FIG. 1.

Growth inhibition caused by hydrogen peroxide (H₂O₂) in E. coli DH5α strains with or without EPA (upper panel) and in EPA-producing S. marinintestina IK-1 and its EPA-deficient mutant IK-1Δ8 (lower panel). To perform growth inhibition tests in petri dishes, 200-μl aliquots of bacterial cultures reaching an optical density at 600 nm of 1.0 were mixed with 10 ml of LB medium containing 0.7% (wt/vol) agar, 50 μg of ampicillin/ml, and 50 μg of chloramphenicol/ml for E. coli recombinants and with 10 ml of LB medium containing 0.5 M NaCl but no antibiotics for S. marinintestina strains, and then the mixtures were layered onto LB plates containing 1.5% (wt/vol) agar and the same components, with a filter paper disc (5 mm in diameter) placed in the center. Ten-microliter aliquots of solutions containing various amounts of H₂O₂ (0.3 to 110 μmol for E. coli DH5α strains and 0 to 110 μmol for S. marinintestina strains) were put on the filter disc. Plates were then incubated at 20°C for 2 days and at 15°C for 3 days for E. coli DH5α strains and S. marinintestina strains, respectively. Narrower areas of growth inhibition caused by H₂O₂ were clearly observed for cells with EPA than for those without EPA, showing that cellular production of EPA could relieve the H₂O₂-induced growth inhibition of bacterial cells. (See references 34 and 35 for details.)