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36) Mitsuhashi, S., Harada, K. and Kameda, M.: 39) Datta, N.: J. Hyg. Camb., 60: 301-310, Nature 189: 947. 1961 1962. 40) Mitsuhashi, S., Hashimoto, H. and Suzuki, K.: J. Bact., 94: 1166-1169, 1967. Drug-resistance of Gram negative Bacteria of Clinical Origin Susumu MITSUHASHI Department of Microbiology, School of Medicine, Gunma University (Chief: Maebashi, Japan Prof. S. Mitsuhashi) Since the discovery of antibacterial agents and their widespread use, some ecological changes occured among the strains of bacteria isolated from clinical sources. S. aureus, Shigella and other gram-negative rod bacteria, E. coli, Proteus, Pyocyaneus, Klebsiella and Aerobacter groups are isolated most frequently, play a leading role in pathological lesions and contribute to the pattern of infections seen today in hospital. The fact that these bacterial strains have easily acquired multiple resistance against antibacterial agents, is mainly responsible for successful parasitism and surviving under circumstances of the widespread and enormous use of antibacterial agents. The R factors are now widespread among the strains of Shigella and other gram-negative rod bacteria, and confer drug-resistance to TC, CP, SM, SA, KM and AB-PC. The quadruply-resistant pattern, (TC, CP, SM, SA) of R factor is encountered most frequently. The genetic properties of R factor and mechanisms of R factor resistant strains are described. According to the demonstration of T factors, the origin of R factor is discussed.