There are two types of sore throat - those caused by viral infections and those caused by bacteria. Those caused by bacteria can by cured by the use of antibiotics.

They are called "strep throat" after the bug that causes them - Streptococcus pyogenes.

However, a study published in The Lancet medical journal found that S. pyogenes may elude the antibiotics by slipping into cells where the drugs are likely to be less concentrated.

The finding helps explain why the infection lingers on in 5-30% of cases, even though penicillin and related antibiotics should eradicate S. pyogenes.

Gene test

The research was carried out by Dr Revital Neeman and colleagues in Tel-Aviv in Israel.

They took samples from patients with strep throat before and then after antibiotic treatment.

They then tested the streptococci that grew for a gene that holds the blueprint for a protein called F-1. F-1 acts as a key for bacteria to enter the cells lining the respiratory tract.

All 67 patients were cured of their sore throats, but 13 of the patients still had S. pyogenes in the samples they gave.

Twelve of the 13 had the gene for F-1, while only a third of the others had it.

The 13 would be considered carriers - although they would have no symptoms themselves, they would be able to pass the bug on to others.

The researchers said that the study showed that the F-1 protein allowed the bacteria access to the cells.

They said: "Our findings suggest that protein-F-1 mediated entry into is involved in the causative process of the carriage state."