Over-the-counter lozenges for a sore throat are fuelling the rise of superbugs, warns new research.
They include Tyrozets pastilles available in the UK that contain a mixture of antibiotics called tyrothricin, according to the study.
Used for mouth infections, they could be contributing to antibiotic resistance – going against World Health Organisation (WHO) goals, say British scientists.
The study, presented at the European Congress of Clinical Microbiology and Infectious Diseases in Amsterdam, warned such products are widely available in the UK and across Europe.
Medicines watchdog NICE (National Institute for Health and Care Excellence) and Public Health England are advising doctors to only prescribe antibiotics for severe sore throats caused by a bacterial infection.
Adrian Shephard, of Reckitt Benckiser Healthcare Ltd, based in Slough, Berks, which commissioned the study, said: “Our work raises doubt about the continued over-the-counter availability of these antibiotics for the treatment of sore throats, especially considering the primarily viral nature of the condition.”
Most sore throats are caused by viral infections, which cannot be treated by antibiotics. But research suggests they are prescribed in 60 per cent of sore throat cases.
NICE has said it is “vital” the medicines were only used when effective because of the rise of antibiotic resistance.
The overuse of antibiotics is making infections harder to treat by creating drug-resistant superbugs.
Acute sore throat, including pharyngitis and tonsillitis, can be caused by a bacterial or viral infection, with symptoms usually improving by themselves within a week.
The Cardiff University team investigated the development of bacterial resistance to four commonly used topical antibiotics legitimately available in sore throat medicines on the high street – gramicidin, neomycin, bacitracin and tyrothricin.
They examined four species of bacteria in which antibiotic resistance is a widespread and significant problem – Staphylococcus aureus, Acinetobacter baumannii, Streptococcus pyogenes and Haemophilus influenza.
Cultures of each were exposed to decreasing concentrations of antibiotic for 24 hours at human body temperature (37°C) – with surviving bacteria sub-cultured and tested for antibiotic susceptibility.
The researchers also looked into cross-resistance, where bacteria exposed to one antibiotic can become less susceptible to a different antibiotic to which they may not have previously been exposed.
They wanted to check whether the in-use concentrations of the antibiotics was above the minimum inhibitory concentration (MIC) – the lowest level of a drug that is still able to prevent bacterial growth.
For S. aureus and A. baumannii the in-use concentrations of neomycin, bacitracin and tyrothricin were all above the MIC – confirming that these products were effective at preventing bacterial growth.
No MIC could be determined for gramicidin – indicating none of the concentrations tested were able to prevent growth of those two species of bacteria.
For S. pyogenes and H. influenza MIC values for all the tested antibiotics were below in-use concentrations in throat medicines.
H. influenza was unable to grow in any of the antibiotics and concentrations tested – and S. pyogenes only exhibited growth in very weak concentrations (5% and 1% of those found in medicines) of neomycin.
When S. aureus was exposed to bacitracin, it eventually showed growth after 144 hours at higher concentrations.
A culture grown in a lower concentration of the drug was discovered to have decreased susceptibility to gentamicin, fusidic acid, and ciprofloxacin, indicating that it had developed cross-resistance.
Mr Shephard said: “We were concerned to find some of the over-the-counter antibiotics used in sore throat preparations were not sufficiently concentrated to prevent growth of common human pathogens and are enabling these pathogens to develop resistance.
“In addition, exposure to both standard and diluted concentrations of bacitracin was associated with clinical cross-resistance to other antibiotics.”
To date, the researchers say only products containing the antibiotic fusafungine have been removed.
A wide range of other over-the-counter products containing antimicrobials remain available across Europe.
Co-author Professor Jean-Yves Maillard, of Cardiff’s School of Pharmacy and Pharmaceutical Sciences, added: “This was an interesting study that showed once again the potential of bacteria to adapt to chemotherapeutic antibiotics highlighting the need for a prudent and perhaps controlled use of antibiotics in practice.”
Reckitt Benckiser is a manufacturer of sore throat medications, but its products do not contain local antibiotics.