Minimum inhibitory concentration of glyphosate and a glyphosate-containing herbicide in salmonella enterica isolates originating from different time periods, hosts, and serovars

Abstract

Glyphosate, the active compound of Roundup, is one of the most used pesticides in the world. Its residues are often detected in animal feed, but the impact on the animal gut microbiota and on pathogens of the intestine has not intensively been investigated. In this study, we analyzed the minimum inhibitory concentration (MIC) of glyphosate isopropylamine salt and a common glyphosate-containing herbicide formulation in 225 Salmonella enterica isolates by broth microdilution. A bacteriostatic effect of glyphosate on Salmonella growth was detected at the concentration range of 10 to 80 mg/mL for both the active ingredient and the ready-to-use formulation. Time/year of isolation, host species, and serovars revealed a statistically significant influence on MIC values. Recently collected Salmonella isolates had significantly higher MIC values for glyphosate and the glyphosate-containing product compared with isolates collected between 1981 and 1990. Isolates from pigs showed significantly higher MIC values compared with isolates from poultry, and isolates of the Salmonella serovar Typhimurium had significantly higher MIC values than Salmonella Enteritidis and Infantis isolates.

No hints at glyphosate-induced ruminal dysbiosis in cows

“…The absence of long-term glyphosate effects in the gastrointestinal tract is not surprising, since excess of amino acids is available and synthesis of aromatic amino acids can possibly be bypassed through transport 19 . However, it was pointed out recently, that Salmonella isolates from before broad usage of glyphosate are more sensitive toward glyphosate than modern isolates 49 . Given that typical ruminal bacteria have been exposed to low concentrations of glyphosate on regular basis, it might be possible that a common ruminal microbiome is adapted to glyphosate exposure.…”

Selection for Resistance to a Glyphosate-Containing Herbicide in Salmonella enterica Does Not Result in a Sustained Activation of the Tolerance Response or Increased Cross-Tolerance and Cross-Resistance to Clinically Important Antibiotics

“…The MIC of Roundup LB Plus (German license 024142-00; Monsanto) was determined by the broth microdilution method described in reference 9 in MHI. Because GBH acidifies medium, the pH was adjusted to neutral with NaOH.…”

“…While the potential toxic effects of glyphosate on humans, animals, and the environment are subjects of heated scientific and public debates ( 5 , 6 ), its effects on gut bacteria have only recently attracted attention, despite its known antimicrobial properties ( 7 ) and frequent exposure through food and feed ( 8 ). Indeed, we recently found that Salmonella enterica sampled after the introduction of GBH into agricultural practice tended to have higher levels of resistance to glyphosate and GBH than the historical isolates from the preglyphosate era ( 9 ), while direct exposure to GBH has been shown to enrich for pathogenic bacteria in the gut ( 10 ) and change susceptibility to antibiotics in S. enterica and Escherichia coli through the activation of AcrAB efflux pumps ( 11 ), which are known to be involved in drug tolerance and resistance ( 12 – 14 ).…”

Effect of a glyphosate-containing herbicide on Escherichia coli and Salmonella ser. Typhimurium in an in vitro rumen simulation system

“…Three isolates of each strain from each fermenter and the last sampling time point from which bacteria could be recovered were further assessed for changes in antimicrobial susceptibility relative to the original parent strains. Prior to the fermenter experiments, the initial MICs of RU and RU supplemented with NaOH (to achieve pH7) for these isolates were determined as described previously [35, 36]. In short, serial twofold dilutions of RU in Mueller Hinton broth ranging from 160 mg/mL to 2.5 mg/mL IPA were prepared in conical 96-well plates and stored at –80 °C until use.…”

“…Sub-lethal glyphosate concentrations could further induce resistances and lead to changing antibiotic susceptibility profiles [31–33], with the possibility of transferring antibiotic resistances between isolates from livestock and humans as another major concern [34]. Our own recent studies showed small but significant increases in minimum inhibitory concentrations (MICs) of glyphosate and a commercial glyphosate-containing formulation in Salmonella enterica isolated in recent years in Germany, when compared to historic isolates [35]. Similarly, this was indicative for glyphosate and E. coli [36].…”