Browsing by Author "Naren N"

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    Experimental evolution of bacterial survival on metallic copper
    (John Wiley and Sons Ltd., 2022-08-22) Xu F; Liu S; Naren N; Li L; Ma LZ; Zhang X-X
    Antimicrobial copper-containing surface materials have a great potential of reducing the risks of healthcare-associated infections (HAIs), but their increased use in hospital facilities may select copper-resistant strains, causing concerns to antimicrobial resistance management. Here, we describe a long-term bacterial evolution experiment wherein a non-pathogenic Pseudomonas strain was subjected to daily transfer in laboratory media with and without copper-mediated contact killing. The copper treatment sequentially involved two surface materials differing in Cu content and thus contact killing effectiveness: first on brass (Cu 63.5%) and then on pure copper (Cu 99.9%). A gradual increase in bacterial survival rate (or a decrease of killing effectiveness) was observed over time on the related copper surfaces. For the final evolved populations after 320 transfers, 37.8% cells of the copper-evolved populations were able to survive 60 min on pure copper, whereas populations in the control lines remained sensitive with a survival rate of 0.09% under the same contact killing condition. Genome re-sequencing revealed ~540 mutations accumulated in the copper lines but only 71, on average, in the control lines (variant frequency > 0.5). The mutagenic activities of Cu+ ions were confirmed by measuring spontaneous mutation rate in a laboratory medium supplemented with copper sulfate at a non-inhibitory concentration. The copper-evolved populations have acquired increased resistance to Cu+ ions and tobramycin (an aminoglycoside antibiotic), but showed decreased production of biofilm, exoprotein, and pyoverdine. Together, our data demonstrate the potential of bacteria to evolve prolonged survival on metallic copper, and the long-term impacts should be considered with increased copper usage in hospital environments.
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    Role of a local transcription factor in governing cellular carbon/nitrogen homeostasis in Pseudomonas fluorescens
    (Oxford University Press on behalf of Nucleic Acids Research, 2021-04-06) Naren N; Zhang X-X
    Autoactivation of two-component systems (TCSs) can increase the sensitivity to signals but inherently cause a delayed response. Here, we describe a unique negative feedback mechanism enabling the global NtrB/NtrC regulator to rapidly respond to nitrogen starvation over the course of histidine utilization (hut) in Pseudomonas fluorescens. NtrBC directly activates transcription of hut genes, but overexpression will produce excess ammonium leading to NtrBC inactivation. To prevent this from occurring, the histidine-responsive repressor HutC fine-tunes ntrBC autoactivation: HutC and NtrC bind to the same operator site in the ntrBC promoter. This newly discovered low-affinity binding site shows little sequence similarity with the consensus sequence that HutC recognizes for substrate-specific induction of hut operons. A combination of genetic and transcriptomic analysis indicated that both ntrBC and hut promoter activities cannot be stably maintained in the ΔhutC background when histidine fluctuates at high concentrations. Moreover, the global carbon regulator CbrA/CbrB is involved in directly activating hut transcription while de-repressing hut translation via the CbrAB-CrcYZ-Crc/Hfq regulatory cascade. Together, our data reveal that the local transcription factor HutC plays a crucial role in governing NtrBC to maintain carbon/nitrogen homeostasis through the complex interactions between two TCSs (NtrBC and CbrAB) at the hut promoter.