tration, MIC) or by the minimum concentration required to kill them within a specified period of time (minimum bactericidal concentration, MBC). Because they are easier to measure and apply to both bactericidal and bacteriostatic drugs, MICs are more frequently used. Tables of typical MICs for many bacterial species/antimicrobial pairs are widely available. When combined with knowledge of the time course of antimicrobial concentrations at various sites in the body, these MICs can be used to guide rational selection antimicrobials for particular infections. Application of this rational approach to selection is still developing and unexpected results do occur.
             Microbial resistance to antibiotics can be inherent or natural resistance. Bacteria may be inherently resistant to an antibiotic. Other microbes developed acquired resistance. This is when bacteria can develop resistance to antibiotics. So bacterial population's previously sensitive to antibiotics become resistant. This type of resistance results from changes in the bacterial genome (Stapleton). Two genetic processes in bacteria drive acquired resistance: mutation and selection (or vertical evolution), or exchange of genes between strains and species (or horizontal evolution) (Garrett 420).
             Vertical evolution is essentiality Darwinian evolution, which is driven by natural selection. So a spontaneous mutation in the bacterial chromosome imparts resistance to a member of the bacterial population. In the presence of the antibiotics then, the wild type (non-mutated) are killed and the resistant mutant is allowed to grow and flourish (Garrett 421). The mutation rate for most bacterial genes is approximately 10-8. This means that if a bacterial population doubles from 108 cells to 2 x 108 cells, and there is likely to be a mutant present for any given gene. Since bacteria grow to reach population densities far in excess of 109 cells, such a mutant could develop from a single g...