What happens when animals lose the microbes they depend on?

7 September 2026

Many animals depend on beneficial bacteria for nutrients they cannot easily obtain from their diet. Some of these partnerships are so ancient that host and bacterium have become inseparable: remove the bacteria, and the animal can no longer survive or reproduce normally. 

But new research from the School of Biological and Behavioural Sciences (SBBS) at Queen Mary University of London shows that this does not always have to be the case. 

In a study published in Nature Communications, researchers from the Henry Lab investigated the relationship between the ant Cardiocondyla obscurior and its bacterial partner Westeberhardia cardiocondylae. Despite being passed from mother to offspring over 70 million years and living inside specialised cells within the ant, Westeberhardia is surprisingly not essential: some ants and even entire colonies can naturally lose the bacterium.  

The research, led by PhD student Phoebe Cunningham, set out to understand how such an ancient but flexible partnership can persist.

A bacterial nutritional backup 

The team discovered that Westeberhardia becomes particularly valuable when ants face poor nutritional conditions. 

Researchers experimentally removed the bacterium from colonies and compared these ants with colonies that retained their symbionts. Both groups were then exposed to diets containing different amounts of protein.  Under severe protein limitation, colonies without Westeberhardia struggled to complete development. Adult emergence declined by 94% in colonies without the symbiont, compared with 57% in colonies that retained it. The bacterium also benefited ants under normal dietary conditions, with symbiotic colonies producing larger workers and experiencing substantially lower colony mortality.  

The key appears to be the nutrients supplied by the bacteria. 

Westeberhardia retains the metabolic machinery needed to produce precursors of tyrosine, an amino acid that insects require in large quantities during development, particularly when constructing their external skeleton. The bacterium can also provide folate, or vitamin B9, another important nutrient involved in growth and development.  

But the ants have a Plan B 

Perhaps the most striking discovery was what happens when the bacterium disappears. 

Rather than simply suffering from the loss of its nutritional partner, C. obscurior can switch metabolic pathways. When Westeberhardia is present, the ant uses compounds produced by the bacterium to make tyrosine. Without the symbiont, however, the ant increases the activity of an alternative pathway that produces tyrosine from other amino acids obtained through its diet.  This flexibility may explain a long-standing evolutionary puzzle. Ancient nutritional symbioses often become obligatory because hosts gradually lose the ability to perform functions that their microbial partners provide. In Cardiocondyla, however, the ants have retained an alternative metabolic route. 

As a result, the bacteria can be extremely beneficial when protein is scarce but potentially dispensable when sufficient nutrients are available from food. 

“What is really interesting about this system is that the ants have not become completely dependent on their bacterial partner,” said Dr Lee Henry, who led the study. “They appear able to adjust their investment in the symbiosis according to their nutritional environment, and if the bacteria are lost, they can switch to an alternative way of obtaining the nutrients they need. It gives us a window into how animals can maintain beneficial microbial partnerships without becoming completely dependent on them.”

Understanding how symbioses evolve 

The findings challenge the idea that ancient, vertically transmitted nutritional symbionts inevitably become indispensable to their hosts. Instead, ecological conditions and the host's own metabolic flexibility may determine whether a partnership becomes permanently integrated or remains flexible.  

This flexibility could also have broader ecological consequences. Cardiocondyla obscurior has spread through tropical and subtropical regions around the world. Being able to draw nutrients either from its bacterial partner or directly from its environment could help colonies withstand variable diets as they encounter new environments.  

The study was conceived and designed by Dr Lee Henry, with PhD student Phoebe Cunningham carrying out the experimental work at Queen Mary, statistical analyses and data visualisation. The research brought together collaborators from the University of Regensburg and the Max Planck Institute for Chemical Ecology and was supported by BBSRC and Leverhulme Trust funding held by the Henry Lab, alongside funding supporting collaborating researchers.  

The study, “Metabolic plasticity supports a flexible nutritional symbiosis in Cardiocondyla ants”, is published in Nature Communications.

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