Kingdom Monera: the structure of bacteria

The smallest and the most common

Every spoonful of garden soil holds millions of bacteria. They live in hot springs, deserts, snow, the deep ocean, the air and inside other organisms. Bacteria are the sole members of kingdom Monera. They are the most abundant of all living organisms.

A bacterial cell is prokaryotic:

• there is no nuclear membrane, so the genetic material lies free in the cell;
• there are no membrane-bound organelles, such as mitochondria;
• there is a cell wall around the cell, but it is not of cellulose.

A bacterium has one cell, much smaller than most cells of animals and plants. Many are only $1$ or $2\ \mu\text{m}$ long.

Four shapes

• Coccus (plural cocci): round, like a ball.
• Bacillus (plural bacilli): rod-shaped.
• Vibrio: comma-shaped, a curved rod.
• Spirillum (plural spirilla): spiral.

How do they reproduce?

Bacteria reproduce mostly by binary fission: one cell divides into two. In good conditions, a cell may divide every twenty minutes or so, so the numbers grow very fast: after $n$ divisions one cell has become $2^n$ cells.

In bad conditions, some bacteria make spores, which can survive for a long time. Sexual reproduction is rare and primitive: a piece of DNA passes from one bacterium to another.

Two big groups

Archaebacteria. These live in extreme places. They have a different cell wall structure, which lets them survive in harsh conditions. There are three kinds.

• Halophiles live in very salty places.
• Thermoacidophiles live in hot, acidic places such as hot springs.
• Methanogens live in marshy areas and in the gut of cattle and other ruminants. They make methane (biogas) from dung.

Eubacteria (the "true" bacteria). These have a rigid cell wall, and some have a flagellum for movement. They are the bacteria that we meet every day, in soil, in water and on us.

You should find that a salt pan at $25\%$ salt suits only the halophiles. A hot spring at $80\ ^\circ\text{C}$ and pH $2$ suits the thermoacidophiles. Take away the oxygen, as in a marsh, and the methanogens appear.

Try it in Python

This program has a list of eight bacteria with their shape and what they do. It counts the shapes.

Practise with new numbers

Common mistakes

• Mixing up vibrio and spirillum. A vibrio is a comma; a spirillum is a full spiral.
• Calling a bacterial wall cellulose. It is not of cellulose.
• Thinking all archaebacteria like heat. Halophiles like salt, and methanogens need no oxygen.
• Thinking bacteria have no DNA. They have DNA, lying free in the cell, with no nuclear membrane.

Remember these

• Bacteria are the only members of kingdom Monera, and the most abundant organisms.
• A bacterial cell has no nuclear membrane, and a cell wall that is not of cellulose.
• The shapes are coccus, bacillus, vibrio and spirillum.
• Binary fission is the common way of reproduction.
• Archaebacteria (halophiles, thermoacidophiles, methanogens) live in extreme places.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which shape is like a comma?

2. Which archaebacteria give biogas?

3. After $7$ divisions, how many bacteria come from one?

4. Why can archaebacteria live in places that kill most other life?