A team of scientists led by Imperial College London has discovered a new subtype of emm89 Streptococcus pyogenes that has contributed to a rise in disease cases in the United Kingdom over the last two decades.

The human pathogen Streptococcus pyogenes (Group A Streptococcus) causes around 600 million infections per year worldwide. Severe infections can cause necrotizing fasciitis, pneumonia, sepsis, or toxic shock.

The researchers noticed a sharp rise in infections caused by one particular strain of Streptococcus pyogenes, called emm89, from 1998 to 2009.

To investigate why, they sequenced the genomes of bacterial samples from patients.

The genetic sequences revealed a new subtype of emm89 whose emergence coincided with the surge in cases.

The subtype has evolved two important differences from other members of the strain: it produces more toxin, and has completely 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 of Imperial College London, lead author of a study published in the journal mBio.

The scientists are still uncertain 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,” Dr Turner 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,” added senior author Prof Shiranee Sriskandan, also from Imperial College London.

“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.”

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Claire E. Turner et al. 2015. 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; doi: 10.1128/mBio.00622-15