Intrinsic resistance is a predictable, species-wide property that is not transmitted between organisms; acquired resistance is a variable property of a subset of isolates that can spread horizontally and can emerge during treatment. The same resistant report means different things depending on which one produced it.
Intrinsic resistance in practice
Klebsiella pneumoniae carries a chromosomal beta-lactamase that hydrolyzes ampicillin, so every isolate is resistant to that drug. Pseudomonas aeruginosa combines low outer-membrane permeability with efflux pumps, making it intrinsically resistant to several agents. Laboratories often omit these drugs from the panel rather than report a predictable result.
Acquired resistance in practice
A previously susceptible Escherichia coli isolate acquires a plasmid carrying an extended-spectrum beta-lactamase and becomes resistant to cephalosporins. The resistance is not shared by all E. coli, it can transfer to other organisms, and it can appear in a patient who was initially infected with a susceptible strain.
Comparing the two origins
Intrinsic resistance
- Present in essentially all isolates of the species
- Encoded in the chromosome and stable
- Not transferred between organisms
- Predictable, so often not tested or reported
Acquired resistance
- Present in a subset of isolates of a normally susceptible species
- Arises by mutation or by acquisition of a resistance gene
- Can spread horizontally on mobile genetic elements
- Must be measured for each isolate and can emerge during therapy
A resistant result is not automatically a therapeutic problem. If the drug was never active against the species, the result simply confirms what was already known. The clinically important signal is acquired resistance in a normally susceptible organism, because that is the situation the drug might otherwise have been expected to cover.