Ants drink their own poison as a disinfectant.

                                

Breaking down the microbiology world one bite at a time


Ants drink their own poison as a disinfectant.

Bacteria are found in nearly every environment on earth and the gut of insects is no exception. Here, bacteria can play a beneficial role in breaking down complex nutrients into simpler ones for the host to take up. Food is an important source of these microbial gut associates, but it could also introduce harmful microbes that are pathogenic or produce toxic chemicals, potentially killing the host.

Some insects that live in a big social community or hive like ants, wasps, bees or termites store food in their stomach, called the crop. Not only do they transport the food, but they distribute it to other members of their colony via trophallaxis. This means that they regurgitate the crop content and feed it to the recipient via mouth-to-mouth feeding. Intimate, right? Although this facilitates the transmission of beneficial microbes, it could open the door to unwanted microbial opportunists that can hijack these transmission routes.

In a recent study, researchers investigated how ants control harmful microbes in their food while the beneficial microbes are allowed to be transported from and with the food. More specifically, they looked at the Florida carpenter ant (Camponotus floridanus), and how it uses its own acidic venom as a defensive weapon! They seem to use the acid to disinfect their food, allowing acid-tolerant bacteria to pass through to their guts.

Carpenter ant (Camponotus sp.) by Muhammad Mahdi Karim. Source: Wikimedia commons.

The researchers first looked at seven different formicine ant species, and measured the acidity of their crop and the midgut. The midgut is like the small intestine, where the absorption of nutrients starts. What they found was that the crop always seemed to be quite acidic, with a pH between 2 and 4. The midgut however, seemed to maintain a higher pH, which means it is less acidic.

Drawing of the ant Kyromyrma neffi, showing the acidopore with fringe of hairs at the end of the body. Source: PNAS https://www.pnas.org/content/97/25/13678 

Although this is not uncommon (in humans the pH in the stomach can reach 2, while the small intestine has a pH of 6), their research demonstrated that ants use a different mechanism compared to humans to keep the pH low: they swallow their own acidic “poison.” This is normally used in defense against predators and is produced by a gland at the end of their bodies. The droplets that come out of this hole called the acidopore, contain 37 chemicals but mostly formic acid (hence the name formicine ants, for their capability to produce this substance). The ants ‘lick’ their acidopore and suck up the poison into their mouths. Yum!

After proving that the acidic poison is indeed the reason for the low pH in the crop, they investigated whether swallowing it allowed for microbial control and if it would kill bacterial pathogens. They fed the Florida carpenter ant honey water with a pathogenic bacterium (Serratia marcescens), which is known to kill the ants in small doses. The researcher then looked at the survivability of the bacteria at different timepoints, and saw that after four hours, no (viable) S. marcescens could be found in the crop, and that barely any bacteria made it to the midgut after even half an hour.  

An image of each area of the ants anatomy is represented by the corresponding graph below. a) Change in the number of bacteria (CFU or colony forming units) surviving in the crop after feeding. b) Change in number of bacteria surviving in the midgut after feeding. Figure from original article.

In a second experiment, the researchers prevented half of the ants from swallowing the acidic poison and gave all ants contaminated food. Next, they looked at the survival rates of the ants. Access to the acid gave a significant survival advantage to the ants, which was comparable to ants that were given non-contaminated food.

The ability to swallow the acidic poison may not only improve survival of formicine ants feeding directly on pathogen contaminated food but also of ants that share the contaminated food via trophallaxis (mouth-to-mouth feeding). To test this, the researchers made two kinds of donor-receiver pairs. In both pairs, the acid glands of the receiving ants were glued shut. In the first pair, the donor would have access to the acid while in the second pair the donor’s acid glands would be glued shut too. As shown in the figure, the receiver ants that got contaminated food through donors which were able to swallow their poison had a higher survivability than both donor and receiver ants that were not able to swallow the acid.

Donor formicine ants with access to their poison gland (FA+) and their receivers (FA-) have a higher survivability than the other donor-receiver pair after being fed pathogenic bacteria. Figure from original article.
a) The Asaia bacteria can survive up to 24 hours in the crop. b) Asaia bacteria are present in the midgut in increasing numbers for 48 hours after feeding the ants. Figure from original article.

The researchers show that the ant poison kills pathogens, but what about bacteria that can withstand this acid? Would the poison act as a filter, letting certain bacteria through? They fed the ants food containing bacteria from the genus Asaia, which is known to colonize the guts of different insects to see if it would withstand the acidic environment. And in contrast to the pathogen, Asaia was not affected much by the acidic environment in the crop, and could even be found alive in increasing numbers in the midgut after 48 hours. So yes, the poison acidified crops might act as a chemical filter to select against harmful bacteria and allow other, potentially beneficial bacteria through.

With these experiments, the researchers were able to show two new functions of the ant poison, which was previously thought to only protect them from predators. The poison is not only a way of killing the harmful bacteria, but can also act as a chemical filter to let (potential beneficial) bacteria through. The acidic crops also act as a barrier to prevent spreading of disease in formicine ant societies.

The researchers also found something curious: the acidity of the crops in ants with and without access to their poison was highly variable. What could be the cause of this? Are there any additional internal or external sources that regulate crop acidity? Are these differences specific to the species? Could they be the result of poison gland secretion composition? And what is the optimal crop acidity? All these questions will need to be addressed in further studies to try and unravel the interactions between ants and their microbes. 


Link to the original post: Simon Tragust, Claudia Herrmann, Jane Häfner, Ronja Braasch, Christina Tilgen, Maria Hoock, Margarita Artemis Milidakis, Roy Gross, Heike Feldhaar, Formicine ants swallow their highly acidic poison for gut microbial selection and control, eLife, 2020

Featured image: Black-beaked meadow ant (Formica pratensis) by Pawel Bieniewski. Source: Wikimedia commons, CC BY-SA 4.0