Chromosome vs plasmid resistance for PK/PD modelers
PK/PD models usually do not simulate DNA base by base. They simulate exposure and a phenotype, often an MIC. Genetics still belongs in the room, because it tells you how stable that phenotype is and how fast the local MIC ecology can shift.
This lesson is a bridge for modelers. Enough bacterial genetics to stop treating every MIC cell in a table as the same kind of object.
Genetics for people who build exposure-response simulations. Not a full genomics course.
Two homes for resistance genes
Resistance can live on the chromosome or on a plasmid (and related mobile elements). Both can raise an MIC. They do not raise it with the same epidemiological behavior.
Chromosomal changes often travel mostly with the lineage: mother cell to daughter cell. Plasmid-borne resistance can move horizontally as well, sometimes across lineages or species. Plasmid copy number can also amplify expression, so “same gene” is not always “same burden.”
| Feature | Chromosomal | Plasmid-borne, typical case |
|---|---|---|
| Main inheritance | Vertical | Vertical and often horizontal |
| Dose / copy effect | Usually limited copies | Copy number can boost expression |
| Speed of ecological change | Often slower within a hospital | Can surge when a plasmid expands |
| Modeling note | Phenotype may be more lineage-stable | MIC priors may age faster |
Why this matters for CFR
CFR needs P(MIC). If that distribution is shaped by a spreading plasmid, last year’s weights can quietly expire. Date-stamp the source and, when possible, separate mechanism strata.Same MIC number, different story
Imagine two isolates with MIC = 8 mg/L to the same drug.
In isolate A, a chromosomal target mutation reduced binding. In isolate B, a plasmid beta-lactamase is chewing the drug. A PTA grid may treat them identically if it only sees “8”. A combination with an inhibitor may help B and do little for A. Your joint-target lesson later depends on knowing which world you are in.
What to change in a PK/PD workflow
- Label MIC provenance with date, region, species, and method.
- Ask whether the mechanism is enzyme-mediated before you lean on inhibitor partners.
- Refresh local MIC weights more aggressively when plasmid epidemics are active.
- Keep genetics in the annotation layer unless you are explicitly building a genotype-to-phenotype model.
Expect more stability inside a clonal background, but still verify species ID and assay method. Target mutations may need a different therapeutic logic than enzyme blockade.
Expect mobility, possible copy-number effects, and faster change in hospital ecology. CFR priors deserve dates and sensitivity checks.
A short example conversation
Someone hands you an MIC table and asks for CFR. Before you run anything, ask three genetics-aware questions:
- Are these isolates dominated by one clone, or is the lab seeing mixed plasmid traffic?
- How old is the table, and has the hospital seen a recent enzyme-gene surge?
- If we later evaluate an inhibitor combination, does the mechanism even match that idea?
You may still proceed with a standard CFR. The difference is that your methods paragraph will say what kind of phenotype ecology you assumed, instead of pretending every MIC cell is interchangeable biology.
If the answer to those questions is “we do not know,” that is still useful. Unknown mobility is itself a reason to run sensitivity CFR under a harsher MIC prior.
Takeaway
Do not ask genetics to replace PK/PD. Ask it to annotate the MIC layer: how mobile is this phenotype, how old is the ecology prior, and which mechanism your target rule quietly assumes?
Next genetics lesson: HGT and MIC ecology · PK/PD spine: Resistance, tolerance, and persistence.