Bacteria maintain resistance even in the absence of antibiotics

Plasmids that provide bacteria with antibiotic resistance offer significant adaptive benefits, but do so at a cost. They also confer a metabolic burden on their carriers, and in the absence of antibiotics, bacteria that have these plasmids should be outcompeted by those that don’t. Reducing the proliferation of antibiotics, then, should reverse or at the very least check the spread of antibiotic resistance.

But a study in Nature Communications reported that bacteria continue to share plasmids at rates high enough to counteract the associated fitness costs even in the absence of antibiotics. Because of extremely high cell density, the bacteria are able to maintain a rate of conjugation that permits them to retain antibiotic resistance. These findings may have significant implications for how the problem of antibiotic resistance is approached.

“We hope this study brings to light the need to develop alternative antibiotic treatment strategies, likely in combination as adjuvants or new therapies all together, that target the ecological and evolutionary dynamics of resistance persistence,” said Allison Lopatkin, the study’s lead author. “Even in the most pristine stewardship environment (e.g. excellent antibiotic management), our results highlight that resistance will likely not go away and at best will be kept at bay.”

The researchers analyzed the rate of gene transfer, or conjugation, in several strains of E. coli for over a month. The plasmids they tested included three that encoded extended-spectrum beta-lactamases (ESBLs), which confer resistance to most beta-lactam antibiotics, including penicillin and cephalosporins.  They found that every single strain maintained resistance even in the absence of antibiotics.

The findings indicate that antibiotic resistance should be approached with drugs that inhibit conjugation, according to the researchers. They tested this approach by using linoleic acid to inhibit conjugation and phenothiazine to increase errors in plasmid segregation. The combination of the two agents reversed resistance in plasmids that were highly efficient at conjugation, and linoleic acid alone was able to destabilize one that had lower conjugation efficiency.

The conjugation inhibitors also did not stop overall bacterial growth, which is an important factor in attempting to remove resistant bacteria from a population. It also means that it’s unlikely they will be able to develop resistance to the inhibition factors, said Lopatkin. “It is however more likely that other consequences of inhibiting conjugation may have downstream effects that warrant investigation,” she said. “For example, perhaps inhibiting conjugation will drive the evolution of even more efficient means of horizontal gene transfer.”

Lopatkin said that she believes that strategic antibiotic treatments such as smart delivery profiles or collaterally synergistic combination therapies should be emphasized in approaching antibiotic resistance. “These approaches when combined with adjuvants that target the ecological dynamics, like the one described, is the best way forward,” she said. “As we understand the many ways in which bacteria circumvent treatment and adapt to survive, we can continue to develop better and smarter drugs to eventually combat resistance.”

Future research could focus on better high throughput screening to find other compounds that can inhibit conjugation or promote plasmid loss, she added.

“Additionally, determining the effects of disrupting an HGT network is another interesting direction that should be investigated in more complex relevant environments such as the mouse microbiome,” she said. “This would elucidate the potential consequences of using this type of intervention strategy so we can evaluate its efficacy in more relevant environments.”

Featured photo: Colorized scanning electron micrograph of E. coli. National Institute of Allergy and Infectious Diseases, NIH. source


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