Abstract
N-Acetylcysteine (NAC) is an antioxidant, anti-adhesive, and antimicrobial compound. Even though there is much information regarding the role of NAC as an antioxidant and anti-adhesive agent, little is known about its antimicrobial activity. In order to assess its mode of action in bacterial cells, we investigated the metabolic responses triggered by NAC at neutral pH. As a model organism, we chose the Gram-negative plant pathogen Xanthomonas citri subsp. citri (X. citri), the causal agent of citrus canker disease, due to the potential use of NAC as a sustainable molecule against phytopathogens dissemination in citrus cultivated areas. In presence of NAC, cell proliferation was affected after 4 h, but damages to the cell membrane were observed only after 24 h. Targeted metabolite profiling analysis using GC–MS/TOF unravelled that NAC seems to be metabolized by the cells affecting cysteine metabolism. Intriguingly, glutamine, a marker for nitrogen status, was not detected among the cells treated with NAC. The absence of glutamine was followed by a decrease in the levels of the majority of the proteinogenic amino acids, suggesting that the reduced availability of amino acids affect protein synthesis and consequently cell proliferation.
Highlights
Bacterial biofilms are responsible for many diseases in plants, animals and humans in which biofilm formation is an essential step for host colonization and disease development[1,2,3,4]
Our findings indicated that NAC interferes with nitrogen metabolism, reducing the availability of amino acids for protein synthesis, which might contribute to reduction in cell proliferation and activation of cell death
We verified that 8 mg/mL of NAC was able to kill X. citri cells after 24 h of growth in a population starting from 104 colony forming unit (CFU)/mL at the beginning of the treatment[17]
Summary
Bacterial biofilms are responsible for many diseases in plants, animals and humans in which biofilm formation is an essential step for host colonization and disease development[1,2,3,4]. Multiple mutants of X. citri impaired in biofilm formation consistently exhibit a decrease in bacterial growth in planta and have reduced ability to elicit canker symptoms in susceptible h ost[15]. This led us to hypothesize that compounds inhibiting biofilm formation may reduce its infection and enhance the control of citrus canker disease. It has been shown that due to the acid trait of NAC (pH < pKa) it can penetrates in the biofilm matrix and eventually kill 100% of the bacteria embedded in the biofilm[9], many authors have shown the antimicrobial effect of NAC even at neutral pH8,17,20,25,26 These results indicate that other factors might be involved with the NAC antimicrobial property. When the metabolomics multivariate data analysis (MVDA) results were integrated in a multi-informational molecular network (MN)[27], this approach highlighted metabolites that differed significantly with NAC treatment
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