A new way to measure contagion: the gut bacterium behind blood poisoning can spread like influenza

Gut bacteria can spread through the population surprisingly rapidly – in some cases as efficiently as influenza viruses. The good news is that the most dangerous and highly resistant strains do not appear to be the most transmissible.

E. coli is typically transmitted through everyday contact – for example through kissing, sharing a household, or via shared objects and food. It is an opportunistic pathogen: it usually lives harmlessly in the gut but can cause urinary tract infections or life-threatening sepsis if it enters the urinary tract and bloodstream, particularly in people with weakened immune systems.

Less resistant strain spreads fastest

The findings are based on a newly published international study in Nature Communications, which showed that three globally circulating Escherichia coli (E. coli) strains differ markedly in their transmissibility.

The most transmissible strain was ST131-A, which is more susceptible to antibiotics. Its basic reproduction number (R₀) was estimated to be 1.47 – comparable to the estimated R₀ of the 2009 swine flu pandemic virus (1.45).

By contrast, the antibiotic-resistant ST131-C1 and ST131-C2 strains spread much more slowly in the general population (R₀ = 1.18 and 1.13). This suggests they are not inherently “super-spreaders” but instead survive better under antibiotic pressure. As a result, they are more likely to circulate in hospitals and long-term care facilities, where antibiotic use is high and patients tend to be more vulnerable – rather than in the wider, healthy population.

A new model for measuring bacterial transmission

The research was carried out in collaboration with Aalto University, the universities of Helsinki, Tampere and Oslo, and the Wellcome Sanger Institute.

‘By combining extensive infectious disease and population datasets from the UK and Norway, we were able to develop a new epidemiological model that, for the first time, allowed us to estimate and compare the transmissibility of three pandemic-like E. coli strains,’ say Professor Pekka Marttinen from Aalto University and Professor Jukka Corander from the universities of Helsinki and Oslo, who led the study.

R₀ – a metric widely publicised during the COVID-19 pandemic – indicates how many new infections a single carrier generates on average. While commonly used to describe viral spread, it has never before been estimated for gut bacteria, which often colonise without causing symptoms.

‘With such comprehensive data, we were able to build a simulation model that predicts the R₀ of E. coli. To our knowledge, this is not only the first time R₀ has been estimated for E. coli but for any bacterium living in the human gut microbiome. Now that we have this model, it could be applied to other bacterial strains to improve our ability to understand, track and hopefully prevent the spread of antibiotic-resistant infections,’ says Fanni Ojala, doctoral researcher at Aalto University and co-first author of the study.

A tool for early detection and prevention

Going forward, the R₀-based approach will help researchers identify which bacterial strains pose the greatest disease risk, guiding surveillance and preventive action. 

Because sepsis-causing bacteria can persist in the body long before symptoms appear, researchers emphasise the importance of developing screening systems that detect high-risk strains at the carrier stage – enabling better understanding of their evolution and spread, and helping prevent infections before they emerge.

More information:

Basic reproduction number varies markedly between closely related pandemic Escherichia coli clones, Fanni Ojala et al. Nature Communications DOI: 10.1038/s41467-025-65301-1

Wellcome Sanger Institute: Advanced disease modelling shows some gut bacteria can spread as rapidly as viruses

This news item was originally published on the Aalto University website on 12.12.2025