Antimicrobial resistance (AMR) is the ability of  microorganisms (from viruses, chlamydia, bacteria and  fungi through to protozoa and even larger organisms such as parasitic worms)  to resist the effects of   antimicrobial agents.  Sometimes such resistance is “innate” (e.g. Stenotrophomonas maltophilia to carbapenem antibiotics), but often  it is acquired either through mutations or acquisition of various transferable genes (e.g. via plasmids, transposons and integrons). When microbes are resistant to an antimicrobial they are able to withstand action of that agent.  A common misunderstanding is that  resistance refers to the patient (rather than the microorganism). 

Antimicrobial agents include “antibiotics”  which strictly speaking are substances produced naturally by microorganisms and are active against other microorganisms, as opposed to substances being synthesised by man,  e.g. trimethoprim.  Fleming’s original discovery of an antibiotic was an observation of the production of what he called penicillin by a fungus which had activity against bacteria e.g. Staphylococcus aureus. Antimicrobial agents active against bacteria are called antibacterial agents. Other antimicrobials include  antifungals, antivirals, and antimalarials. 

Antimicrobial resistance is a problem of increasing magnitude. As standard treatments are becoming ineffective, there is a threat of a return to the pre-antibiotic era, when infections could not be treated and were associated with higher morbidity and mortality. Moreover antimicrobial resistance is a public health threat, as it spreads through transmission of the resistant microorganisms  to other patients and through transfer of the resistant genes to other microorganisms.

The evolution of resistant strains is a natural phenomenon that happens when microorganisms are exposed to antimicrobial drugs, and resistant traits can be exchanged between certain types of bacteria. The overuse and misuse of antimicrobials accelerates this natural phenomenon through increasing selection pressure. Poor infection prevention and control practices encourages the spread of resistant organisms. Typing techniques are needed to explore the epidemiology of dissemination as they are able to identify resistant clones and can also plot how such clones  further evolve.

As mentioned above, antimicrobial resistance in bacteria may occur as a result of natural evolutionary changes when inherent characteristics or natural mutations are passed on through vertical gene transfer, or as a result of acquired resistance mechanism, via horizontal gene transfer:

  • Intrinsic resistance is the innate ability of a bacterial species to resist activity of a particular antimicrobial agent through its inherent structural or functional characteristics, which allow tolerance of a particular drug or antimicrobial class.   This can also be called “insensitivity” since it occurs in organisms that have never been susceptible to that particular drug.

Such natural insensitivity can be due to:

  • lack of affinity of the drug for the bacterial target
  • inaccessibility of the drug into the bacterial cell
  • extrusion of the drug by chromosomally encoded active exporters
  • innate production of enzymes that inactivate the drug

Acquired resistance occurs when a susceptible microorganism becomes resistant through a number of mechanisms, including mutations and transfer of resistance genes from another microorganism.



Link to European IC/HH Core Competencies: Area 4. Infection control activities