Some words commonly used in this section:

Gram-negative/positive : refers to a staining developed by the Danish bacteriologist Gram. It is a staining of the cell wall of bacteria and is a major classification characteristic.

Rod/cocci : refers to the shape of the bacteria. Cocci are sphere-shaped, rods are rod-shaped.

Flagellum/flagella : refers to special thread-like structures, used by bacteria to move or adhere to surfaces.

Family/Genus/Species/Strain : refers to the position of the bacterium in the structure of nature. A family is a large group of related organisms (here bacteria), and is subdivided in genera (singular : genus). A genus is a group of more closely related bacteria and is subdivided in species. Species are different, but very closely related organisms. Strains refer to individual bacteria, which belong to a species. Serotypes are strains with a common immunological property.

Analogy : Dogs (family) – Canis (genus) – species : Canis familiaris (dog) and Canis lupus (wolf); the breeds of dogs are comparable to strains.

From the analogy it can be seen that strains have different characters (compare the breeds of dogs). Some are harmless, some are pathogens, some are benefical. Both for pathogenic and beneficial bacteria it is the strain that determines the effects, not the species (like some dogs are killers, others are friendly etc).