Introduction to antibiotic resistance

Resistance simply means that a specific antibiotic is not effective against a specific bacterium. In clinical practice, strict definitions are used to determine if bacteria growing in collected patient samples (e.g., blood or urine) are susceptible, intermediate or resistant to selected antibiotics that can be used to treat infections caused by these pathogens.

The results are used to guide the treating physician in the choice of antibiotic therapy. ”Susceptible” means that the patient is likely to respond well to therapy with this antibiotic, ”intermediate” means a high likelihood of therapeutic success under certain circumstances (e.g. with higher dosing or concentration at the infection site) and ”resistant” that is not likely that patient will be cured with this antibiotic. European countries use similar definitions for antibiotic susceptible testing (produced by The European Committee on Antimicrobial Susceptibility Testing; EUCAST) but other methods and breakpoints may be used in other countries.

For each bacterium, a panel of antibiotics will be tested to which the bacteria can be susceptible to all, some or none of the tested drugs. Resistance to multiple antibiotics that are considered standard therapy is referred to as ”multidrug resistance” and resistance to all available options as ”pandrug resistance”. Importantly, different bacterial species are naturally resistant to many antibiotics, which are therefore not considered treatment options for these pathogens in the first place. Also, keep in mind that resistant bacteria are not more likely to cause an infection and are not more virulent or aggressive than susceptible strains.

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