Inhibitory effect of norharmane on Serratia marcescens NJ01 quorum sensing-mediated virulence factors and biofilm formation

Abstract

Serratia marcescens NJ01, a Gram-negative bacterium, can infect tomato leaves and cause chlorosis and wilting. The present study evaluated the quorum sensing (QS) and biofilm inhibitory effects of seven carboline compounds against S. marcescens NJ01 at 20 μg ml⁻¹, and subsequently focused the study on norharmane as this had the best inhibitory activity. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis confirmed the down-regulation of QS and biofilm related genes bsmA, bsmB, fimA, fimC, flhD, pigA, pigC and shlA on exposure to norharmane. Fourier-Transform Infrared Spectroscopy (FT-IR) analysis showed a reduction in the major components of the exopolysaccharide (EPS) matrix such as nucleic acids, proteins and fatty acids, which are involved in forming the tertiary structure of biofilms. Norharmane exposure also enhanced the susceptibility of the biofilm to ofloxacin. Hence, norharmane has the potential for use as an antibiotic adjuvant to enhance the efficacy of conventional antibiotics to reduce pathogenic bacterial infections.

Keywords:

• Norharmane
• carboline derivatives
• Serratia marcescens
• quorum sensing
• quorum sensing inhibitors
• anti-biofilm

Acknowledgements

The authors thank Prof. Yongyu Li for the kind gift of the S. marcescens strain (Fujian Agriculture and Forestry University).

Disclosure statement

The authors declare that they have no conflict of interest.

Additional information

Funding

This work was supported by grants from the National Natural Science Foundation of China (41766006), the National Key Research and Development Program of China (2017YFD0201400), and the Six Talent Peaks Project in Jiangsu Province.