Scitech | For the good of bacterial kind

Researchers study survival strategies of antibiotic-resistant E. coli

Although one wouldn’t expect to see caring tendencies in bacteria, a new study conducted at Boston University shows the behaviour of Escherichia coli organisms, commonly referred to as E. coli, in a surprising new light.  
Published in the journal Nature on September 2, the study, “Bacterial charity work leads to population-wide resistance”, shows how certain antibiotic-resistant E. coli individuals share their resilience with their more vulnerable neighbours, unexpectedly acting to benefit others bacteria, even to the point of exhibiting something similar to altruism. 
The researchers observed cultures of E. coli bacteria as they increased the levels of an antibiotic called norfloxacin present in their environment. Lead author Henry Lee says that “most bacteria were less resistant by themselves than the population as a whole.” Lee says that it was a surprise to find that only a few rare mutated individuals were capable of resisting the antibiotic on their own, and were helping the other bacteria survive by producing indole, an organic compound that stimulates the bacteria’s protective mechanisms and helps them pump out the fatal drugs. 
Though all E. coli organisms are capable of making their own indole, their production mechanisms usually shut down when a stressor is introduced to their environment. Some mutated bacteria are never adversely affected by the antibiotics’ presence and never stop producing indole. Lee’s study suggests that they produce higher levels of indole than necessary in order to release it into their environment, enhancing the survival of the overall population. 
The population-saving indole production comes at a cost for the resistant constituents. According to Hyun Youk, a biophysicist at MIT, by helping their non-resistant neighbours and focusing their efforts on making the indole, the mutated bacteria had fewer opportunities to grow. “They could have invested that energy for their own growth instead of using it to help out their non-resistant neighbours,” he said.

The researchers sequenced the genomes of the antibiotic-resistant E. coli and found that the compulsion to share the indole seemed to be an innate feature, programmed into the bacteria at the genetic level. This natural altruism, though often observed in more complex animals and even in several plant species, is not commonly associated with the bacterial world. According to Lee, though there have been cases of resource sharing found among bacteria, particularly those residing together in aggregated biofilms – layers of bacteria – there is still a tendency to “discard the idea…, expecting [the bacteria] to fend for themselves.” 
Although self-preservation is a biological imperative, “altruism is good for the survival of the species,” said Youk, and many species’ individuals will value the evolutionary fitness of their kind over their own. Kin selection, for example, refers to individuals that help their relatives reproduce successfully, even at the cost to their own personal reproduction.

Lee thinks that many aspects of his research point to kin selection, though the researchers are still exploring the possibility. The mutated bacteria’s indole production seemed to benefit those individuals in their vicinity, as well as those with common genetic material.

Regardless of the purposefulness to the individual bacteria, the altruistic strategy may shed light on how doctors can counter antibiotic-resistant bacteria, and block indole pathways.

“The work is important because it shows how such a cunning strategy develops over a period of a few days,” explained Youk.

Lee added that helping doctors’ diagnoses is “definitely the goal,” and that their methodology could be helpful, since “testing a whole population versus testing individual bacteria is not the same.” Their study and ongoing research will hopefully help physicians understand the bacteria’s behaviour, bringing them closer to determining how much antibiotic is sufficient and how to better treat patients.