A Taste of Their Own Medicine
How Formic Acid Helps Ants Fight Bacteria
Background
I have found that growing up on the Gulf Coast unites people in one painful and unexpected way: ant bites. The invasive species Solenopsis invicta, or fire ant, is plentiful in states like Louisiana, Texas, and Florida. An encounter with a fire ant is not one you soon forget, as many of us unfortunate southern children know from sitting right down on a hill during recess. If you have ever wished that ants could get a taste of their own medicine, you’re in luck. That’s just what German researchers have examined and uncovered in this 2020 study.
Formicine ants are united by the formic acid present in their poison secretions. Formic acid is not only used as a defensive weapon but is also spread around the ant’s environment to protect the nest and limit the spread of disease. It can also be used to protect against the alkaline poison of ants like Solenopsis. In order to spread the poison around, ants engage in a behavior called acidopore grooming. During acidopore grooming, ants will take the acidopore (which is the opening of the poison gland) to their mouths to spread the poison around. Ants actually have two stomachs, a crop and a second stomach, instead of only one like humans do. No digestion takes place in the crop, as it is a place for food to be stored until it either passes to the second stomach or is regurgitated for another ant. Because the crop lacks the same digestive properties of the second stomach, it must find another way to vanquish harmful bacteria.
Summary: Grooming Away Microbes
This study explores whether formicine ants ingest their own poison during acidopore grooming and what, if any, benefit it may provide. The scientists hypothesized that the ingestion of their own poison during acidopore grooming may protect the ant from bacteria present in their environment.
To see whether the ants were swallowing the poison, they fed the ants 10% honey water with a pH of 5 while allowing the ants to acidopore groom. Acid levels were recorded in the crop lumen of the ants over time. They concluded that the natural pH of the crop lumen was about pH 2. To be sure that the ingestion of the formic acid was responsible for the acidity, they then repeated the experiment without allowing the ants to acidopore groom. This resulted in a much lower acidity in the crop lumen. Interestingly, the acidity of the crop lumen did not extend into the midgut where most digestion occurs. This suggested that the function of this acidity was not digestion, but microbial control.
To better understand how food passes through the ants, the scientists placed fluorescent particles in the ant’s food and measured how long it stayed in the crop before moving to the midgut. They found that the food lingered in the crop before passing to the midgut. This would allow time for the formic acid to work to destroy pathogenic bacteria. To test the potential antimicrobial function of the acidopore grooming, the scientists introduced Serratia marcescens, a bacterium known to infect insects. Almost immediately, the bacteria was not found in the crop, and after several hours, it was found at very low levels in the midgut. The scientists also tested the effects of formic acid with the following in vitro experiment.
Over a two-hour time period, the scientists placed Serratia marcescens in 10% honey water that was acidified by formic acid. They found that the presence of the formic acid significantly decreased the levels of the bacterial colonies when compared to their experiment without formic acid. They concluded that the formic acid present in the poison of the ants would be able to significantly affect the pathogenic bacteria.
To see just how important the bacteria-destroying properties of the ant’s poison are, the scientists conducted another experiment. They introduced Serratia marcescens into the food of the ants and allowed one group to acidopore groom while preventing the others from acidopore grooming. When Serratia marcescens was introduced into the ants allowed to perform acidopore grooming, the survival rate was much higher than in the ants that were not allowed to acidopore groom. In fact, ants that acidopore groomed survived just as well as the control group that was not introduced to Serratia marcescens. Not only do the ants ingest their own poison, but it serves an important function as an acidic antibacterial.
