How Bacteria Grow and Reproduce

A bacterial cell usually contains one main circular chromosome, along with smaller rings of DNA called plasmids in some species. Before division, the main DNA molecule is copied so each new cell can receive a complete chromosome.

As the cell grows longer, the two DNA copies move apart. The cell membrane and cell wall then begin to grow inward between them. Eventually the original cell separates into two daughter cells.

The daughter cells are genetically very similar to the parent because they received copies of the same chromosome. Small differences can still appear through mutation, and bacteria can also gain genetic material through processes that move DNA between cells. Reproduction and genetic variation are therefore related but not identical ideas.

If every cell divides and both daughter cells survive, a bacterial population can double repeatedly. One cell becomes two, two become four, four become eight and the numbers rise quickly. This is exponential growth.

The doubling time differs between species and conditions. Some bacteria can divide rapidly in a warm, nutrient-rich environment, while others grow much more slowly. Temperature affects enzyme activity, nutrients provide raw materials and energy, and water allows chemical reactions to occur.

Rapid growth cannot continue forever. As the population becomes crowded, food is used up, waste products accumulate and space becomes limited. The growth rate slows because the environment can no longer support endless doubling. Scientists often describe bacterial growth in phases. A population may first adjust to new conditions, then grow rapidly, then level off as resources become scarce. If conditions continue to worsen, the number of living cells can fall. This pattern helps explain why growth is fast but never unlimited.

You can predict faster bacterial growth when several basic needs are available together. Changing even one condition can slow reproduction, which is why cooling food and keeping surfaces dry can reduce the growth of many unwanted bacteria.

Useful factors to check include:

• •A suitable source of nutrients.
• •Enough available water.
• •A temperature within the species' useful range.
• •Suitable oxygen conditions for that species.
• •Enough time for repeated cell divisions.

Different species prefer different conditions, so there is no single temperature or oxygen level that suits every bacterium. Some require oxygen, some grow without it and others can switch between conditions. When you study bacterial growth, always connect the environmental factor to the particular organism.

Bacteria are far too small to count one by one, so microbiologists use serial dilution. A sample is diluted in steps, often tenfold each time, and a small measured volume from a dilute stage is spread on a nutrient agar plate. Each living cell that lands on the plate grows into a visible colony over a day or a few days. Counting the colonies and multiplying by the dilution gives an estimate of the living cells in the original sample. A quicker method shines light through a liquid culture and measures how cloudy it has become, which tracks growth.

Some bacteria, including species of Bacillus and Clostridium, have a survival option when conditions turn bad. They form endospores, tough dormant structures with a protective coat and very little water inside. Endospores are not a way of reproducing, but they can withstand boiling, drying, radiation and many disinfectants, and can stay viable for very long periods before germinating when conditions improve. This is why hospitals and laboratories sterilise equipment in autoclaves using pressurised steam at around 121 degrees Celsius, a temperature that reliably destroys spores that ordinary boiling would leave alive.

Bacteria have three main ways of picking up DNA from outside. In transformation, a cell takes in loose DNA from its surroundings, often released by dead cells. In transduction, a virus that infects bacteria accidentally carries a piece of bacterial DNA from one host to the next. In conjugation, one cell forms a direct connection with another and passes across a copy of a plasmid. These routes help explain why a gene for antibiotic resistance can spread through a population, and sometimes between species, far faster than mutation alone could manage.

The first clue came in 1928, when the British scientist Frederick Griffith worked with two strains of pneumococcus bacteria in mice. A smooth, coated strain was deadly and a rough strain was harmless. Heat-killed smooth bacteria were also harmless, but mixing them with living rough bacteria killed the mice, and living smooth bacteria were recovered. Something had passed from dead cells to living ones. In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty showed that the transforming substance was DNA, an important step towards recognising DNA as genetic material.

Numbers show how quickly doubling escalates. In a warm, rich laboratory broth, the gut bacterium Escherichia coli can divide roughly every 20 minutes. Starting from one cell, that gives 8 cells after an hour, over 500 after three hours and around a billion after ten hours, since 30 doublings multiply the starting number by just over a billion. If that rate somehow continued for two full days, the bacteria would outweigh the entire Earth many times over. The calculation is deliberately absurd, and it shows why limits on food, space and waste always cut exponential growth short.

Doubling times vary enormously between species. The bacterium that causes tuberculosis, Mycobacterium tuberculosis, takes most of a day to divide even in good conditions. That slow pace has practical consequences. Laboratory cultures used to confirm an infection can take weeks to form visible colonies, and treatment has long lasted around six months, partly because slowly dividing cells are harder for many drugs to kill. Fast growers create the opposite problem in kitchens. Food left warm for a few hours can give a small number of contaminating bacteria time to multiply to levels that cause illness.

Binary fission needs a cell to split in exactly the right place, or one daughter would end up without a chromosome. Most bacteria solve this with a protein called FtsZ, a distant relative of tubulin, the protein that forms the spindle fibres in your own dividing cells. FtsZ molecules link into a ring just under the membrane at the future division site. This Z ring then recruits a team of other proteins that build new cell wall inwards, gradually pinching the cell in two like a tightening drawstring.

The name stands for filamenting temperature-sensitive, because mutant strains with a faulty version grew into long, undivided filaments when warmed, which is how the gene was found. In Escherichia coli, the ring is steered to the centre by Min proteins that oscillate from one end of the cell to the other, roughly once a minute, and block ring formation wherever their average concentration is highest, which is near the poles. A second system stops the ring forming over the chromosome. Because FtsZ is essential and differs enough from its human relative, researchers study it as a possible target for new antibiotics.

Most bacteria reproduce by binary fission, copying their DNA before one cell divides into two. Population size can rise rapidly when nutrients, water and temperature are suitable, but growth slows as resources become limited. Separate the cell division process from the environmental conditions, and bacterial growth becomes much easier to explain.