Glyphosate affects persistence and tolerance but not antibiotic resistance

Is there a link between exposure to pesticides and antibiotic resistance in Gram-negative bacteria isolated from Thai farmers?

BACKGROUND

The organophosphate pesticides have the potential to impact microbial diversity, but their influence on antibiotic resistance (AR) in bacteria remains understudied.

OBJECTIVES

The objective of our study was to evaluate the impact of exposure to acetylcholinesterase inhibitors on glyphosate tolerance and AR in Gram-negative bacteria isolated from the digestive tracts of Thai farmers.

METHODS

Human fecal samples from Thailand, grouped by pesticide exposure level measured by acetylcholinesterase blood concentration, were cultured on MacConkey (McK) agar with or without 7 g/L of a glyphosate-based formulation (GBF). Antibiotic susceptibility and glyphosate minimum inhibitory concentration (MIC) of isolated strains were assessed using the disk diffusion and broth microdilution methods, respectively.

RESULTS

A total of 547 GNB were isolated from 112 human fecal samples. GBF medium predominantly selected Escherichia coli, Klebsiella pneumoniae, and Citrobacter freundii. GBF MICs ranged from 2 g/L to 16 g/L with K. pneumoniae species harboring the highest median MIC (16 g/L). AR rates were not significantly different between exposed and not exposed groups to pesticides. In contrast, six mobile colistin resistance (MCR)- and/or extended spectrum beta lactamase (ESBL)-producing E. coli strains were isolated from pesticide-exposed group, while only one colistin-resistant K. pneumoniae strain was isolated from a sample which was not exposed to pesticides.

CONCLUSIONS

The results of our study underscore the need for further research, particularly on the impact of glyphosate exposure on colistin resistance and the prevalence of ESBL-producing strains. Additionally, we emphasize the importance of testing a broad range of pesticides to better understand their impact on AR.

Revealing novel protein interaction partners of glyphosate in Escherichia coli

Despite all debates about its safe use, glyphosate remains the most widely applied active ingredient in herbicide products, with renewed approval in the European Union until 2033. Non-target organisms are commonly exposed to glyphosate as a matter of its mode of application, with its broader environmental and biological impacts remaining under investigation. Glyphosate displays structural similarity to phosphoenolpyruvate (PEP), thereby competitively inhibiting the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), crucial for the synthesis of aromatic amino acids in plants, fungi, bacteria, and archaea. Most microbes, including the gut bacterium Escherichia coli (E. coli), possess a glyphosate-sensitive class I EPSPS, making them vulnerable to glyphosate's effects. Yet, little is known about glyphosate's interactions with other bacterial proteins or its broader modes of action at the proteome level. Here, we employed a quantitative proteomics and thermal proteome profiling (TPP) approach to identify novel protein binding partners of glyphosate in the E. coli proteome. Glyphosate exposure significantly altered amino acid synthesizing pathways. The abundance of shikimate pathway proteins was increased, suggesting a compensatory mechanism. Extracellular riboflavin concentrations were elevated upon glyphosate exposure, while intracellular levels remained stable. Beyond the target enzyme EPSPS, thermal proteome profiling indicated an effect of glyphosate on the thermal stability of certain proteins, including AroH and ProA, indicating interactions. Similar to the competitive binding between PEP and glyphosate at EPSPS, one reason for the interaction of AroH and ProA with the herbicide could be a high structural similarity between their substrates and glyphosate. Overall, glyphosate induced metabolic disturbances in E. coli, extending beyond its primary target, thereby providing new insights into glyphosate's broader impact on microbial systems.

Non-Canonical Aspects of Antibiotics and Antibiotic Resistance

The understanding of antibiotic resistance, one of the major health threats of our time, is mostly based on dated and incomplete notions, especially in clinical contexts. The "canonical" mechanisms of action and pharmacodynamics of antibiotics, as well as the methods used to assess their activity upon bacteria, have not changed in decades; the same applies to the definition, acquisition, selective pressures, and drivers of resistance. As a consequence, the strategies to improve antibiotic usage and overcome resistance have ultimately failed. This review gathers most of the "non-canonical" notions on antibiotics and resistance: from the alternative mechanisms of action of antibiotics and the limitations of susceptibility testing to the wide variety of selective pressures, lateral gene transfer mechanisms, ubiquity, and societal factors maintaining resistance. Only by having a "big picture" view of the problem can adequate strategies to harness resistance be devised. These strategies must be global, addressing the many aspects that drive the increasing prevalence of resistant bacteria aside from the clinical use of antibiotics.

RpoS role in antibiotic resistance, tolerance and persistence in E. coli natural isolates

BACKGROUND

The intrinsic concentration of RpoS, the second most abundant sigma factor, varies widely across the E. coli species. Bacterial isolates that express high levels of RpoS display high resistance to environmental stresses, such as temperature, pH and osmolarity shifts, but are less nutritional competent, making them less capable of utilising alternative nutrient sources. The role of RpoS in antibiotic resistance and persistence in standard laboratory domesticated strains has been examined in several studies, most demonstrating a positive role for RpoS.

RESULTS

Using disk diffusion assays we examined bacterial resistance to 15 different antibiotics, including β -lactams (penicillins, monobactams, carbapenems and cephalosporins), aminoglycosides, quinolones and anti-folates, in a representative collection of 328 E. coli natural isolates displaying a continuum of different levels of RpoS. There was an overall trend that isolates with higher levels of RpoS were slightly more resistant to these antibiotics. In addition, the effect of RpoS on bacterial tolerance and persistence to 3 different antibiotics - ampicillin, ciprofloxacin and kanamycin was evaluated through time-kill curves. Again, there was a small beneficial effect of RpoS on tolerance and persistence to these antibiotics, but this difference was not statistically significant. Finally, a K-12 strain expressing high levels of RpoS was compared with its isogenic RpoS-null counterpart, and no significant effect of RpoS was found.

CONCLUSION

Based on a representative collection of the species E. coli, RpoS was found to have a very small impact on antibiotic resistance, tolerance, or persistence.