Liquorice root could be effective against upper respiratory tract infections - while protecting ‘good’ microbes

The research, by Barbara Kimani at Birmingham City University, won a poster prize at the Minoritised Life Scientists Future Forum at the ICC in Birmingham in April 2025. Following her success, Barbara has now signed up as an MLSFF ambassador for the conference in March 2026.

“Liquorice root extract has antimicrobial properties that are able to selectively target pathogenic upper respiratory tract bacteria, while causing minimal disruption to the commensal flora, meaning that lozenges supplemented with liquorice root extract could be an effective remedy against upper respiratory tract infections,” she summarised.

“The upper respiratory tract is made up of bacteria, among other microorganisms, that exist non pathogenically and limit colonization by other pathogenic bacteria by maintaining immune homeostasis. Some of these bacteria are Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes and Streptococcus mutans. 

“Factors such as competition for available resources ensures that these bacteria do not overpopulate the niche, thus becoming pathogenic. Use of antibiotics disrupts this delicate balance through killing important resident bacteria, leading to dysbiosis and pathogenesis. Staphylococcus aureus, Streptococcus pneumoniae and Streptococcus pyogenes can also be acquired in the community and hospitals and lead to potentially fatal infections.”

Susceptible people

The most susceptible individuals to infections by these bacteria are children, the elderly and immunocompromised due to an underdeveloped immune system, loss of immune diversity due to ageing and a weakened immune system, she explained.

People with cystic fibrosis are also highly susceptible due to a weakened immune system and a microenvironment that favours bacterial overgrowth.

Antibiotics are highly prescribed in the UK for treating respiratory tract infections which can lead to antimicrobial resistance as well as the aforementioned dysbiosis. Therefore it is important to identify alternative therapies that limit antibiotic use.

“This project was aimed at investigating whether an extract made of liquorice root extract could be effective against pathogenic upper respiratory tract bacteria and investigating the effect on commensal bacteria, such as Streptococcus mutans,” Barbara said.

Study methods

The first step was to prepare the extract by mixing ground liquorice root with 100% ethanol and storing it for a few days, after which it was centrifuged, filtered and stored in aseptic conditions.

“I then prepared a broth microdilution assay to establish the concentration of liquorice root extract that was effective against each organism. I used pure 100% ethanol as control to ascertain that any bactericidal effect was as a result of the extract and not the ethanol that had been used to prepare the extract,” Barbara explained.

“I used the results from the broth microdilution assay to calculate the amount of extract to use in the antimicrobial kinetics experiment. These experiments were conducted to establish bacterial sensitivity to the extract by using the results to establish the amount of time taken for a reduction in bacterial viability to be observed.

“I discovered that the extract was fast and effective in killing the pathogenic bacteria, while Streptococcus mutans, which is part of the normal oral flora, was least affected by the extract and remained viable for longer than 20 minutes, which is longer than the amount of time taken for a lozenge to dissolve (~10 minutes).”

Surprising finding

One surprise was that even when using the highest antibacterial concentration calculated for the organisms, disruption to the commensal bacteria was minimal, meaning that a lozenge meant to target all these pathogenic bacteria would not be disruptive to the microflora.

“This means that lozenges supplemented with liquorice root extract can be an alternative treatment for upper respiratory tract infections, which would reduce chances of antimicrobial resistance in general population and in those with underlying conditions such as cystic fibrosis and chronic obstructive pulmonary disease (COPD) who are prone to upper respiratory tract infections, and prevent dysbiosis especially in individuals with reduced microbial diversity and a weakened immune system,” Barbara said.

“Further tests are needed to investigate how the extract affects other microorganisms commensal to the upper respiratory tract microflora.

“All the above findings were done in vitro, therefore, there is a need for in vivo tests to establish whether observed results are similar in live models.”

The study was led by Barbara’s project supervisor, Dr. Martin Goldberg and was supported by Birmingham City University.

MLS Future Forum 2026 takes place at the Edinburgh International Conference Centre from March 23 to 25 - to find out how you can involved, click HERE.