The Protective Microbiome
The phylum Actinobacteria is one of the most ancient groups of bacteria and includes deadly human pathogens like Mycobacterium tuberculosis and a group of filamentous "actinomycete" bacteria which includes Pseudonocardia and Streptomyces species. These actinomycetes are notable because they do not grow as single cells like most bacteria but instead have filamentous growth like fungi, only about 100 times smaller. They are important because they produce two thirds of all the antibiotics and anti-cancer compounds used in human medicine.
Humans and attine ants both use the antibiotics made by actinomycete bacteria but attines have been doing it for much, much longer. In attine ant colonies, Pseudonocardia bacteria are vertically transmitted by the attine queens and grow on the surfaces of the worker ants. However, it seems that attine ants can recruit additional actinomycete strains from the environment and we usually find they are also associated with Streptomyces bacteria. This suggests that attine ants use multiple antibiotics from a mixture of actinomycete strains to prevent drug resistance arising in the microbes that infect their fungus gardens.