Abstract: A colorimetric assay for antibacterial susceptibility testing of clinical isolates (Escherichia coli, Pseudomonas aeruginosa, Shigella dysenteriae, Staphylococcus aureus, Bacillus cereus, and Streptococcus pneumoniae) is described based on the reduction of a novel tetrazolium salt, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS), in the presence of phenazine methosulfate (PMS) as an electron-coupling agent. The combination of 200 μg/mL MTS with 25 μmol/L PMS resulted in production of large amounts of formazan within 1 h of exposure. In this setting, fractions extracted from Chinese Masson pine (Pinus massoniana Lamb.) needles damaged by the pine caterpillar Dendrolimus punctatus Walker were found to have enhanced levels of antibacterial activity. These fractions, which were designated “Master”, “Technique”, and “Strength”, were isolated and identified by reverse-phase C18 cartridge concentration, gel filtration, and affinity chromatography. Two fractions purified from healthy and undamaged needles were designated H1 and H2, respectively. For all test bacteria species. Technique produced the lowest minimal inhibitory concentrations (MICs), ranging from 2 to 32 μg/mL, and H2 produced the highest values, with four of the six MICs being higher than 128 μg/mL. We found that the Rmax model fitted the data well in that the r2 ranged between 0.87 and 0.96 (median, 0.92) and no statistically significant deviations from the model were found (P = 0.23). The median coefficient of variation of the log RC50 values and the slope m of the fitted model for all six strains among the replicates were 38 and 41%, respectively. In the course of the investigation, the physiological and functional factors involved in pest damage to plants were also explored. In summary, the MTS-PMS colorimetric assay has advantages over existing methods for the examination of antibacterial activity, and could be developed further such that it would be suitable for screening new antibiotic molecules.（**Author for correspondence. Tel(Fax): +86 (0)10 8259 2310. Email: liugs@ ibcas.ac.cn）

Key words: antibacterial activity, clinical isolate, colorimetric assay, Masson pine needle, tetrazolium salt.