Transferable antibiotic resistanc

Abstract

Extract

Antibiotic resistance of bacteria may broadly and conveniently be classified into three major groups:

1. Inherent or natural resistance. To this group belong micro-organisms that have never responded to a particular drug, for instance, the enterobacteria with their natural resistance to penicillin.

2. Acquired resistance. This resistance can develop in a population of organisms owing to selection pressure by an antibiotic that is in contact with the population at a level inadequate to bring about the death of all the organisms involved in a microbial infection or of the micro-organisms on the skin, mucous membranes, or in the environment associated with herds or communities. The resistance of the selected bacteria may be achieved by a variety of mechanisms. For example, resistant strains of staphylococci are able to produce the enzyme penicillinase which destroys penicillin. In addition, resistant Gram-negative bacteria can produce ß-lactamase, an enzyme capable of destroying a number of antibiotics. Another way in which resistance may be acquired by bacteria is the alteration in cell wall permeability to certain antibiotics such as the sulphonamides or the tetracyclines.

3. Transferable or infectious drug resistance. Transfer of this type of drug resistance is by a process in which cytoplasmic elements composed of DNA are transferred from, a resistant to a sensitive strain of organism simply by contact and conferring on the sensitive organisms a similar resistance pattern. The contact between these cells need only be of short duration. These extra-chromosomal genetic elements are made up of resistance determinants which are transferable when they are associated with a resistance transfer factor (R.T.F.). The combination of resistance determinants and R.T.F. is called the resistance factor (R factor). The transfer of the R factor from the donor cell to the recipient is along cytoplasmic appendages or fimbriae. These R factors may be transferred from the bacteria of animals to those inhabiting man or vice versa; they may be transferred from a non-pathogenic organism to a pathogenic one or vice versa; the transferred resistance may be to a single antibiotic but most commonly several antibiotics up to eight at a time. The existence of this transferable drug resistance can be demonstrated in vivo and in vitro. A general description of the mechanism of infectious drug resistance is given by both Watanabe (1967) and Smith (1967). A more detailed account of the mechanrism and other aspects of transferable drug resistance is given by Anderson (1968). Infectious drug resistance is not only a subject of special interest to the microbial geneticist, but is of great potential importance to the clinician, and will therefore be considered more fully. Organisms apt to acquire this type of resistance usually inhabit the gastro-intestinal tract of man or animals and belong to the Shigella, Salmonella or Escherichia groups. Recently Vibrio cholera, Pasteurdla pestis, Klebsiella and Proteus spp. have also been found with this type of resistance. Infections with these oganisms usually occur in the intestinal, respiratory or urinary tracts of man and animals.