Antibiotics and their effects on Plants

Soil bacteria and fungi are a rich source of natural antibiotics, but the prevalence of human-made antibiotics and antibiotic resistance genes in soils, is an emerging concern. Antibiotics are widely used to promote livestock growth in modern non-organic agriculture, with poultry, cattle and pigs, being regularly treated with these antibacterial drugs. Millions of kilograms of antibiotics are released into the environment annually, much in the excrement of grazing animals, or through application of manure to agricultural fields(1). Discharge of human waste into waterways and the use of contaminated irrigation water or sewerage sludge to fertilise crops in many countries, is also a contributory cause. As a result, a higher level of antibiotic resistance is now apparent in conventional agricultural versus natural forest soils(2).

Soil and water-containing antibiotics constitute a potential route of human exposure to antibiotic resistance genes through their uptake by plants(3-8).  Uptake by plants can also have other effects, such as the accumulation of nitrofuran-type antibiotics in the edible parts of spring onions, and the subsequent metabolism of these into genotoxic and potentially carcinogenic hydrazine-containing metabolites(9).

The other consideration is the effects these human-made antibiotics have on the soil or plants themselves.  With human and animal health being intrinsically connected to that of plants and soil, and increasing research showing the many symbiotic and complex relationships between living organisms and their environment, effects of human-made antibiotics on plant health, should also be considered.

The high level of contamination with antibiotic residues and transferable resistance genes in pig manure applied to soil, has been shown to change the antibiotic resistant gene reservoir of the plant microbiome(10).  Carrots and lettuce can uptake amoxicillin and tetracycline(4), and tetracycline residues have toxic effects on both root and stems of germinating lettuce seedlings(11).  Oxytetracycline residues from cattle manure have also been shown to affect the diversity and type of nitrogen-fixing soil bacteria communities(12).

A recent European study has shown that even small amounts of antibiotics can have a range of potentially negative effects on plant traits(13). The comprehensive study examined the effects of three antibiotics (penicillin, tetracycline and sulfadiazine), on germination and growth of four plant species. These included two cultivated species (rapeseed, Brassica napus and common wheat, Tricicum aestivum), and two non-crop (herb) species (Shepherd’s purse, Capsella bursa-pastoria and Common Windgrass, Apera spicaventi). In farmland fertilised with manure containing antibiotic concentrations as typically found in agricultural soils, various effects on the plants were observed.

Main effects were delayed germination or reduced plant biomass. These effects varied markedly depending on the plant species concerned, but were most pronounced in the two herb species, particularly by penicillin and sulfadiazine. This suggests that different antibiotics could potentially affect the prevalence and types of species, and the diversity of natural plant communities near agricultural fields. Furthermore, these species-specific responses may not only alter the competitive abilities and makeup of the plant community, but also have secondary effects on other species such as pollinating and herbivorous insects(13).

Petrochemical residues and the use of non-organic agricultural pesticides and insecticides, are also starting to come under the spotlight as likely contributors to multi-drug antibiotic resistance among soil bacteria. A recent Chinese study has demonstrated that petrochemical residue -polluted soils were more than 15 times more likely than less-contaminated ones, to contain antibiotic resistance genes. This strong association of soil pollution with polycyclic aromatic hydrocarbons, suggests these may also be contributing to the growing amounts of antibiotic resistant genes in human-impacted environments(14).

In non-organic agriculture, soil bacteria can be continuously exposed to synthetic pesticides at sub-lethal concentrations, and a recent Indian study has found that insecticide-contaminated soil may have contributed to development of resistance to a range of different antibiotics, by several Bacillus species(15).

Silver nanoparticles are also now widely used in antibacterial products, and these inevitably discharge into aquatic environments and have been shown to affect the nitrogen cycle in phytoplankton and aquatic plant life(16).

Antimicrobial chemicals such as triclosan and triclocarban, which are used in some liquid soaps and toothpastes, can take a long time to break down in the environment and have been shown to have detrimental effects on aquatic organisms, and potentially contribute to antimicrobial resistance(17-19).

Soil and plant health are pivotal to the health of the planet and all its living organisms, and antibiotic drugs have saved many millions of lives. However, the widespread use of antibiotics in non-organic agricultural production systems particularly those involving animals, should be curtailed.

Refs:

  1. Popova IE et al, J Environ Sci Health B 2017; 52(5):298-305.
  2. Popowska M et al, Antimicrob Agents Chemother 2012; 56(3):1434-1443.
  3. Grote M. et al, Landbauforschung Volkenrode 2007; 57: 25-32.
  4. Azanu D et al, Chemosphere 2016; 157:107-114.
  5. Rahube TO et al, Can J Microbiol 2016; 62(7):600-7.
  6. Pan M et al, J Agric Food Chem 2014; 62:11062-11069.
  7. Kang DH et al, J AGric Food Chem 2013; 61:9992-10001.
  8. Kumar K et al, J Environment Qual 2005; 32:2082-2085.
  9. Wang Y et al, J Agric Food Chem 2017; 65(21):4255-4261.
  10. Wolters B et al, Appl Microbiol Biotechnol 2016; 100(21):9343-9353.
  11. Pino MR et al, Environ Sci Pollut Res Int 2016; 23(22):22530-22541.
  12. Sun J et al, Bioresour Technol 2016; 801-807, epub May 21.
  13. Minden V et al, AoB Plants 2017; 9(2):plx020.
  14. Chen B et al, Environ Pollut 2017; 220(Pt B):1005-1013.
  15. Rangasamy K et al, Microb Pathog 2017; 103:153-165.
  16. Jiang HS et al, Environ Pollut 2017; 223:395-402.
  17. Falisse E et al, Aquat Toxicol 2017; 189:97-107.
  18. McNamara PJ, Levy SB. Antimicrob Agents Chemother 2016; 60(12):7015-7016.
  19. Tremblay Louis, Environmental toxicologist, Cawthron Institute, Nelson, New Zealand Herald, 23 June 2017.
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Manuka and Myrtle Rust

Last week I attended a two day workshop organised by scientists at Plant and Food Research Ltd and Massey University in Palmerston North, to discuss a range of recent scientific and biosecurity developments, concerning Manuka (Leptospermum scoparium), an important plant in New Zealand’s natural environment and economy. As with the two day Hui on ‘Manuka and More’ in Ruatoria and Te Araroa in November last year, this was an excellent event in which more than 30 scientists working actively on Manuka research presented on a diverse range of subjects and discussed where there could be gaps in our knowledge or research needs for this plant. While Manuka Honey and essential oil are currently the main two medicinal products produced from Manuka, numerous other therapeutic applications and potential contributions to preserving our environment, are found within this plant.

Jacqui Horswell and colleagues from the Institute of Environmental Science and Research, have shown that Manuka and other myrtaceaeous plants seem to be capable of killing the faecal bacterial pathogen Enterobacter coli (E. coli), by enhancing the die-off of this and other pathogenic organisms that pass through their root systems. A field trial involving riparian planting of Manuka is just getting going, to see whether laboratory results extend to helping to reduce animal effluent flows into a polluted lake. A lake which was once pristine and a treasured swimming area, but in recent years has changed into a green and dirty waterway due largely to dairy industry runoff, has been selected for this trial.

Hayley Ridgway from Lincoln University presented some interesting findings concerning novel and potentially useful mycorrhizae (fungi) and endophytic bacteria associated with the roots of Manuka, some of which I wrote about in my previous blog. Inoculation of Manuka plants with different mycorrhizae causes significant alterations in their growth rates and essential oil composition, highlighting the complex inter-relationships between microbes associated with Manuka, and its production of phytochemicals including some with bioactive properties.

Other presentations were made on experiences to date involving plantations of Manuka which have been established at a number of North Island sites in recent years. Challenges include site access, weeds, pests, and the relative attractiveness of different genetic lines to bees. A comment made by one of the presenters that while humans have had multiple generations of experience with cultivation and enhancing performance characteristics of crops such as wheat and rice, our experience with Manuka plantations spans less than 10-15 years to date.

The hottest topic at the workshop, however, was the recent finding of isolated outbreaks of Myrtle Rust (Austropuccinia psidii) in New Zealand nursery and garden grown specimens of Manuka and the native tree, Ramarama (Lophomyrtus bullata). This pathogenic fungi originated from Brazil where it causes guava rust, but spread internationally into North America in the 1880’s, and was first reported in Australia in 2010.  Australia is home to around half of the world’s Myrtaceae (Myrtle family) plant species, including Eucalyptus (850 species), Melaleuca (176 species) and Callistemon species.

Outbreak of Myrtle rust has had a devastating effect on much of the east coast as well as other areas of Australia, where it has resulted in ecosystem collapse for certain plant species. To date it has only been found in isolated locations in Northland, Waikato, Bay of Plenty and Taranaki, although it is widespread on Raoul Island in the Kermadec group, about 1,100km to the north-east of New Zealand.

Myrtle rust spores can easily spread across large distances by wind, or via insects, birds, people, or machinery, and it is thought the fungus arrived in New Zealand carried by strong winds and significant weather events from Australia.

The Myrtle Rust Strategic Science Advisory Group is working hard to assess and try to ameliorate the widespread environmental, economic, social and cultural impacts this plant pathogen could have on New Zealand. Apart from Manuka and Ramarama, other indigenous Myrtaceae species such as Pohutakawa (Metrosideros spp) and Swamp Maire (Syzygium maire), are under risk. Priorities including acceleration of scientific research into the biology of the pandemic strain detected here, pathways of spread, surveillance, management, exploring plant susceptibility and resistance, and coordinating and communicating a management plan that has widespread engagement by communities, scientists, industry and Maori stakeholders and landowners, councils and government.

The Ministry for Primary Industries (MPI) and the Department of Conservation (DOC), with the help of local iwi, the nursery industry, and local authorities are running an operation to determine the scale of the situation and to try and contain and control myrtle rust in the areas it has been found. However, emergence of the infection and appearance of the distinctive yellow or brown leaf discolouration may not become fully apparent until the spring, and a better assessment of the number of infection sites and their extent, may not be possible until then.

The arrival of Myrtle Rust in New Zealand means that the task of collecting and storing seed of New Zealand indigenous Myrtaceae including Manuka, has now become urgent. The NZ Indigenous Flora Seed Bank (NZIFSB), a collaborative project between Massey University, AgResearch, Landcare and the Department of Conservation, with support from the NZ Plant Conservation Network and the Millennium Seedbank at Kew in the UK, was established in 2013. NZFISB has been doing some really valuable work to collect and store seeds aimed at preserving a wide range of biodiversity within New Zealand native plant species. More than 130 volunteer seed collectors have been trained to date, and plans are underway to extend this and the level of community participation, to try to better protect our native plants for generations to come.

Refs:

http://www.nzpcn.org.nz/page.aspx?conservation_seedbank

http://www.mpi.govt.nz/protection-and-response/responding/alerts/myrtle-rust/