R (resistance) plasmids are extrachromosomal, self-replicating, double-stranded circular DNA elements found in bacteria that carry genes conferring resistance to one or more antimicrobial agents (and sometimes heavy metals).
Structure: an R plasmid typically has two components — the Resistance Transfer Factor (RTF), which carries genes for plasmid replication and conjugative transfer (including the tra genes and sex pilus formation in Gram-negative bacteria), and the r-determinant, which carries the actual antibiotic-resistance genes (often clustered as transposons, so multiple resistance genes can be acquired/lost together).
Significance:
R plasmids are transferable between bacteria (even across species/genera) by conjugation, transformation, or transduction, allowing rapid horizontal spread of multidrug resistance through a bacterial population — a major mechanism behind the emergence of multidrug-resistant Enterobacteriaceae.
Resistance genes commonly carried include those conferring resistance to ampicillin, chloramphenicol, tetracycline, streptomycin, and sulfonamides.
Because r-determinants often reside on transposons (“jumping genes”), they can move between plasmids and the chromosome, further amplifying the spread and persistence of resistance.
Clinically important in nosocomial infections and in the rise of extended-spectrum beta-lactamase (ESBL)-producing organisms.
