A harmless nose bacterium supplied genes that helped make the Canterbury outbreak strain invasive

The comparison set is what makes this a genetic finding and not a coincidence. Martin Maiden's group at the University of Oxford sequenced whole genomes of live *Neisseria meningitidis* from blood samples taken from six of the patients, including the two who died ^[5]. The six sequences were almost identical, which the team took as evidence that one person seeded the cluster ^[6]. One of those genomes then went up against 48,000 other Neisseria sequences, collected during earlier outbreaks and from people with no known health problems ^[7]. That archive works as the control: it shows what related strains normally carry, and it points to the donors ^[8].

Two sorts of change turned up. Some of the acquired genes improved the bacterium's uptake of iron from blood, which it needs to survive and replicate ^[12]. Others reduced the sugar coating on the pilus, a tail-like structure on the cell surface, and the cells then clumped together ^[13]. Emma Wall at Queen Mary University of London, who was not involved in the discovery, said: "Neutrophils will try and come and eat an individual bacteria, but they can't eat a clump; it's too big" ^[14][18]. Exchanges like this are routine among bacteria, sometimes through tiny tunnels formed between cells, through which copies of genes pass ^[11].

Nine of the 21 known cases needed intensive care, about 43 per cent, and two of the patients died ^[3][25][4]. The denominator there is detected cases in one student cluster, not infections. The same species lives harmlessly in the nose and throat of 5 to 10 per cent of people without health problems, and it usually moves between them through saliva, in kissing or shared drinks and vapes ^[10][23].

Speed was the other unusual feature. Every case appeared within about a week ^[3]. Wall said UK outbreaks usually "roll over two or three weeks and there's one or two cases that end up in hospital" ^[17]. Maiden ties the abrupt end to the same catalogue of changes: people ill enough for intensive care stop circulating. "If you're ill, you're not going to be going around transmitting organisms, so the outbreak ended very quickly," he said ^[16].

When the gene acquisitions happened in the person thought to have started the cluster is unclear; Maiden thinks they accumulated gradually, until the right combination of genetics and social mixing was in place ^[19]. He called the bacteria "inherently highly unpredictable" ^[20].