Pseudomonas aeruginosa is a leading cause of nosocomial bloodstream infections, ranking third among gram-negative bacteria, after Escherichia coli and Klebsiella species. Intrinsic resistance has been traditionally attributed to the low permeability of cellular envelopes together with the presence of chromosomally-encoded detoxification systems such as multidrug efflux pumps or antibiotic inactivating enzymes. Some strains acquire increased resistance via the horizontal transfer of resistance determinants and mutations in chromosomal genes.
