Selective Advantage of Resistant Strains at Trace Levels of Antibiotics: a Simple and Ultrasensitive Color Test for Detection of Antibiotics and Genotoxic Agents

Abstract

Many studies have examined the evolution of bacterial mutants that are resistant to specific antibiotics, and many of these focus on concentrations at and above the MIC. Here we ask for the minimum concentration at which existing resistant mutants can outgrow sensitive wild-type strains in competition experiments at antibiotic levels significantly below the MIC, and we define a minimum selective concentration (MSC) in Escherichia coli for two antibiotics, which is near 1/5 of the MIC for ciprofloxacin and 1/20 of the MIC for tetracycline. Because of the prevalence of resistant mutants already in the human microbiome, allowable levels of antibiotics to which we are exposed should be below the MSC. Since this concentration often corresponds to low or trace levels of antibiotics, it is helpful to have simple tests to detect such trace levels. We describe a simple ultrasensitive test for detecting the presence of antibiotics and genotoxic agents. The test is based on the use of chromogenic proteins as color markers and the use of single and multiple mutants of Escherichia coli that have greatly increased sensitivity to either a wide range of antibiotics or specific antibiotics, antibiotic families, and genotoxic agents. This test can detect ciprofloxacin at 1/75 of the MIC.

Changes in Enterococcal Populations and Related Antibiotic Resistance along a Medical Center-Wastewater Treatment Plant-River Continuum

“…The concentration of fluoroquinolones (0.14 ng/ml) and cephalosporins (0.16 ng/ml) in hospital effluents was far below the MICs for susceptible enterococci (MIC greater than 16 g/ml for cephalosporins and equal to 1 or 2 g/ml for ciprofloxacin, the most active fluoroquinolone [29]) and was much farther below those for resistant enterococci. However, it has been postulated that concentrations below MICs may have selective effects leading to enrichment of the environment by antibiotic-resistant bacteria (30,31). In a recent publication, minimum selective concentrations for resistant E. coli and Salmonella spp.…”

Review of Antimicrobial Resistance in the Environment and Its Relevance to Environmental Regulators

“…The MSC is a useful term that refers to the minimal concentration of chemical required to provide a selective advantage to a microorganisms carrying the resistance gene relative to the same bacterium that is sensitive to the chemical, i.e., not containing the resistance gene (Gullberg et al, 2011; Hughes and Andersson, 2012; Lundström et al, 2016). The MSC is a theoretical threshold that can be carefully determined in the laboratory for any microorganism and chemical pair (Liu et al, 2011), but takes on a different meaning when applied to more realistic scenarios of multiple species and multiple chemicals. Efforts to determine MSCs for more complex systems are ongoing, and as a result of their more realistic conditions might offer greater insights into the ecological relevance of low concentrations of antibiotics in the environment (Yergeau et al, 2010, 2012).…”

Spread of Antibiotic Resistance in the Environment: Impact on Human Health

“…It is well accepted that antibiotics at therapeutic concentrations select for resistant microbes; however, there is only scarce information and in some cases, contradictory data are available on the effect of antibiotics at subtherapeutic concentrations or concentrations below the minimal inhibitory concentrations (MICs; Rodríguez-Rojas et al 2013;Andersson and Hughes 2012;Hughes and Andersson 2012;Gullberg et al 2011;Liu et al 2011).…”