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5-AMINOSALICYLIC ACID SELECTIVELY MODULATES BACTERIAL GENE EXPRESSION AT CONCENTRATIONS WHICH ARE NOT INHIBITORY TO GROWTH

KP Rioux, S Ness, MG Surette⁺
Department of Medicine, Division of Gastroenterology, Gastrointestinal Research Group and Department of Microbiology and Infectious Diseases⁺

BACKGROUND AND AIMS: Mesalamine (5-aminosalicylic acid, 5ASA) is widely used to treat inflammatory bowel diseases. Although not traditionally considered to have antibiotic properties, we have previously demonstrated that 5ASA alters the community of intestinal bacteria in healthy subjects, and proposed that this may be relevant to its mechanism of action in IBD. Quorum sensing refers to the ability of small molecules to potently and specifically modify bacterial gene expression and thus alter an organism's competitiveness and affect community structure. In this study, we explored the impact of 5ASA on bacterial promoter activity.
METHODS: A high-throughput screen was carried out to identify genes that responded to 5ASA at concentrations where no growth effect was observed. This screen was based on established methods used to identify transcriptional responses to sub-inhibitory concentrations of antibiotics (Goh et al PNAS 99:17025-30, 2003). Briefly, a Salmonella typhimurium strain ATCC 14028 random promoter library consisting of ~6500 clones coupled to a luciferase reporter (luxCDABE) was screened in the presence of various concentrations of aminosalicylates.
RESULTS: 5ASA induced specific transcriptional responses in a small number of clones (<2% of the library) in a concentration-dependent manner, and at doses that are not inhibitory to Salmonella growth but are relevant to intra-luminal concentrations of 5ASA in IBD patients. The effects of 5ASA on promoter activities were distinct from its parent compound sulfasalazine, or the therapeutically inactive compounds 4-ASA and sulfapyridine. The affected promoters are involved in a variety of functions including acid resistance, multi-drug resistance, and a known regulator of virulence in Salmonella.
CONCLUSIONS: In this model system, 5ASA potently affects bacterial gene regulation at concentrations that do not impact growth of the organism. The resulting changes in physiological functions of specific groups of bacteria may translate to a measurable impact of 5ASA at the population level within complex bacterial communities such as those that exist in the human intestinal tract.

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