s that mediate antibiotic resistance. Genes are often present at a single copy per genome and expressed at low levels; thus resistance is to low concentrations of antibiotics. Intrinsic resistance, particularly when multiple, can however account for the success of new pathogens (Courvalin, 1996).
Acquired resistance, resulting from mutations, are due to acquisition of foreign DNA; "qualitative alterations mainly occur in the primary sequence of structural genes that direct the production of proteins that are strongly preferred targets for certain antibiotics" (p. 857). Bacteria can acquire DNA or genetic information by transduction, transformation, and conjugation (Courvalin, 1996).
Transduction may be limited to closely related species due to the high degree of specificity of adsorption in bacteriophage invasion. Transformation may be limited to intrageneric transfer; DNA fragments are transferred and stabilized in the genome of the naturally transformable transcipient by monologous recombination. This is the mechanism that resulted in the build up of mosaic genes that are responsible for penicillin resistance with a production of hybrid penicillin binding proteins. Plasmids and transposons are genetic element types that are self-transferable by conjugation (Courvalin, 1996).
Conjugative plasmids transfer among, but not between, Gram-positive or Gram-negative bacteria which belong to different genera. The host range for replication is more narrow than that for transfer; they can conjugate between two groups of bacteria, but can't establish themselves by replication into the new host. Conjugative transposons of Gram-positive cocci demonstrate a mode of transfer of antibiotic resistance genes between phylogenetically distant bacterial genera. These elements have a broad host range of transfer to include Gram-positive species and Gram-negative bacilli. This type of element can confer resistance in up to three fa...