Polysaccharide cloaks called capsules adorn bacteria to help them evade immune system detection and adapt to changing environments.1 Pathogens like Escherichia coli produce dozens of capsular types, but only a few associate with invasive diseases. In a paper published in Nature Communications, researchers mapped the genetic history of one particularly potent E. coli cloak—the K1 capsule—for understanding the secrets to its success.2 

Alex McCarthy, a microbiologist at Imperial College London, and his team analyzed global whole-genome sequencing data from more than 5,000 clinical samples of bloodstream infections, some predating the antibiotic era. They reconstructed the evolutionary history of the E. coli populations and mapped the emergence of genes encoding the K1 capsule. The results surprised McCarthy’s team.

Approximately 25 percent of the sampled bacteria encoded genes for the K1 capsule, and the genetic locus for the capsule emerged in multiple lineages over the last 500 years. “This was really exciting and interesting,” said McCarthy. “We didn't grasp beforehand the way in which this capsule has emerged independently in multiple different clones of bacteria.”

“One of the strengths that allowed them to get to this result is the use of unbiased databases,” said Olaya Rendueles, a microbiologist at the Pasteur Institute who was not involved in the study. McCarthy’s team used a blend of datasets, which allowed them to take a snapshot of all pathogenic E. coli.

The team also tested K1 capsule survival in human serum in vitro. When they added a bacteriophage enzyme to a blend of human serum and E. coli, it stripped the bacteria of its protective cloak, allowing the human immune system to attack the pathogen. 

In the future, McCarthy and his team hope to understand how the K1 capsule evolves to inform new measures to control and prevent infections. 


  1. Nucci A, et al. Nat Commun. 2022;13:4751.
  2. Arredondo-Alonso S, et al. Nat Commun. 2023;14:3294.