Exogenous methionine contributes to reversing the resistance of Streptococcus suis to macrolides

Streptococcus suis (S. suis) has been increasingly recognized as a porcine zoonotic pathogen that threatens the health of both pigs and humans. Multidrug-resistant Streptococcus suis is becoming increasingly prevalent, and novel strategies to treat bacterial infections caused by these organisms are desperately needed. In the present study, an untargeted metabolomics analysis showed that the significant decrease in methionine content and the methionine biosynthetic pathway were significantly affected by the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis in drug-resistant S. suis. The addition of L-methionine restored the bactericidal activity of macrolides, doxycycline, and ciprofloxacin on S. suis in vivo and in vitro. Further studies showed that the exogenous addition of methionine affects methionine metabolism by reducing S-adenosylmethionine synthetase activity and the contents of S-adenosylmethionine, S-adenosyl homocysteine, and S-ribose homocysteine. Methionine can decrease the total methylation level and methylesterase activity in multidrug resistant S. suis. The drug transport proteins and efflux pump genes were significantly downregulated in S. suis by exogenous L-methionine. Moreover, the exogenous addition of methionine can reduce the survival of S. suis by affecting oxidative stress and metal starvation in bacteria. Thus, L-methionine may influence the development of resistance in S. suis through methyl metabolism and metal starvation. This study provides a new perspective on the mitigation of drug resistance in S. suis.IMPORTANCEBacterial antibiotic resistance has become a severe threat to human and animal health. Increasing the efficacy of existing antibiotics is a promising strategy against antibiotic resistance. Here, we report that L-methionine enhances the efficacy of macrolides, doxycycline, and ciprofloxacin antibiotics in killing Streptococcus suis, including multidrug-resistant pathogens. We investigated the mechanism of action of exogenous methionine supplementation in restoring macrolides in Streptococcus suis and the role of the methionine cycle pathway on methylation levels, efflux pump genes, oxidative stress, and metal starvation in Streptococcus suis. It provides a theoretical basis for the rational use of macrolides in clinical practice and also identifies a possible target for restoring drug resistance in Streptococcus suis.

IraM remodels the RssB segmented helical linker to stabilize σs against degradation by ClpXP

Upon Mg2+ starvation, a condition often associated with virulence, enterobacteria inhibit the ClpXP-dependent proteolysis of the master transcriptional regulator, σs, via IraM, a poorly understood antiadaptor that prevents RssB-dependent loading of σs onto ClpXP. This inhibition results in σs accumulation and expression of stress resistance genes. Here, we report on the structural analysis of RssB bound to IraM, which reveals that IraM induces two folding transitions within RssB, amplified via a segmented helical linker. These conformational changes result in an open, yet inhibited RssB structure in which IraM associates with both the C-terminal and N-terminal domains of RssB and prevents binding of σs to the 4-5-5 face of the N-terminal receiver domain. This work highlights the remarkable structural plasticity of RssB and reveals how a stress-specific RssB antagonist modulates a core stress response pathway that could be leveraged to control biofilm formation, virulence, and the development of antibiotic resistance.

Resistance to Bipyridyls Mediated by the TtgABC Efflux System in Pseudomonas putida KT2440

Resistance-nodulation-division (RND) transporters are involved in antibiotic resistance and have a broad substrate specificity. However, the physiological significance of these efflux pumps is not fully understood. Here, we have investigated the role of the RND system TtgABC in resistance to metal ion chelators in the soil bacterium Pseudomonas putida KT2440. We observed that the combined action of an RND inhibitor and the chelator 2,2'-bipyridyl inhibited bacterial growth. In addition, the deletion of ttgB made the strain susceptible to 2,2'-bipyridyl and natural bipyridyl derivatives such as caerulomycin A, indicating that TtgABC is required for detoxification of compounds of the bipyridyl family. Searching for the basis of growth inhibition by bipyridyls, we found reduced adenosine triphosphate (ATP) levels in the ttgB mutant compared to the wild type. Furthermore, the expression of genes related to iron acquisition and the synthesis of the siderophore pyoverdine were reduced in the mutant compared to the wild type. Investigating the possibility that 2,2'-bipyridyl in the ttgB mutant mediates iron accumulation in cells (which would cause the upregulation of genes involved in oxidative stress via the Fenton reaction), we measured the expression of genes coding for proteins involved in intracellular iron storage and the response to oxidative stress. However, none of the genes was significantly upregulated. In a further search for a possible link between 2,2'-bipyridyl and the observed phenotypes, we considered the possibility that the ion chelator limits the intracellular availability of metabolically important metal ions. In this context, we found that the addition of copper restores the growth of the ttgB mutant and the production of pyoverdine, suggesting a relationship between copper availability and iron acquisition. Taken together, the results suggest that detoxification of metal chelating compounds of the bipyridyl family produced by other bacteria or higher ordered organisms is one of the native functions of the RND efflux pump TtgABC. Without the efflux pump, these compounds may interfere with cell ion homeostasis with adverse effects on cell metabolism, including siderophore production. Finally, our results suggest that TtgABC is involved in resistance to bile salts and deoxycholate.