Streptomyces avermitilis is a microorganism of scientific and medical importance. This soil-dwelling bacterium produces avermectins. The discovery and development of these natural products, particularly ivermectin, have impacted global health and agriculture. Its significance stems from its role in combating parasitic diseases in both humans and animals, as well as its utility in crop protection.
Biological Characteristics
Streptomyces avermitilis is a bacterium in the genus Streptomyces, part of the phylum Actinomycetota. These microorganisms are found in soil, where they decompose organic matter. Like other streptomycetes, S. avermitilis exhibits a filamentous growth pattern, resembling fungi, forming a mycelium.
A feature of Streptomyces species, including S. avermitilis, is their life cycle involving spore formation. These spores are produced in chains on aerial filaments. This bacterium is also known for its metabolic versatility, synthesizing a wide array of secondary metabolites that possess biological activity.
The Discovery of Avermectins
The discovery of avermectins began in the early 1970s through a collaborative effort between Satoshi Ōmura of Japan’s Kitasato Institute and William Campbell at Merck & Co. in the United States. Ōmura isolated a strain of Streptomyces avermitilis from a soil sample collected near a golf course in Japan. This strain was identified for its potential to produce novel compounds.
Ōmura sent cultures of this bacterium to Merck, where Campbell and his team screened them for antiparasitic activity. In 1975, this screening process revealed that a fermentation broth from S. avermitilis possessed activity against parasitic worms in mice. The active compounds were subsequently isolated and identified as the avermectins, a new family of macrocyclic lactones. Further chemical modification of avermectin led to the development of ivermectin, which proved to be a more effective and safer compound for widespread use. Both Ōmura and Campbell were awarded the Nobel Prize in Physiology or Medicine in 2015 for their discoveries.
Diverse Applications
The avermectins, particularly ivermectin, have found extensive applications across human health, animal health, and agriculture, owing to their broad-spectrum antiparasitic properties. Ivermectin functions by interfering with the nerve and muscle functions of parasites, causing paralysis and death. Its selective action against invertebrate-specific ion channels contributes to its safety in mammals.
Ivermectin has had a transformative impact on human health, especially in controlling neglected tropical diseases. It is a primary treatment for onchocerciasis, commonly known as river blindness, a debilitating disease prevalent in many parts of Africa. A single annual dose can reduce the levels of parasitic larvae in the skin and prevent disease progression. Ivermectin is also crucial in global campaigns to eliminate lymphatic filariasis, or elephantiasis, another disfiguring parasitic disease. Its importance is further underscored by its inclusion on the World Health Organization’s List of Essential Medicines.
In veterinary medicine, ivermectin is widely used to control a variety of internal and external parasites in livestock and companion animals. For cattle, sheep, and pigs, it treats gastrointestinal roundworms, lungworms, mites, and lice. In companion animals such as dogs and cats, it is a preventative measure against heartworm disease, a severe parasitic infection. Its introduction revolutionized parasite control in animal agriculture, becoming a top-selling veterinary medicine for decades.
Beyond medicine, avermectins also serve as tools in agriculture. They are utilized as insecticides and miticides to protect crops from various pests. These compounds offer an effective way to manage insect and mite infestations, contributing to higher crop yields and reduced crop damage. The use of avermectins in crop protection provides a targeted approach to pest control, benefiting agricultural productivity worldwide.