The resistance of bacteria to antibacterial drugs: a selective review.

Bacteria may be resistant to antibacterial drugs because of some alterable property of the cell or the environment ('phenotypic resistance') or some alteration of the usual genetic properties of the cell ('genotypic resistance'). Genotypic resistance can be mediated by a mutation of the bacterial chromosome or the infection of the cell by an extrachromosomal element, e.g. an R factor. The properties of these resistances are summarized and those of the R factors of enteric bacilli are reviewed in detail. The resistance of certain bacteria to antibacterial drugs has been appreciated since these drugs were introduced into the medical armamentarium. The basis for bacterial drug resistances was not defined until somewhat later, however. Bacteria are now known to be drug resistant either because of some alterable property of the bacteria or the environment ('phenotypic resistance') or a more permanent property mediated by a chromosomal or extrachromosomal resistance gene ('genotypic resistance'). Certain aspects of each of these phenomena are summarized in this review but particular emphasis is given to the resistance mediated by the extrachromosomal resistance (R) factors of the enteric bacilli. GENETIC BASIS OF RESISTANCE Certain examples of alterable or 'phenotypic' drug resistance of bacteria are presented in Table 1. All antibacterial drugs, with the exception of the polyrnyxins, inhibit only bacteria actively synthesizing macromolecules. For example, concentrations of streptomycin (Str) that are bactericidal for actively growing bacteria have no effect on the sanie cells if their protein synthesis is stopped by chloramphenicol (Cml) or the deprivation of a required nutrient1. Although the composition and quantity of the bacterial envelope is genetically regulated, these properties can be altered by manipulations of the environment, which, in turn, may alter the susceptibility of the cell to certain