Inside the bacterial cell

Conjugation is the transfer of DNA from one bacterium to another through direct contact, and the structure that makes the first contact is the sex pilus, a thin protein tube built by the donor cell. The classical donor carries a conjugative plasmid such as the F plasmid of Escherichia coli. The sequence runs as follows. First, the donor builds the pilus and uses it to attach to a recipient that lacks the plasmid; the pilus then retracts and draws the two cells together until their surfaces touch. Second, the donor nicks one strand of its plasmid DNA at a defined site called the origin of transfer, and a protein complex called the relaxosome separates the nicked strand while the relaxase enzyme remains bound to its end. Third, the free single strand is threaded through a channel into the recipient, while the strand left behind in the donor is used as a template to rebuild a complete double-stranded plasmid, a process called rolling-circle replication. Fourth, the transferred strand closes into a circle in the recipient and is copied into double-stranded DNA, so the recipient becomes a donor in its own right; it is now called a transconjugant. The original donor has not lost the plasmid — it made a copy — which is why conjugation spreads a gene through a population instead of diluting it. Some plasmids can even move between species, so a resistance gene first seen in one organism can appear months later in another. In hospital practice this is the mechanism behind the quiet spread of extended-spectrum beta-lactamases and carbapenemases among Gram-negative bacteria. Nursing actions act on the same steps: hand hygiene before and after contact, gloves and gowns, single-use equipment, cleaning of shared surfaces, isolating or cohorting colonised patients, and careful antimicrobial stewardship all reduce the chance that a donor and a recipient ever meet.

The difference between the chromosome and a plasmid is the difference between what a cell must have and what it merely finds useful. The chromosome is the primary replicon, the unit of DNA with its own origin of replication; it carries the housekeeping genes for growth, energy and division, and the cell cannot lose it and survive. A plasmid is a secondary replicon with its own origin, but its genes are accessory: resistance, virulence factors, or enzymes for unusual nutrients. That status explains why losing a plasmid usually costs the bacterium nothing. In the absence of the antibiotic or the special nutrient that the plasmid genes address, the plasmid is a small metabolic burden — its DNA must be copied and its proteins made — so a plasmid-free daughter cell can actually grow slightly faster and, over many generations, outgrow the plasmid-carrying line. This is why resistance can fall when antibiotic use falls, although it does not always disappear: plasmids can carry toxin–antitoxin addiction systems that kill any daughter cell that loses them, and resistance genes can be rescued by transposons that jump into the chromosome. Plasmids are not lost at random either. A low-copy plasmid such as F is partitioned actively by a par system that ties copies to the dividing cell, whereas high-copy plasmids rely partly on chance; even so, a curing event occurs in perhaps one cell in a thousand to one in a hundred thousand divisions. In the laboratory, plasmids are deliberately removed with curing agents such as acridine orange or ethidium bromide, or by growth at a hostile temperature, which is how researchers prove that a trait is plasmid-borne. Two plasmids that share the same replication and partitioning system are incompatible and cannot be maintained together stably, which is why plasmid families are classified by incompatibility groups. For nursing, the practical lesson is that a resistance plasmid is not permanent furniture: reducing unnecessary antibiotic use removes the advantage of carrying it, but contact precautions remain necessary because the plasmid can return the moment selection pressure returns.

Mesosomes deserve their own paragraph because they show how a textbook fact can turn out to be a preparation artefact. In the 1960s, electron micrographs of chemically fixed bacteria showed membrane whorls folded deep into the cytoplasm, and these were given the name mesosome and the status of organelles, with roles in division and respiration. The problem is the preparation. Fixatives such as osmium tetroxide and glutaraldehyde, followed by dehydration, remove water from a delicate membrane and can collapse it into artificial spirals; when bacteria are instead frozen very rapidly and examined by cryo-electron microscopy, the cytoplasmic membrane is smooth. Some local membrane invaginations are real, and the membrane certainly carries the respiratory chain, but the mesosome as a permanent organelle comparable to a mitochondrion is not supported by the evidence. Two solid facts replace the disputed one. The first is that a prokaryote has no mitochondrion: the electron transport chain and ATP synthase sit in the cytoplasmic membrane itself, and the cell makes its ATP there by chemiosmosis, using a gradient of protons across the membrane to drive the enzyme. The second is that this exposed membrane is exactly what polymyxins attack. Polymyxin B and colistin are cationic peptides, positively charged, that bind the negatively charged lipopolysaccharide of the Gram-negative outer membrane, displace the magnesium and calcium ions that stabilise it, and insert into the membrane, making it leaky; the cell loses its contents and dies. Colistin is a last-resort drug for carbapenem-resistant Acinetobacter baumannii and Pseudomonas aeruginosa, and it is toxic to human cells as well: nephrotoxicity and neurotoxicity are its classic dose-limiting effects, so nurses monitor urine output and creatinine, ask about numbness, dizziness or weakness, and respect the prescribed interval. Resistance can arise when the bacterium alters the lipid A part of lipopolysaccharide, and the plasmid-borne mcr-1 gene does exactly that — a reminder that the plasmid story and the membrane story meet.

Bacteria and human cells both make protein, so a drug that shuts down protein production ought to poison both. Antibiotics that attack the ribosome avoid this because the two machines are built from different parts. The bacterial ribosome is a 70S particle formed from a 30S and a 50S subunit; the human ribosome is an 80S particle formed from a 40S and a 60S subunit. The difference lies mostly in the ribosomal RNA and in a few proteins, and it is enough for a drug to bind the bacterial machine tightly and the human one weakly. The aminoglycosides — gentamicin, tobramycin, amikacin, streptomycin — bind the 30S subunit and distort the site where the message is read, so the ribosome misreads the instructions and inserts the wrong amino acid; the abnormal proteins disrupt the membrane, which is why aminoglycosides are bactericidal, meaning they kill rather than merely stop growth. The tetracyclines — tetracycline, doxycycline, oxytetracycline — also bind the 30S subunit, but they block the entry of the next transfer RNA, so the chain simply stops; these drugs are bacteriostatic, meaning they hold the organism in check and depend on the patient’s defences to clear it. Selectivity is not absolute, and that explains the side effects. Aminoglycosides can damage the kidney and the inner ear, causing hearing loss or balance disturbance, particularly when blood levels run too high or treatment is prolonged; nurses monitor trough levels, urine output and creatinine, and ask about ringing in the ears, dizziness or unsteadiness. Tetracyclines bind calcium and are deposited in growing teeth and bones, so they are avoided in pregnancy and in children under about eight years, and they can stain developing teeth yellow-brown. Because the ribosome is essential and ancient, resistance appears here too: bacteria can make an enzyme that modifies the drug, pump the drug out, or alter the ribosomal target itself. A nurse who understands the target can therefore predict what a drug will do, which laboratory result matters, and which harm to watch for.