Antimicrobial Activities of 2-Methyl-8-Hydroxyquinoline and Its Derivatives against Human Intestinal Bacteria

Abstract

The survival of bacterial microbiota in the human intestine is largely dependent on the equilibrium that it is able to establish with its environment. Loss of this equilibrium is associated with gastrointestinal disease in humans [Canche-Pool et al., 2008]. Humans interact with a great deal of microorganisms that are classified in two groups, beneficial and harmful intestinal bacteria. The gut microbiota contributes to the host’s immune system development, angiogenesis, fat storage, and nutrition. Thus, the balance between potentially beneficial and nonbeneficial bacteria is very important [Myllyluoma, 2007]. Clostridium perfringens, a Gram-positive, spore-forming, non-motile, anaerobic rod-shaped bacterium producing a variety of virulence factors, is one of the main causes of food-borne illnesses [Angelotti et al., 1962; Byrne et al., 2008]. In contrast, bifidobacterium and lactobacillus of the beneficial bacteria play important roles in the protection against infection, cleaning the inside of intestines, and supplying of nutrients [Kim et al., 2006b; Kim and Yi, 2008]. Recently, antibiotics such as ampicillin, erythromycin, tetracycline, and vancomycin have been used against the exogenous pathogens. Antibiotics, however, tend to disturb the health of the intestinal microorganisms [Van Den Bogaard Jr and Weidema, 2008]. Furthermore, over-applied antibiotics can cause damages, such as microorganism susceptibility, vomiting, diarrhea, and stomachache [Chen et al., 1989; Kim et al., 2006a]. Therefore, interest on the development of new effective techniques to modulate the intestinal bacteria has been growing. In our previous study, the antimicrobial activities of 8-quinolinol isolated from the roots of Sebastiania corniculata were evaluated for selectivegrowth inhibition toward the human intestinal bacteria [Kim et al., 2006b]. Therefore, we reasoned that 8quinolinol could be a lead to the selective antimicrobial agent. In this regard, the growth inhibitory effects of the 8-quinolinol derivatives including 2-methyl-8-hydroxyquinoline on the intestinal bacteria were evaluated with regard to their ability to inhibit the human intestinal bacteria. Bacterial strains used in this study were Bifidobacterium bifidum ATCC 29521, Bifidobacterium longum ATCC 15707, Clostridium difficile ATCC 9689, Clostridium perfringens ATCC 13124, Escherichia coli ATCC 11775, Lactobacillus acidophilus ATCC 4356, and Lactobacillus casei ATCC 393. Stock cultures of these strains were stored routinely on the eggerth gagnon (EG) liver extractField’s slants at −80oC, and subcultured on EG agar (Eiken Chemical, Tokyo, Japan), when required. The plates including the subculture of these strains were incubated for 2 days at 37oC in an anaerobic chamber (Hirayama, Tokyo, Japan) in an atmosphere of 80% N2, 15% CO2, and 5% H2. The bacteria were subsequently grown in the BHI broth (pH 7.6) and the MRS broth. Growth responses varied according to the tested chemicals and dose, as well as the bacterial strains. Antimicrobial activities of the test samples were used for the paper disc agar diffusion method. To determine the effects of the chemicals on the growth inhibition of the evaluated bacteria, one loopful of bacteria was suspended in 1 mL sterilized physiological saline. An aliquot (0.1 mL) of the bacterial suspension was seeded on the EG agar. The desired dose of the sample was then dissolved in 0.1 mL methanol, which was subsequently applied to a paper disc (Advantec, diameter 8 mm and thickness 1 mm, Toyo Roshi, Japan) using a Drummond glass microcapillary tube. After evaporation of the solvents, the disc was then placed on the surface of the agar plates that had been inoculated with the test bacteria. All plates were then anaerobically incubated for 2 days at 37oC. The control samples exerted no adverse effects against the tested organisms. All growth inhibition tests were conducted in triplicate, and the antimicrobial activity was determined by assigning one of the following values *Corresponding author Phone: +82-63-270-2544; Fax: +82-63-270-2550 E-mail: hoiseon@chonbuk.ac.kr