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Open Access Research article

Isolimonic acid interferes with Escherichia coli O157:H7 biofilm and TTSS in QseBC and QseA dependent fashion

Amit Vikram1, Palmy R Jesudhasan2, Suresh D Pillai12* and Bhimanagouda S Patil1*

Author affiliations

1 Vegetable and Fruit Improvement Center, Department of Horticultural Sciences, Texas A &M University, Texas, 77843-2119, USA

2 Food Safety & Environmental Microbiology Program, Texas A&M University, College Station, College Station, Texas, 77843-2472, USA

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Citation and License

BMC Microbiology 2012, 12:261  doi:10.1186/1471-2180-12-261

Published: 15 November 2012

Abstract

Background

E. coli O157:H7 (EHEC) is an important human pathogen. The antibiotic treatment of EHEC reportedly results in release of Shiga toxin and is therefore discouraged. Consequently, alternative preventive or therapeutic strategies for EHEC are required. The objective of the current study was to investigate the effect of citrus limonoids on cell-cell signaling, biofilm formation and type III secretion system in EHEC.

Results

Isolimonic acid and ichangin were the most potent inhibitors of EHEC biofilm (IC25=19.7 and 28.3 μM, respectively) and adhesion to Caco-2 cells. The qPCR analysis revealed that isolimonic acid and ichangin repressed LEE encoded genes by ≈3 to 12 fold. In addition, flhDC was repressed by the two limonoids (≈3 to 7 fold). Further studies suggested that isolimonic acid interferes with AI-3/epinephrine activated cell-cell signaling pathway. Loss of biofilm inhibitory activity of isolimonic acid in ΔqseBC mutant, which could be restored upon complementation, suggested a dependence on functional QseBC. Additionally, overexpression of qseBC in wild type EHEC abated the inhibitory effect of isolimonic acid. Furthermore, the isolimonic acid failed to differentially regulate ler in ΔqseA mutant, while plasmid borne expression of qseA in ΔqseA background restored the repressive effect of isolimonic acid.

Conclusions

Altogether, results of study seem to suggest that isolimonic acid and ichangin are potent inhibitors of EHEC biofilm and TTSS. Furthermore, isolimonic acid appears to interfere with AI-3/epinephrine pathway in QseBC and QseA dependent fashion.

Keywords:
Quorum sensing; Natural products; Limonoids; Citrus; Escherichia coli O157:H7; LEE; Epinephrine