Bacteria are classified in order to distinguish one organism from another and to group similar organisms together, depending on what is of use to microbiologist.

There are many ways to classify bacteria and the chart above provides a few examples of how different bacteria are classified: for example, according to shape, gram staining or oxygen requirements for growth.

Classification of bacteria has a number of applications in the food industry, such as helping to create processes that can exclude certain bacteria from growing in food and beverage products – for example, removing oxygen from packed foods to prevent the growth of aerobic spoilage and disease causing bacteria.

In the food industry, we tend to classify bacteria into 3 further groups: pathogenic, spoilage and commensal.

Gram positive or negative?

Microbiologists often use the terms gram positive or gram negative in relation to bacteria. The name comes from a Danish physician named Hans Christian Gram, who used ultra violet dye, iodine and alcohol to classify different types of bacteria.

Hans Christian Gram

Gram staining is a common technique used to differentiate two large groups of bacteria based on their different cell wall constituents. The Gram stain procedure distinguishes between Gram positive and Gram negative groups by colouring these cells red or violet, as seen in the image below. Gram positive bacteria stain violet due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet these cells are stained with. Alternatively, Gram negative bacteria stain red (or pink, below), which is attributed to a thinner peptidoglycan wall, which does not retain the crystal violet during the decolouring process.

Classification by shape

Bacteria are also classified according to their shape, with the names reflecting different shapes: for example, rod shaped bacteria are termed bacilli and contain rod like organisms, such as Salmonella, E. coli, Bacillus species.

Spiral shapes (spirilla) include Campylobacter jejuni, Vibrio species.

Spherical (cocci) include species such as Staphylococcus aureus.

There are also variations on each of these basic types.

Classification by oxygen demand

Bacteria can be further classified by their requirements for oxygen to survive and grow.

Aerobic bacteria require the presence of oxygen so that they can survive and grow – oxygen powers the metabolic activities of the cell, such as oxidising sugar and fats to generate energy.

Some, called obligate aerobes, need oxygen to survive and will die without it.

There are other types of aerobic bacteria:

Facultative aerobes: these bacteria use oxygen when it is available to them, but also have anaerobic methods of energy production.

Microaerophiles: require oxygen for energy production, but high oxygen concentrations are harmful to them. They also use fermentation for energy production.

Aerotolerant aerobes: these bacteria do not require oxygen for metabolism, but are not harmed by its presence.

Anaerobic bacteria survive in the absence of oxygen – their metabolic processes do not require oxygen, instead using fermentation which can produce secondary gaseous molecules such as methane or alcohol.

Obligate anaerobes are incapable of aerobic metabolism – some cannot tolerate oxygen at all and will die if exposed to an environment which has a high concentration of oxygen; others have a low to moderate tolerance of oxygen.

Facultative anaerobes can survive and grow in oxygenated environments, but can also metabolise in the absence of oxygen.

Microaerophilic anaerobes can only multiply in low oxygen concentrations (2-10%) or in an atmosphere with high Carbon Dioxide concentrations (>10% CO2).

Aerotolerant anaerobes can tolerate atmospheric oxygen for a limited time.