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R Factors for Aminoglycoside Antibiotics

1969; Oxford University Press; Volume: 119; Issue: 4-5 Linguagem: Inglês

10.1093/infdis/119.4-5.378

1537-6613

David H. Smith,

Salmonella and Campylobacter epidemiology

The definition of R factors by Japanese scientists 10 years ago opened a new chapter in the saga of antibiotic resistance among enteric bacteria [1]. This package of extrachromosomal genes, comprised of the resistance determinants, which mediate resistance to a variety of environmental hazards, and the resistance transfer factors (RTF), which mediate the potential to transfer between bacteria by the process of conjugation, is widespread in nature. Studies in Japan [1], Europe [2], and the United States [3] have now indicated that, although originally found primarily among Shigella, R factors are widespread throughout enteric bacteria. R factors were initially found to mediate resistance to sulfonamide, streptomycin, tetracycline, and chloramphenicol, but their genetic potential is now known to include resistance to ampicillin, kanamycin, neomycin, spectinomycin, gentamicin, heavy metals, ultraviolet light, bacterial viruses, and colicins. Furthermore, most studies, like the one shown in table 1, indicate that R factors constitute the primary basis for drug resistance among most of the enteric bacteria, with the exception of Pseudomonas. The strains indicated in table 1 were isolated from patients seen in our hospital with genitourinary tract infections; more than 507o of the resistant isolates, particularly Escherichia coli and Klebsiella, contained R factors. A resistance determinant for streptomycin resistance has been found in all types of R factors and dembnstrated in all types of enteric bacteria. The minimum inhibiting concentration (MIC) mediated by most of the streptomycin-resistance determinants depends on the host species and on the size of the inoculum. For example, the same R factor will mediate an MIC of 20 ^g/milliliter in E. coli but more than 1,000 /ug/milliliter in Salmonella or Shigella. Although strains of E. coli with a high level of streptomycin resistance, presumably chromosomal mutants, have been observed during therapy of patients in the past, all streptomycin-resistant isolates of coliform bac-