Microorganism-mediated resistance refers to antimicrobial resistance that results from genetically encoded traits of the microorganism. Organism-based resistance can be divided into two subcategories, intrinsic or inherent resistance and acquired resistance.
Intrinsic Resistance
Antimicrobial resistance resulting from the normal genetic, structural, or physiologic state of a microorganism is referred to as intrinsic resistance (Table 1). Such resistance is considered a natural and consistently inherited characteristic associated with the vast majority of strains in a particular bacterial group, genus, or species. Therefore, this resistance pattern may be predictable, leading to identification of the organism. Intrinsic resistance profiles are useful for determining which antimicrobial agents should be included in the battery of drugs tested against specific types of organisms. For example, referring to the information given in Table 1, aztreonam would not be included in antibiotic batteries tested against gram-positive cocci. Similarly, vancomycin would not be routinely tested against gram-negative bacilli. intrinsic resistance profiles are also useful markers to aid the identification of certain bacteria or bacterial groups.
Table1. Examples of Intrinsic Resistance to Antibacterial Agents
Acquired Resistance
Antibiotic resistance resulting from altered cellular physiology and structure caused by changes in a microorganism’s genetic makeup is known as acquired resistance. Unlike intrinsic resistance, acquired resistance may be a trait associated with specific strains of a particular organ ism group or species. Therefore, the presence of this type of resistance in any clinical isolate is unpredictable. This unpredictability is the primary reason laboratory methods are necessary to detect resistance patterns in clinical isolates.
Because acquired resistance mechanisms are all genetically encoded, the methods for acquisition involve genetic change or exchange. Therefore, resistance may be acquired by:
• Successful genetic mutation
• Acquisition of genes from other organisms via gene transfer mechanisms
• A combination of mutational and gene transfer events
