Physical Characteristics of Bacteria

A bacteria is a tiny, single-celled prokaryote microorganism. They have many shapes and external features, and are usually only a few micrometers long. Bacteria live almost everywhere on earth, from water to radioactive waste, and are some of the most persistent life forms known to exist.

Bacterial cells are surrounded with a membrane, or lipid membrane, that acts as a skin. It keeps proteins, nutrients and cytoplasm inside the cell where it belongs. Unlike eukaryotes, bacteria tend not to have a true nucleus, chloroplasts or even mitochondria.

Instead they have fewer, larger cellular organelles. Some bacteria have a prokaryotic cytoskeleton, a structure that arranges the bacteria’s organelles into a more organized way.

Many bacteria use their external cellular membrane as a means of generating energy. Concentration gradients run the length of the skin, and act as an inbuilt conductor for potential difference, such as that found in a battery.

Bacteria generally do not have internal membranes and so consequently electrons can travel across the cell membrane. In photosynthetic bacteria, the cell membrane is folded and covered in a special light-collecting membrane.

Bacterial cell walls are made of peptodoglycan, or murein. There are two kinds of cell walls in bacteria, and they are Gram-positive and Gram-negative. This refers to the reaction of cells to a Gram stain test. Positive bacteria have a thick cell wall, and negative bacteria have a thin one. Most bacteria are negative, and therefore less resistant to antibiotics.

Some kinds of Gram-positive bacteria, such as Bacillus or Clostridium, can form endospore structures, which are a kind of highly insulated hibernation structures. By forming an endospore, a bacteria can survive with no food or water for a very long period of time until a suitable host comes along.

Endospores can survive gamma radiation, ultraviolet light, heat, freezing, desiccation, and even hard vacuum. A bacteria that has gone dormant in this way could theoretically survive for millions of years unharmed.

Bacteria move by moving their flagella. A flagella is a whip-like structure attached to the bacteria at one end, which lets the bacteria move by undulating the flagella and “swimming”. They are tiny, barely more than two to twenty nanometers, and resemble hairs. Some bacteria also have a capsule or slime layer on their surfaces that help to protect them from being eaten by other bacteria.

They can also be involved in cell recognition. How these parts are assembled on an individual bacteria depends on its bacterial secretion systems, which also influence the effectiveness of pathogens: that is, how effective an individual bacteria is at eating its host while avoiding being killed by the immune system.