aerial hyphae of some Pseudonocardia species have been described as squarish.

Growth primarily occurs in this genus through acropetal budding, where new hyphae grow from a bud structure formed by constriction and enlargement of a hyphal tip, or basipetal septation, where existing hyphae are segmented by the growth of interior cell walls called septa. Mature hyphal structures form reproductive spores, which are essentially semi-dormant bacterial cells--aerial hyphae sometimes differentiate into spore chains, though spores usually grow singly or in pairs on substrate and aerial hyphae. In addition to spore growth on acropetal buds or newly septated cells, aging hyphae also irregularly produce spores. Spores vary in size and morphology by species, but are on the order of 0.5-5μm in diameter. Pseudonocardia sp generally grow well on standard media for hyphae-forming actinomycetes, such as Czapek medium or yeast extract agar, usually forming substrate and aerial hyphae which vary in color depending on growth conditions.

Metabolism

Pseudonocardia is typified by extensive metabolic diversity, with different species able to degrade a wide range of toxic environmental contaminants as well as produce exotic secondary metabolites with antifungal, antibiotic, or other biotechnologically relevant properties. All known species are aerobic, meaning they use diatomic oxygen as a terminal electron acceptor. Most are chemo-organo-heterotrophic, deriving energy from the breakdown of organic compounds, though some are capable of facultative autotrophy in certain conditions. A notable number of species in this genus are named for their ability to use atypical compounds as sole sources of carbon for growth; Pseudonocardia dioxanivorans, for example, can fulfill all its carbon and energy needs by breaking down the significant pollutant 1,4-dioxane. Similarly, Pseudonocardia carboxydivorans can use carbon monoxide (CO) as its only source of carbon, and one species has been identified which grows on the pollutant 1,2,3,5-tetrachlorobenzene as a sole carbon source. Other species are capable of breaking down a wide variety of other chlorinated compounds, while others can utilize dimethyl disulfide and dimethyl sulfide, and yet more can break down simple hydrocarbons. Because of this metabolic diversity, as well as their high tolerance to other common pollutants such as tetrahydrofuran (THF), Pseudonocardia are being actively researched for their potential to remove persistent pollutants from the environment.

In addition to breaking down a diverse set of molecules, Pseudonocardia sp also produce a wide variety of secondary metabolites, and some species are increasingly used in biotechnology research for their potential to synthesize biologically active compounds. Most notably, Pseudonocardia species produce many novel antifungal and antibacterial compounds. These form the basis of their mutualism with leafcutter ants, who need these inhibitory chemicals to protect the fungi they cultivate for food against various pathogenic "pests". Multiple classes of novel antifungals have already been isolated from this genus, including the depsipeptide gerumycins and the polyene nystatin/nystatin-like molecules, as well as antibiotics such as branimycins B and C. Many of these compounds display multiple functions, such as various diketopiperazines (DKPs) which exhibit antiviral, antitumor, and antimicrobial activities, while also acting as quorum-sensing molecules for bacteria and signaling molecules for plant growth. Additionally, whole-genome characterizations of biosynthetic gene clusters are underway across this genus, which aim to identify more biologically active secondary metabolites, as well as the enzymes which facilitate their manufacture.

Life Cycle

Pseudonocardia sp have been isolated from a variety of natural and industrial environments, including plant material, soil, marine sediment, cattle manure, and biofilter sludge. Pseudonocardia are likely to be abundant in hydrogen-rich habitats, as species have been isolated from high-hydrogen environments such as rhizobial nodules or decomposing plant matter, and many species can oxidize dihydrogen and fix carbon dioxide.

Ecology

Pseudonocardia sp. are generally found in a symbiotic relationship with Attine ants of the Atta and Acromyrmex families. These ants are known as leaf-cutting or fungus-growing ants due to their defining characteristic of growing fungi in large farms for consumption. The ants normally grow fungi of the leucoagaricus and leucoprinus families, which have evolved to rely of the ants for propagation. While the ants grow and harvest these fungi, their farms are susceptible to infection by another parasitic fungus called Escovopsis. Escovopsis is known to infect the farms in ant colonies and result in massive losses of fungal biomass. This results in a negative effect to the ants who need the farms for nutrition. In response to the pathogenesis of Escovopsis, the ants have formed a mutualistic relationship with specific strains of Pseudonocardia which grow on a special region on their cutucle as a biofilm. The ants provide the pseudonocardia with nutrition and protection while the pseudonocardia produce anti-fungal compounds that the ants use to protect their farms from escovopsis.

References

1. Pseudonocardia https://en.wikipedia.org/wiki/Pseudonocardia

2. Pseudonocardia acaciae https://en.wikipedia.org/wiki/Pseudonocardia_acaciae

3. Pseudonocardia Symbionts of Fungus-Growing Ants and the Evolution of Defensive Secondary Metabolism https://www.frontiersin.org/articles/10.3389/fmicb.2020.621041/full#B50

4. Pseudonocardia sp. EC080625-04 https://www.genome.jp/kegg-bin/show_organism?org=psee

5. Variable genetic architectures produce virtually identical molecules in bacterial symbionts of fungus-growing ants https://www.pnas.org/doi/10.1073/pnas.1515348112

6. Genome Analysis of Two Pseudonocardia Phylotypes Associated with Acromyrmex Leafcutter Ants Reveals Their Biosynthetic Potential https://www.frontiersin.org/articles/10.3389/fmicb.2016.02073/full

7. Taxonomy, Physiology, and Natural Products of Actinobacteria https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4711186/

8. Pseudonocardia, Bergey's Manual of Systematics of Archaea and Bacteria https://sci-hub.se/https://doi.org/10.1002/9781118960608.gbm00184

9. Morphological Identification of Actinobacteria https://www.intechopen.com/chapters/49285

10. Genus Pseudonocardia: What we know about its biological properties, abilities and current application in biotechnology https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/jam.15271

Author

Page authored by Caleb Hill, Sannnoong Hu, and Abby Jackson, students of Prof. Jay Lennon at Indiana University.