Some strains of bacteria have acquired resistance to almost all antimicrobials. Acquired resistance is also a variable phenomenon depending on the microorganisms ability to express the acquired trait. Therefore, the antimicrobial concentration becomes an important expression of susceptibility. Individual isolates of Enterobacteriaceae, Staphylococci vary in their susceptibility to a variety of antimicrobials. Some bacterial genre are noted for exceptional resistance (e.g., Pseudomonas aeruginosa)