A new variant of streptococcal bacteria has contributed to a rise in disease cases in the UK over the last 17 years, scientists have discovered.

A sharp rise in infections caused by one particular strain, called emm89, from 1998 to 2009, was noticed by researchers at Imperial College London and clinicians at Imperial College Healthcare NHS Trust, together with colleagues at Public Health England. To find out why, the genomes of bacterial samples from patients were sequenced.

The results revealed a new subtype of emm89 streptococcus whose emergence correlated with the spike in cases.

Group A streptococcus bacteria causes about 600 million infections per year worldwide. More severe infections can result in pneumonia, sepsis, necrotising fasciitis, or toxic shock. One in four people who suffer an invasive infection do not survive.

Surges in the same strain type have been reported by Japan, Canada, France and Sweden, raising the possibility that the new variant is spreading globally.

The new variant has evolved two key differences from others of the strain. It produces more toxin, and has totally lost its outer capsule.

“The fact that it had lost its capsule was a complete surprise, because it was believed that the capsule was essential for group A streptococcus to cause invasive disease,” said Dr Claire Turner, a Junior Research Fellow at Imperial College London who led the study.

Researchers are still unclear about why the new variant without capsule has become so dominant.

“We know that without capsule, they stick better to surfaces, so that may help them to transmit more easily. Another possibility is that they can more easily get inside human cells, which makes them harder to treat,” said Dr Turner.

Senior author Professor Shiranee Sriskandan from the Department of Medicine at Imperial College London, said:

“We know very little about how group A streptococcus is transmitted from person to person. We need to look into this more deeply and think about better ways to prevent transmission.

Luckily, the new variant remains exquisitely sensitive to penicillin and related antibiotics. But we also need to think about whether our treatment strategies are as good as they can be.”

Claire E. Turner, James Abbott, Theresa Lamagni, Matthew T. G. Holden, Sophia David, Michael D. Jones, Laurence Game, Androulla Efstratiou, Shiranee Sriskandan Emergence of a New Highly Successful Acapsular Group A Streptococcus Clade of Genotype emm89 in the United Kingdom mBio vol. 6 no. 4 e00622-15 14 July 2015 doi: 10.1128/mBio.00622-15

Illustration: Milos Kalab, Wellcome Images, Creative Commons by-nc-nd 4.0

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