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Spontaneous mutations that produce new resistance or strengthen existing resistance occur readily in bacteria. Bacteria may acquire resistance genes through a number of routes. They may inherit the genes from their resistant forerunners. Alternatively they may acquire resistance genes through transfer via a virus or plasmid vector, or by free DNA uptake.
Source: Levy, SB. The challenge of antibiotic resistance. Sci Am. March 1998:46-53. One of the last major new classes of antibiotics, introduced in 1980, was the quinolones. Subsequent research efforts to combat the problem of resistance have focused largely on modification of existing compounds, producing new generations of current treatments. Only one new class of antibiotics has been introduced in the last 20 years, although numerous new generations have been approved.
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