Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.


The Structure of bacterial cells

Use the mouse to label these diagrams of a bacterial cell.

Labels should include the following - but also read the notes which follow.

single "chromosome" - also called a nucleoid

cytoplasm (perhaps containing plasmids - circular sections of DNA independent from the main "chromosome"

surface membrane (cell membrane)

cell wall


Bacterial cell - component parts

Although bacteria are fairly simple organisms, there is much variation in the structure of the various components, and most textbook diagrams are therefore generalisations.

Which cell components listed are not likely to be found in all bacterial cells?

> flagellum/flagella,

>slime capsule

Bacterial cell walls are normally composed of protein and / or lipid (fat) substances, perhaps with carbohydrate (polysaccharide), and the slime capsule, if present, is normally a complex carbohydrate.

Which carbohydrate is unlikely to be in the cell wall, however?

> cellulose ( also starch)

Variations in these cell wall components may provide protection for the bacterium, according to their environment.

List below some "environments" in which bacteria may find themselves.

water - from rivers, lakes, sea, taps!
blood and other internal body fluids - if infected
gut - alimentary canal, especially the colon (large intestine); all of gut if infected
faeces and (stale) urine
skin (even washed skin!) of humans and animals
food, especially those composed of small particles and those processed by Man
milk, beer and other drinks

What sort of things might bacteria need protection from?

In bacterial cells, the nuclear material is not contained in a structure like a true chromosome - there is no membrane round it - it is merely one long strand or loop of nucleic acid (DNA), somewhat spread out in the cell cytoplasm.

However, there may also be structure called plasmids in the cytoplasm, which carry genes which code for characteristics like resistance to antibiotics, and which can occasionally be passed from one bacterial cell to another, even between different species.

The main nuclear material is referred to as a "chromosome" or nucleoid. In what ways does the nuclear material of a bacterium differ from the nuclear material in typical plant or animal cells?

In bacteria genetic material is single loop of DNA, not many/several (pairs of) chromosomes (plasmids - " optional extras")
loose in cytoplasm, not organised into chromosomes
not in a distinct "blob " (nucleus), confined inside nuclear membrane

In what ways does the cytoplasm of a bacterium differ from the cytoplasm in typical plant or animal cells?

There is no endoplasmic reticulum or mitochondria (but ribosomes are also smaller)

(May contain plasmids)

Some species of bacteria have flagella - single or many - at one or both ends, or all round.

What is the function of flagella to bacteria?

organs of locomotion - in order to propel the organism - avoid saying "to swim with"

In what sort of environment do flagella function?


In the space below, list the similarities (shared components) between the structure of a bacterium and a typical animal cell.

ribosomes (although smaller size)
cell membrane
glycogen found in liver & muscle cells

In the space below, list the similarities between the structure of a bacterium and a typical plant cell.

rigid cell wall (although not made of cellulose)

Shape of bacterial cells

Rod-shaped bacteria are called bacilli (bacillus in singular); spherical ones are called cocci (coccus) ; other spiral shapes are less common. Cells may stay together after division, resulting in (short) chains or groups of distinct appearance.

Cocci may be further described according to the way cells are grouped: diplococci, staphylococci, streptococci.

Helically coiled bacteria may be called spirilla (spirillum in singular), and comma shaped cells are called vibrios.

Use the mouse to show some outline diagrams illustrating these forms and groupings, with some specific examples (i.e. scientific names).

Range of shapes of bacterial cells

These names are used in classification as generic names as well as for general descriptions.

Certain bacilli (in the strictest sense,i.e. Bacillus and Clostridium spp.) may form spores. These are extremely resistant to high temperatures and chemicals, etc., which can kill ordinary bacterial cells.

Other examples of bacteria

Salmonella, Shigella, Vibrio cholerae, Mycobacterium tuberculosis, Lactobacillus spp, Agrobacterium tumefaciens, and aerobic and anaerobic bacteria in sewage treatment

bacteria -

Single bacilli
Agrobacterium tumefaciens causes crown gall disease in plants - used in gene
Lactobacillus spp
(L. bulgaricus)
used in production of yoghurt, cheese etc
Bacilli in chains
Bacillus anthracis causative organism of anthrax
Mycobacterium tuberculosis causative organism of TB (tuberculosis)
Bacilli with spores (Bacillus spp, Clostridium spp only)
Bacillus subtilis soil bacterium
Bacillus thuringensis kills bacteria - genes used in genetic engineering
Clostridium welchii causative organism of gangrene
Clostridium tetani causative organism of lockjaw (tetanus)
Flagellated forms
Escherichia coli normal inhabitant of human gut (colon)
- some enteropathogenic forms e.g. 0157
Salmonella typhi typhoid, food poisoning
Shigella spp bacillary dysentry
forms -

Diplococci (cocci in pairs)
Streptococcus pneumoniae
causative organism of pneumonia
Neisseria meningitidis causative organism of bacterial meningitis
Neisseria gonorrhoeae
causative organism of gonorrhoea
Streptococci (in chains)
Streptococcus thermophilus used in production of yoghurt
Streptococcus faecalis normal inhabitant of human gut (colon)
Streptococcus spp scarlet fever, respiratory infections
Staphylococci (in clusters)
Staphylococcus albus normal skin microflora
Staphylococcus aureus may cause boils on skin, abscesses etc
Staphylococcus spp (staphylococcal) food poisoning
Staphylococcus spp septicaemia ("blood poisoning")
Vibrio cholerae causative organism of cholera
Leptospira causative organism of Weil's disease
Treponema pallidum causative organism of syphilis
Size of bacterial cells
In general, most commonly found bacteria range between 5 and 0.5 Ám.

1 Ám = 10-6 m, or 10-3 mm.

Bacteria can only be seen using a good light microscope - and then only in outline.

The largest bacteria are bacilli - which may reach 5-8 Ám (micrometres) in length.

Cocci are usually smaller. Pneumonia-causing bacteria may be less than 0.5 Ám in diameter.

Bacteria as agents of human disease

Bacteria have many important roles in nature, but they are chiefly known for the diseases they cause, not only in Man but also in other animals and plants.

In the case of animals, digestive juices - especially hydrochloric acid in the stomach - deal with many bacteria on food. The dry skin is an effective barrier to most micro-organisms, and body secretions, e.g. tears, contain various anti-microbial compounds. If an infecting micro-organism enters the body or the blood, an attempt is made to fight off infection through the workings of the immune system - using substances called antibodies, and specialised white blood cells which destroy the foreign bacteria.

The main human disease of bacterial origin can be classified as either:

1) food poisoning - including water-borne bacteria (i.e. ingested bacteria)

2) infections - including wound infections.



a) Botulism - caused by Clostridium botulinum, normally a spore-forming soil bacterium which can grow in the absence of air (anaerobic) and which can therefore very occasionally become established in canned food. It produces a very powerful toxin (1 mg will kill 20 million mice!), which is broken down by heating (heat labile). Thus food eaten cold (e.g. infected tinned salmon, hazel-nut flavoured yoghurt), is especially at risk. A favourite of crime story writers!

b) Certain Staphylococci can cause similar but less drastic food poisoning, but have a heat stable toxin, so cooking contaminated food will kill the bacteria, but not prevent upset stomachs, etc.

c) Cholera (causative organism Vibrio cholerae), bacillary dysentery (caused by Shigella spp.) and other water-borne diarrhoea-causing bacteria can be spread by infected water. Often, these bacteria are flagellated and so can swarm on wet surfaces. This underlines the fact that water supply and sewage must be kept separate - a problem in war zones, floods and earthquake areas.

Edexcel d) Salmonella is a genus of rod-shaped bacteria which are quite common in the gut of poultry, especially those which are intensively reared, and many birds stand on their accumulated faeces, so that the bacteria spread onto their skin, and the meat (which is fibrous) is consequently often contaminated with bacteria and toxins. With the increased popularity of frozen poultry (also larger birds, and expectations of faster food preparation), there are greater chances of bacterial growth in the time spent at intermediate warm temperatures between incomplete thawing and cooking. It may be helpful to look up proper ways to defrost frozen meats to prevent contamination. It would be helpful with other items too, like frozen salmon and seafood, or even frozen vegetables that can carry bacteria.

e) Listeria and other types of bacteria may become established on cheeses (especially made from unpasteurised milk) and foods such as pate. These organisms are not usually life-threatening, but they may present problems for some groups within the population, e.g. the old, and pregnant women.

f) Escherichia coli (E. coli) is a very common species - a normal component of the bacterial population (microflora) inhabiting the colon (large intestine) of Man - and it exists in many different varieties or strains, most of which are fairly harmless, although their mere existence is the main reason for elementary hygiene precautions after visiting the toilet. Some enteropathogenic strains, e.g. 0157, are able to produce severe food poisoning, especially in vulnerable individuals. In fact, biochemically speaking, there is a very fine line between this bacterium and Salmonella and Shigella.


a) Gangrene - caused by Clostridium welchii (another anaerobic organism similar to, and in the same genus as the organism causing botulism), causes deep-seated wound infections- powerful toxins, nasty stench - frequent amputations.

b) lockjaw or tetanus - caused by a similar organism Clostridium tetani - spores are widespread, especially in manure and soils. Thus any dirty cut or scratch can result in lockjaw. Bacteria in the bloodstream produce a toxin blocking nerve action, causing involuntary contraction of muscles and spasm, eventually death as chest muscles etc used in breathing cease to function (the first muscle to react is the masseter muscle of the jaw).

Bacteria in Nature

As well as their roles in causing diseases in plants and animals (including Man), bacteria also have an important part to play in nature.

Many bacteria are free-living (i.e. not inside any living organism), and they obtain their nutrition from complex substances of plant or animal origin, e.g. dead parts of plants such as leaves & twigs, and dead animals and waste and metabolic products of live ones. These are broken down by external digestion, then their digestion products are absorbed into the bacterial cells, and may be respired to give energy.

As a result of this, some of the elements in the original material are released into the surrounding area, e.g. the soil.

This saprophytic feeding therefore has the effect of recycling plant nutrients (mainly inorganic ions - "minerals" e.g. N, P, K,) within ecosystems, e.g. woods and lakes.

Nitrogen fixation

This is the process of converting gaseous nitrogen into soluble nitrogen compounds such as nitrates (NO3-) which can be trapped in the soil, and used as fertilisers for plant growth.

This may be carried out industrially to make synthetic fertilisers, an expensive process - mostly only carried out in industrialised countries - and potentially wasteful, or "naturally", as a result of nitrogen and oxygen gases combining in the air when lightning discharges occur - only really important in the tropics.

Nitrogen is a limiting factor in many ecosystems, including agriculture, so biological nitrogen fixation is also very important:

Some free-living bacteria and algae fix nitrogen in aerobic conditions.

Other types of bacteria live inside special structures called root nodules on the root systems of leguminous plants (peas, beans, clover, etc). These are said to be symbiotic associations because both the bacteria and the plants gain an advantage; the bacteria are protected and provided with "food" - products of the plant's photosynthesis, and the plant receives nitrogen compounds which the bacteria produce by fixing atmospheric nitrogen. The nitrogen may be released into the soil for other crops when the legume is ploughed in ("green manure"), or as part of a system of "crop rotation".

Symbiosis is defined as a permanent association between two unrelated organisms, for their mutual benefit.

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