Incompatibility between R factors has been reported by several authors, and four incompatibility groups have already been described by Datta and Hedges among Rfifactors. The stability of 12 plasmids in pairs was studied after 116 crosses, and five new groups were found, designated 5, 6, 7, 8, and 9. Each plasmid studied belongs to one single group. Incompatibility between plasmids in pairs is a clear-cut phenomenon, is easy to observe, and can provide a reliable method for recognizing and classifying resistance factors, and for tracing their spread among bacterial species. Previous studies on drug resistance factors have shown that these plasmids can be classified in two groups, Rfi-and Rfi-, according to their effects on the fertility of male strains of Escherichia coli (Watanabe et al., 26). Rfi+ factors, when present in an Hfr strain, are also able to prevent the lysis of this strain by malespecific phages. This property is frequently used to distinguish Rfiand Rfifactors, and it was shown by Egawa and Hirota (9) that Rfi' factors repress the synthesis of F pili by male strains of E. coli. Bacteria carrying transmissible R factors synthetize external appendages called R pili. On the basis of morphological studies of these pili, it was suggested that Rfipili were related to F pili and Rfi-pili to I pili (i.e., pili synthetized by bacteria carrying a ColI factor). Consequently Rfifactors were also called "Flike" or "R(f)," and Rfifactors "I-like" or "R(i)." These facts have been extensively discussed by Meynell, Meynell and Datta (15) and by Novick (17). However, it was observed
Gram-negative bacilli isolated from clinical specimens submitted for culture in two Paris hospitals during 1974 were studied for susceptibility to six currently used aminoglycosides: kanamycin, neomycin, paromomycin, lividomycin, gentamicin, and tobramycin. Resistance patterns of strains of various species including those of Enterobacteriaceae, Pseudomonas, and Moraxella were determined, and the strains were grouped into eight resistance "phenotypes." In comparative studies of 807 strains belonging to different phenotypes, amikacin was markedly more active than any of the six other antibiotics; at concentrations of less than or equal to 4 mug/ml, it inhibited about 88% of the strains, including those resistant to gentamicin and tobramycin. Some amikacin-resistant strains were found among different species. The mechanism of resistance to amikacin of strains of Serratia and Moraxella group II was related to an N-acetylating enzyme. Amikacin can be expected to be useful as an alternative treatment of infections due to gram-negative bacilli sensitive to aminoglycosides and also, more particularly, for the treatment of patients infected with multiresistant strains.
Resistance to aminoglycoside antibiotics is frequently observed among enteric bacteria harbouring transferable drug-resistance factors (R factor). Streptomycin or kanamycin can be inactivated by various en-zymes1^1^but the activity of commercially available gentamicin is generally unaffected.
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