plasmid

**Structure and features** - Circular, double-stranded, extrachromosomal DNA that is autonomously replicating (has its own origin of replication, ori). Linear plasmids are rare (e.g., Borrelia burgdorferi). - Not required for basic bacterial survival, but they confer selective advantages. - Can be present in multiple copies per cell. Different plasmids with the same replication machinery are incompatible and cannot coexist stably in one cell.

**Major plasmid types / gene content** - **F (fertility) plasmid:** encodes the sex pilus (tra genes) and enables conjugation. F+ (donor) x F- (recipient) transfers only the plasmid, so the recipient becomes F+. The chromosome is not transferred. - **Hfr (high-frequency recombination) cell:** the F plasmid is integrated into the chromosome. Conjugation transfers part of the chromosomal DNA, usually not the whole F plasmid, so the recipient generally stays F- but may recombine donor chromosomal genes. F' (F prime) plasmid: excised imprecisely, so it carries some chromosomal genes and transfers them at high efficiency. - **R (resistance) plasmids:** carry antibiotic resistance genes (e.g., beta-lactamases, aminoglycoside-modifying enzymes) and are often transferred by conjugation. This is a major driver of multidrug resistance, especially in gram-negative enteric bacteria. - **Virulence plasmids:** examples include the toxin genes of *Bacillus anthracis* (pXO1 encodes the toxins, pXO2 encodes the capsule), *Clostridium tetani* (tetanospasmin), and enterotoxigenic *E. coli* (heat-labile and heat-stable toxins). Others: *Yersinia* (pCD1), *Shigella* invasion plasmid, and *Borrelia*. - **Col plasmids:** encode bacteriocins (colicins) that kill competing bacteria. - **Degradative/metabolic plasmids:** e.g., *Pseudomonas* plasmids for degrading toluene and other hydrocarbons.

**Horizontal gene transfer (high-yield comparison)** - **Conjugation:** direct cell-to-cell contact through a sex pilus. Plasmid DNA moves (F, R plasmids). - **Transformation:** uptake of naked DNA from the environment (e.g., *Streptococcus pneumoniae*, *Haemophilus*, *Neisseria*; "SHiN"). - **Transduction:** bacteriophage-mediated transfer. Generalized transduction packages random bacterial DNA. Specialized transduction involves faulty excision of prophage plus adjacent genes. Phage-encoded toxins ("ABCDE'S"): Group A strep (erythrogenic toxin), Botulinum, Cholera, Diphtheria, Shiga toxin/Shiga-like toxin (EHEC), and others (e.g., *S. aureus* toxins such as TSST-1 and PVL). - **Transposons ("jumping genes")** can move between plasmid and chromosome and contribute to resistance gene spread.

**Clinical significance** - Rapid spread of antibiotic resistance in hospitals (e.g., ESBL- and carbapenemase-producing Enterobacterales, VRE via plasmid-borne vanA, MRSA's mecA on a mobile cassette, SCCmec, which is not a plasmid). - Plasmid-encoded virulence determines toxin production in several organisms. - Plasmid curing (loss of plasmid) can attenuate virulence or restore susceptibility.

**Biotechnology (also tested)** - Plasmids are the workhorse cloning vectors. A typical plasmid vector has an ori, a selectable marker (e.g., ampR), and a multiple cloning site (MCS) with restriction sites. Recombinant plasmids (e.g., human insulin gene in *E. coli*) are used to produce proteins such as insulin and growth hormone. - Blue-white screening (lacZ disruption) identifies colonies that carry an insert. - Capacity: plasmids hold about 1–10 kb inserts (compare lambda phage ~ 20 kb, cosmids ~ 40 kb, BACs ~ 100–300 kb, YACs up to ~1 Mb).

**Distinguishing points** - Plasmid ≠ episome in the strict sense: an episome is a plasmid that can integrate into the chromosome (the F plasmid can act as an episome). - Bacterial chromosome: usually a single circular chromosome essential for survival, whereas plasmids are dispensable. - Eukaryotes: plasmids are mostly a prokaryotic feature, with rare exceptions such as yeast 2-micron plasmid.