No collateral antibiotic sensitivity by alternating antibiotic pairs

Abstract

In a retrospective study of antibiotic susceptibility tests, Andrew Beckley and Erik Wright1Beckley AM Wright ES Identification of antibiotic pairs that evade concurrent resistance via a retrospective analysis of antimicrobial susceptibility test results.Lancet Microbe. 2021; 10: 545-554Summary Full Text Full Text PDF Scopus (10) Google Scholar reported low evidence of disjoint resistance in antibiotic pairs. Although disjoint antibiotics resistance is nearly absent in other species, Enterococcus faecalis was reported to possess varying degrees of disjoint resistance in specific antibiotic classes, especially the rifampicin–ciprofloxacin and rifampicin–tetracycline pairings. However, it is unclear if E faecalis could be trapped in susceptible/resistant and resistant/susceptible states for both pairings in experimental settings, where alternating between antibiotic pairs can induce collateral sensitivity. We tested if collateral sensitivity to antibiotics could be achieved in the E faecalis clinical isolate OG1RF with disjoint resistance of the rifampicin-resistant/tetracycline-susceptible state.2Sirichoat A Flórez AB Vázquez L et al.Antibiotic resistance-susceptibility profiles of Enterococcus faecalis and Streptococcus spp from the human vagina, and genome analysis of the genetic basis of intrinsic and acquired resistances.Front Microbiol. 2020; 111438Crossref PubMed Scopus (4) Google Scholar Since experimental evolution is commonly used to study antibiotic adaptation,3Cooper VS Experimental evolution as a high-throughput screen for genetic adaptations.mSphere. 2018; 3: e00121-e00218Crossref PubMed Scopus (38) Google Scholar we used this strategy in alternating antibiotic pairs by which the rifampicin-resistant E faecalis was exposed to prolonged sublethal tetracycline concentrations over in brain-heart infusion at 37°C by serial transfer for 100 generations. We then performed minimum inhibitory concentrations (MICs) testing of rifampicin and tetracycline on E faecalis populations before and after prolonged exposure of tetracycline, as determined according to the Clinical & Laboratory Standards Institute guidelines.4Clinical and Laboratory Standards InstitutePerformance standards for antimicrobial susceptibility testing (M100-S24).https://clsi.org/standards/products/microbiology/documents/m100/Date: April, 2021Date accessed: September 4, 2021Google Scholar With prolonged tetracycline treatment, we observed a 4-fold increase in tetracycline MIC of the evolved bacterial strain, suggesting a gain of tetracycline resistance after prolonged tetracycline exposure (table). However, we did not observe any sign of susceptibility to rifampicin, suggesting that the evolved bacteria had adopted the rifampicin-resistant/tetracycline-resistant state. Since the bacterium was also rifampicin -resistant/ciprofloxacin-susceptible, we used a similar assay for prolonged ciprofloxacin treatment, but found that the evolved populations achieved a rifampicin-resistant/ciprofloxacin-resistant state (table).TableMIC of Enterococcus faecalis OG1RF clinical isolate in a 100-generation experimental evolution assay with half-MIC tetracycline and ciprofloxacin treatmentRifampicinTetracyclineCiprofloxacinAbsence of tetracycline stress0 generations256·00·5..100 generations256·00·5..With tetracycline stress0 generations256·00·5..100 generations256·02·0..Absence of ciprofloxacin stress0 generations256·0..2·5100 generations256·0..2·5With ciprofloxacin stress0 generations256·0..2·5100 generations256·0..10·0Generation numbers are after experimental evolution. MIC=minimal inhibitory concentration. Open table in a new tab Generation numbers are after experimental evolution. MIC=minimal inhibitory concentration. Although E faecalis can display evidence of disjoint resistance, our findings suggest that collateral sensitivity of antibiotic pairs may be more complex than previously thought and strategy of alternating between antibiotic pairs requires in-depth investigation of inherent mechanisms. As the experimental evolution strategy typically favours gain-of-function and loss-of-function genomic mutations,5Bailey SF Bataillon T Can the experimental evolution programme help us elucidate the genetic basis of adaptation in nature?.Mol Ecol. 2016; 25: 203-218Crossref PubMed Scopus (41) Google Scholar preferential gain or loss of resistance plasmids by bacteria in clinical settings could be another possible reason why collateral sensitivity of antibiotic pairs occur. Although we retain hope for antibiotic cycling, we raise the need for further studies detailing the mechanism of disjoint antibiotic resistance, which may improve future clinical strategy on antimicrobial treatment of bacterial diseases. This research was supported by the Departmental Startup Grant (reference BE2B), State Key Laboratory of Chemical Biology and Drug Discovery Fund (1-BBX8), and Environment and Conservation Fund (48/2019). We declare no competing interests. Identification of antibiotic pairs that evade concurrent resistance via a retrospective analysis of antimicrobial susceptibility test resultsThe high frequency of concurrent resistance shows that bacteria have means of counteracting multiple antibiotics at a time. The almost complete absence of disjoint resistance at the species level implies that treatment strategies based on alternating between antibiotics might require subspecies level pathogen identification and be limited to a few antibiotic pairings. 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