The bacterial cell: what makes it different

A bacterium and one of your cells are both alive, both wrapped in membrane, both running on DNA and protein. Past that, they are built on different plans. The bacterium keeps its chromosome loose in the cytoplasm with no nucleus around it, carries no mitochondria, and translates protein on a smaller ribosome.

Those are not curiosities. Every difference in this table is a place where a drug can hit the microbe and miss you.

Bacteria are small because small cells trade with their surroundings faster. Everything a cell eats crosses its surface, and everything it must supply lives in its volume. For a sphere, that ratio is

$\frac{S}{V} = \frac{4\pi r^{2}}{\tfrac{4}{3}\pi r^{3}} = \frac{3}{r}$

Surface per unit volume rises as the radius falls. Halve the radius and you double the surface available to each unit of interior. A one micrometre bacterium therefore has roughly ten times the relative surface of a ten micrometre human cell.

That is the engine behind everything in the next lesson. A cell that absorbs nutrients this efficiently, and has no nucleus to disassemble before dividing, can copy itself in twenty minutes.

Outside its membrane, a bacterium wears a mesh called peptidoglycan: sugar chains cross-linked by short peptides into one enormous molecule that wraps the whole cell. It is what stops the cell bursting when water rushes in.

How much of it a species builds splits the bacterial world in two, and a stain from 1884 still reports which side a cell is on.

• Gram-positive: thick peptidoglycan layer on the outside, no outer membrane.
• Gram-negative: thin peptidoglycan layer sandwiched under a second, outer membrane.

Key idea: Human cells have no peptidoglycan and no enzymes that build it. A drug that breaks that chemistry has nothing to damage in you. This is the cleanest example of selective toxicity in all of medicine, and it is why penicillin was such a startling drug.

The stain is a sequence of four reagents. The alcohol wash is the step that decides: a thick wall holds the trapped dye complex, a thin wall lets it escape and takes the pink counterstain instead.

A bacterial genome is usually a single circular chromosome, coiled into a region called the nucleoid with no membrane around it. It is compact and densely coding, with little of the non-coding sequence that fills animal genomes.

• Escherichia coli K-12 carries about 4.6 million base pairs and roughly 4,300 protein-coding genes (NCBI Genome).
• Your genome holds about 3.1 billion base pairs and roughly 20,000 protein-coding genes.
• So the bacterium packs a comparable gene count into a genome about 670 times smaller.

Alongside the chromosome sit plasmids: small independent DNA circles carrying accessory genes. A cell can gain one, lose one, or hand a copy to a neighbour without touching its chromosome. Hold on to that, because it is how resistance travels.

Both kinds of cell translate messenger RNA into protein, on machines built to different specifications. The bacterial ribosome is 70S, assembled from a 30S and a 50S subunit. Yours is 80S, from 40S and 60S. The shapes differ enough that a molecule can jam one and ignore the other, which is why a whole family of antibiotics targets translation.

But the boundary has a leak in it.

Without a nucleus to dismantle or a spindle to build, bacterial division is a much shorter procedure than mitosis:

1. Replication starts at a single fixed point on the circular chromosome, the origin, and runs in both directions.
2. The two finished copies are pulled toward opposite ends of the cell as it elongates.
3. A protein ring assembles at midcell and pulls inward, building new membrane and wall as it closes.
4. The septum completes and two cells separate, each with one chromosome.

Gotcha: Fast-growing bacteria start a new round of replication before the previous one has finished, so a single cell can hold several partly-copied chromosomes at once. That is how a division cycle can be shorter than the time it takes to copy the genome.

For most of the twentieth century life was sorted into bacteria and everything else. In 1977 Carl Woese and George Fox compared ribosomal RNA sequences across organisms and found that a group then filed under bacteria was as distant from them as it was from us. They named it the archaea.

Gotcha: Archaea look like bacteria under a microscope and are routinely called them in casual writing. They are a separate domain, and because they build no peptidoglycan, antibiotics such as penicillin do nothing to them.

In 1998 Whitman, Coleman and Wiebe added up prokaryotic cells habitat by habitat in PNAS and put the global total at 4 to 6 x 10^30 cells. Most of them are nowhere near a host or a hospital.

The same paper estimated that carbon held in prokaryotes equals 60 to 100 percent of the carbon held in all plants. The subsurface entries dominate, and they are also the least sampled, which is the honest reason these figures are given as ranges rather than a single number.

Every structure in this lesson reappears as a target or a loophole.

The peptidoglycan wall is what beta-lactams break. The 70S ribosome is what a second drug family jams. The folate pathway, which bacteria run themselves because they cannot import the vitamin the way you do, is what a third family starves. The plasmid is how a cell acquires the gene that defeats any of them.

And the speed, the twenty minute division that small size and a simple genome make possible, is why a population can explore enough mutations to find that defence in a single patient, in a single course of treatment. The next lesson makes that speed concrete.