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Bacteria Diagram Labeled

Bacteria / September 6, 2016

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


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).

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
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 µm.

1 µm = 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 µm (micrometres) in length.

Cocci are usually smaller. Pneumonia-causing bacteria may be less than 0.5 µm 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.

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