Israeli researchers find new mechanism of mutation in bacteria exposed to antibiotics

In experiments with the bacterial genus Bartonella, Israeli researchers discovered a new mechanism of mutation that the pathogen uses to survive antibiotic attack. 

The discovery has potential groundbreaking significance in the battle against the growing number of antibiotic-resistant pathogens as well as in helping to establish a still-controversial hypothesis known as “adaptive mutation.”

Bartonella is a small, slow-growing bacterium which often infects domestic pets and farm animals as well as humans. Most strains are transmitted by fleas, ticks or other arthropods.

The newly discovered mutational mechanism proved extremely effective at evading the antibiotic rifampicin, and partially so against mitomycin C. As rifampicin and mitomycin C are often part of the treatment against Bartonella, the results have practical as well as important theoretical significance. 

The mutations were not found effective against the antibiotics enrofloxacin, chloramphenicol or doxycycline, drugs commonly used to treat many strains of Bartonella in dogs, cats, and humans. 

The results of the study, a collaboration of microbiologists, medical and veterinary specialists at nine different institutions, was published in the journal Molecular Biology and Evolution in October 2021. 

New mechanism of mutation

“Adaptive mutation” is a still-controversial hypothesis which proposes that rather than occurring only randomly and spontaneously during replication, some mutations may arise in the non-actively dividing cells of an organism subjected to stressful conditions. Other ways to describe this mechanism of mutation include programmed adaptation or replication independent mutation.

Previously, survival mutations in bacteria were thought to involve a mechanism known as the SOS response, in which the normal cell cycle is interrupted, and DNA repair begins. The SOS repair system is error prone and contributes to significant mutations in the DNA. 

One of the reasons for choosing the Bartonella species as the subject of study is that they are slow growing and do not exhibit the SOS response exhibited in other species.

Describing their discovery, the authors write: “We found that Bartonella species exposed to gradually increasing antibiotic concentrations elicit a mutagenic state that yields resistant mutations, even in the absence of known error-prone polymerases, and the conventional SOS response.” 

The new mechanism is not yet clearly characterized. The authors suspect that single mutations on a protein called rpoB may allow the bacteria to survive in the presence of rifampicin because the antibiotic cannot bind and block RNA synthesis. 

“The fact that emerging resistant mutants accumulated a single mutation only at the rpoB gene and no other mutation in the whole genome is remarkable,” they state.

Dose dependent

The authors call special attention to the new mutational phenomenon that is extraordinarily dependent on the antibiotic dose. 

The mutations do not appear when the bacterial colonies are immediately exposed to dosages above a certain minimum inhibition concentration. However, exposure to a sub-lethal concentration of rifampicin allowed the mutations to emerge. 

“Our data suggest that gradual exposure to RIF [rifampicin] concentrations, which includes a 24-hour sublethal period before supra-inhibitory concentrations are reached, is critical for the acquisition of resistant mutations,” the authors state.

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R. Gutiérrez et al. Adaptive Resistance Mutations at Suprainhibitory Concentrations Independent of SOS Mutagenesis, Molecular Biology and Evolution (October 2021). DOI:  https://doi.org/10.1093/molbev/msab196