Acromyrmex is an ant species that lives in obligate symbiosis with Leucoagaricus, a fungus. Two species are obligate symbionts if they are of mutual benefit to each other, and need each other to survive.
In this case, the ants chop up leaves into fine pieces and feed them to the fungus, grown in specialized chambers (gardens) within their colonies. The fungus is the major food source for the ants, completing the symbiotic relationship.
This relationship is disrupted by another fungus, Escovopsis. This fungus is detrimental to the health and survival of the Leucoagaricus (helpful) fungus gardens, and consequently to the health and survival of the ant colony.
The ants carefully clean their gardens to protect against fungal invaders. The ants also secrete antifungal and antibacterial agents as a further defense.
Bacteria also play an important role here, but the chemical basis is unknown. In other words, what molecule is responsible for antifungal activity by these bacteria?
Dieter Spiteller (Max Planck Institute for Chemical Ecology, Germany) and coworkers have investigated this mystery. They have shown that the bacteria secrete candicidin molecules, which are toxic to the pathogenic fungus, but not the beneficial fungus.
Ants, fungus samples, and bacteria.
The ants and fungus samples were obtained from ant colonies in Gamboa, Panama. The ants were of three different Acromyrmex species.
Ants were touched with sterile toothpicks to collect bacteria, which were subsequently cultured in the laboratory. Bacteria were also obtained from the fungus gardens, identified by morphology, and isolated before further culturing.
The scientists focused on isolating Streptomyces bacteria. This is because this bacteria was found to be highly inhibitory towards Escovopsis, the pathogenic fungus.
Identifying the active molecule.
The scientists wanted to determine what molecule was responsible for the observed antifungal activity of the bacteria. Various instrumental techniques applied against the bacteria, as well as genetic analysis of the bacteria, identified the molecule as candicidin.
The scientists found that six (all Streptomyces bacteria) of the 19 bacterial species isolated from the ants synthesize candicidin or similar molecules. Additionally, each of the three ant species possessed at least one bacterial species that synthesized candicidin, which is suggestive of the importance of this molecule to the ants.
Fungus inhibition by candicidin.
Now that the scientists had determined that candicidin is the antifungal agent, they tested its effectiveness against Escovopsis, the pathogenic fungal species. Only a small amount was required to inhibit the growth of the fungus.
It is especially notable that candicidin seems to be specific against the pathogenic fungus; Leucoagaricus, the beneficial fungus, is not inhibited by the molecule. Thus, candicicin can selectively attack the ants' enemies, and not their symbiont.
Future directions.
These scientists have shown that candicidin, produced by Streptomyces bacteria, are an effective weapon against fungi pathogenic towards Acromyrmex ants' source of food. Other protective agents are likely used as well; candicidin is only one option in the toolkit.
Further studies may head in the direction of determining the molecular identity of these other protective agents. A possibly more challenging topic to address would be elucidating the chemical basis of how Acromyrmex ants are able to select for the growth of beneficial bacteria on their bodies, as weapons against pathogenic fungal invaders.
for more information:
Haeder, S.; Wirth, R.; Herz, H.; Spiteller, D.
Candicidin-producing Streptomyces support leaf-cutting
ants to protect their fungus garden against the pathogenic fungus
Escovopsis.
Proc. Natl. Acad. Sci. USA 2009, 106, 4742-4746.