Genes can encode proteins or ribosomal RNA that enables bacteria to evade the actions of antibiotics. Such antibiotic resistance may either be intrinsic or acquired (Box 2).3 Intrinsic resistance is associated with the “usual” chromosomal genes or determinants of the organism's characteristics. Thus, an organism can be resistant to a specific class of antibiotics by virtue of its inherent genetic makeup. This form of resistance is predictable, which makes antibiotic selection straightforward. For example, all streptococci are intrinsically resistant to aminoglycosides (e.g., gentamicin and tobramycin), and all gram-negative bacilli are intrinsically resistant to vancomycin.

Figure 2: Figure 2: Effect of selective antibiotic pressure in bacteria.. Image by: Lianne Friesen and Nicholas Woolridge

In the past decade, several antibiotic-resistant pathogens have been identified as causes of serious infections among patients in hospital (Box 1). These organisms are typically resistant to multiple classes of antimicrobial agents and are therefore called multidrug-resistant organisms. Clostridium difficile, the major cause of diarrhea associated with antibiotic use, may also be considered a nosocomial antibiotic-resistant organism. In this article, we review important mechanisms of antibiotic resistance and discuss the clinical impact of key multidrug-resistant bacteria in hospital settings.

Figure 1: Figure 1: Sites of action and potential mechanisms of bacterial resistance to antimicrobial agents. Modified with permission from the American Association for the Advancement of Science (Science 1992;257:1064–73).3 . Image by: Lianne Friesen and Nicholas Woolridge