Media pH and media type can significantly affect the reliability of in vitro copper tolerance assessments of Pseudomonas syringae pv. tomato

Effects of the quorum sensing related luxS gene and lsr operon on Klebsiella michiganensis resisting copper stress

Industrial wastewater is a major environmental concern due to its high copper content, which poses significant toxicity to microbial life. Autoinducer-2 (AI-2) can participate in the inter- and intra-species communication and regulate the physiological functions of different bacterial species by producing AI-2 signal molecules. However, there are few research reports on the luxS gene and lsr operon functions for AI-2 in bacteria with a certain tolerance to copper. This study delves into the potential of quorum sensing mechanisms, particularly the AI-2 system, for enhancing microbial resistance to copper toxicity in Klebsiella michiganensis (KM). We detail the critical roles of the luxS gene in AI-2 synthesis and the lsr operon in AI-2 uptake, demonstrating their collective impact on enhancing copper resistance. Our findings show that mutations in the lsr operon, alongside the knockout of the luxS gene in KM strain (KMΔluxSΔlsr), significantly impair the strain's motility (p < 0.0001) and biofilm formation (p < 0.01), underscoring the operon's role in AI-2 transport. These genetic insights are pivotal for developing bioremediation strategies aimed at mitigating copper pollution in wastewater. By elucidating the mechanisms through which KM modulates copper resistance, this study highlights the broader ecological significance of leveraging microbial quorum sensing pathways for sustainable wastewater management.

Transcriptomic analysis reveals resistance mechanisms of Klebsiella michiganensis to copper toxicity under acidic conditions

The aim of this study was to elucidate the effect of pH on bacterial resistance mechanisms to copper (Cu) stress by genomic and transcriptomic analysis. Klebsiella michiganensis cells were exposed to 0.5 mM CuCl₂ at pH 4 and 5. Lower pH (pH < 4) strongly inhibited K. michiganensis growth, while Cu stress and higher pH (pH > 5) induced Cu precipitation in the medium. Transcriptomic analyses indicated that two groups of genes related to quorum sensing (QS) systems (lsrABCDFGKR) and type II secretion systems (T2SS) (gspCDEFGHIJKLM) were significantly up-regulated at pH 4 only. These results suggest that T2SS may be induced and controlled by QS, thereby contributing to the formation of extracellular polymeric substances (EPS) and the secretion of proteins to prevent Cu ions from entering cells. Six Cu resistance genes (cusABF, copA, cueO, and gene05308) were more significantly up-regulated at pH 4 than at pH 5. In addition, the relative expression (log₂|FC=) of the sulfur assimilation genes cysHJIK was relatively higher at pH 4 than at pH 5, while the gene encoding organic sulfur metabolism, tauB, was also significantly up-regulated at only pH 4. These results indicate that the Cu efflux system can remove intracellular Cu ions from cells, and that the sulfur assimilation system is related to the detoxification of Cu ions. Furthermore, increased free Cu ions at lower pH (4) could induce communication signals among cells, thereby stimulating the response of T2SS-related genes in K. michiganensis to tolerate Cu stress. Consequently, the resistance of K. michiganensis to Cu stress is a multisystem collaborative process composed of intracellular and extracellular components.