Antibiotic Resistance is More Complex Than You Think

Adding to the troubling data worldwide on antibiotic resistance, researchers at the University of York in the United Kingdom have found that even trace concentrations of antibiotics, such as those found in sewage, are sufficient for bacteria to maintain resistance to most broad-spectrum agents.

Their findings were published online in January by the journal Antimicrobial Agents and Chemotherapy (AAC).

The research team from the departments of Mathematics and Biology at York analyzed a plasmid called RK2, which encodes what they described as “cooperative” resistance (affecting the resistant cell and the surrounding cells, whether they are resistant or not) to ampicillin and “selfish” resistance (affecting only the resistant cell) to tetracycline, in Escherichia coli (E. coli), a bacterium known to cause infectious diarrhea. They determined that “selfish” resistance occurs at very low concentrations—only 1.3 percent of the minimum inhibitory concentration (MIC) of sensitive cells—roughly equivalent to the residue levels of antibiotics found in contaminated sewage—while cooperative resistance occurred at concentrations exceeding 10 percent of the MIC of the plasmid-free strain.

The authors of the AAC article were not available for comment at press time. However, of these findings, they wrote, “[At] concentrations of tetracycline above 10 percent of the MIC of sensitive E. coli, the percent resistant plasmid bearers competitively excluded the plasmid-free bacteria, with no plasmid-free cells observable. This is despite the fact that plasmid free E. coli could survive at these tetracycline concentrations when grown alone.”

In addition, they noted that earlier studies have demonstrated that environmental antibiotic resistance genes (ARGs) are a major source of clinical drug resistance, and that ARGs are thought to be positively selected at antibiotic concentrations exceeding the MIC of sensitive cells in monoculture—or the “conventional selective window.” To determine whether the “sociality of resistance” affected the selection window for the RK2 multi-drug resistant plasmid, they estimated the relative fitness of plasmid bearing versus isogenic plasmid free cells by direct competition and found that in the absence of antibiotics the plasmid imposed “a significant cost of carriage,” reducing the fitness of E. coli by 19 percent. They also demonstrated that sensitive cells were able to maintain positive growth in mixed cultures at ampicillin concentrations that completely inhibited their growth in monoculture.

They wrote, “Cooperative resistance permits persistence of a sensitive subpopulation beyond the sensitive MIC due to the inactivation of the antibiotic, potentially allowing reinvasion by sensitive cells once the antibiotic concentration is sufficiently reduced by the action of resistant cells. Our data suggest that selfish tetracycline resistance is positively selected in the sub-MIC selective window at very low tetracycline concentrations, similar to those observed in the natural environment.”

Furthermore, when ampicillin and tetracycline were applied in combination, the researchers found “no significant interaction indicating that… their selective effects were independent and additive,” meaning that very low concentrations of tetracycline were needed to completely mask the population-level effects of cooperative ampicillin resistance. Indeed, with increasing tetracycline concentrations, the ampicillin concentration positively selecting for the multi-drug resistant plasmid shifted to lower, sub-MIC levels, reducing the window of selective conditions where sensitive cells could persist.

They authors concluded, “Combining the two antibiotics—at concentrations that would not normally select for resistance individually—selects for both resistances and spread of the multi-drug resistant plasmid. Taken together [our] findings suggest that selfish efflux-mediated drug resistances are likely to be especially important for the selective maintenance and spread of multi-drug resistant plasmids.”

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.