A new study led by the University of Oxford has shed light on why certain species of bacteria carry astonishing arsenals of weapons. The findings, published today in the journal Nature Ecology & Evolution, could help us to engineer microbes that can destroy deadly pathogens, reducing our reliance on antibiotics.

Many species of bacteria possess multiple weapons to attack competitors. These include both short-range weapons that require direct contact with neighbouring cells, and long-range weapons, such as toxins that are released into the environment. Up to now, why bacteria have evolved to carry such a wide array of weapons has been a mystery.

Study co-author Professor Kevin Foster (Departments of Biology and Biochemistry, University of Oxford), said: ‘Unlike animals, which tend to carry a single weapon type such as horns, antlers, or tusks, bacterial species commonly carry multiple weapons. But it was unclear what the evolutionary basis for this was – why not just invest in a single type? One theory was that bacteria carry multiple weapons because they serve different functions during competition.’

The researchers tested this using the opportunistic pathogen Pseudomonas aeruginosa, a priority one pathogen by the World Health Organization, due to the rapid emergence of multidrug-resistant strains. P. aeruginosa possesses diverse weapons, including the ability to produce various toxic molecules (a long-range weapon), and toxin-loaded filaments anchored to its outer membrane (a short-range weapon).

The team designed a series of experiments to determine under which conditions short- versus long-range weapons give a greater advantage. They used genome editing to generate P. aeruginosa strains that lacked and were susceptible to either the toxin-loaded filaments or long-range toxins called tailocins. The susceptible strains were then grown on agar plates with control P. aeruginosa over two days, at a series of different ratios. Because the strains each expressed a different fluorescent protein, the researchers could quantify the ratio of attacker vs susceptible bacteria.

The results clearly demonstrated that the two weapons perform best under different conditions. Tailocins, the long-range weapon, only became effective when the attacking bacteria were at a high density and more common than the competition. On the other hand, carrying toxin-loaded filaments gave a competitive advantage over a much greater range of conditions. This included situations when the attacking bacteria were only present in low initial numbers and had to compete with a larger population of susceptible bacteria.

The researchers then challenged the two engineered strains in direct head-to-head competitions. When the strains started at an equal frequency, the bacteria carrying toxin-loaded filaments had a distinct advantage. However, both weapon users were able to win when they started in the majority.