What Are Bacteria and What Do They Do?


Bacteria are a vast group of unicellular microorganisms, visible only under the microscope. They populate virtually every natural environment and interact with essentially every living organism. They are actually one of the first life forms and have evolved into an incredibly wide range of microorganisms. For the most part, bacteria are either harmless or beneficial and co-exist with living organisms. However, a small number are pathogenic and produce disease in various life forms. Bacteria that are pathogenic to humans are usually classified as gram-negative and gram-positive bacteria, differentiated by shape, peculiarities of cell structure and life-sustaining activities.

What are bacteria?
By definition, bacteria denominates one of three major divisions of cellular life and refers to a large and varied group of generally microscopic microbes. Almost all forms are microscopic, meaning they are visible only under the microscope, but not with the unaided eye. There are two exceptions to this: Thiomargarita namibiensis and Epulopiscium fishelsoni. Bacteria are unicellular, or single-cell organisms, although they can organize and form multi-cellular structures. Bacteria that organize in multiple-cell structures are more efficient and resilient to antibacterial agents. Because they exhibit life-sustaining activities, they are considered a type of life form.

What is bacteria

One of their distinctive traits is their lack of a nucleus, a central element in the cell that contains the cell’s genetic material and regulates DNA replication. Bacteria also don’t have specialized cell subdivisions enclosed in separate membranes to carry out different functions. Those who do have some semblance of such subdivisions do not have them separated from the rest of the cell material (cytoplasm). In addition to a cell nucleus, bacteria also lack a subdivision called mitochondria, the powerhouse of the cell responsible for generating energy for cell processes that regulate its life-sustaining activities. They will, however, have different types of such subdivisions with similar functions. After all, bacteria too need to convert some sort of food or fuel into energy to be able to grow and multiply. They are essentially simpler, one-cell life-forms.

Bacteria types and shapes

Bacteria can be successfully classified according to their morphology into several categories.
Bacteria according to cell wall structures are commonly classified as gram negative and gram positive. The classification is obtained as a result of the Gram staining test.
1) Gram positive bacteria have a thick, outer cell wall made of what is known as peptidoglycan which surrounds the cell membrane. This thick outer layer serves a protective role. During the Gram stain test, the peptidoglycan retains a purple dye, which is confirmation of the bacteria being gram positive. Read more about the difference between gram negative and gram positive bacteria.

2) Gram negative bacteria have an outer membrane, a thin layer of peptidoglycan and a cell membrane. The outer membrane serves as protection against some antibiotics and prevents them from reaching the peptidoglycan layer and causing it do malfunction. The thin layer of peptidoglycan of these bacteria does not retain the initial purple dye applied in the first phase of the Gram stain test, only the second pink dye. This result is what allows for the classification as gram negative.

Bacteria according to shape:
1) Spherical or ovoid (coccus, cocci)
2) Rod-shaped (bacillus, bacilli)
3) Spiral-shaped (spirillum, spirilla)
4) Filamentous, elongated shapes.

Bacteria according to organizational structures. Bacteria are single-cell organisms, but have the ability to organize in structures for more efficient use of resources, better survival rates and increased resistance to antibacterials. They are known as micro-colonies.
Lesser organizational structures of bacteria can be called arrangements and include pairs, chains, bunches, clusters, threadlike filaments, helix arrangements and others.


Bacteria according to size. Microscopic forms are usually between 0.5 micrometers (half of a millionth of a meter in length) and 5 micrometers (5 millionths of a meter in length). Bacteria smaller than 0.1 to 0.3 micrometers are called ultramicrobacteria. Thiomargarita namibiensis (not microscopic) is the largest bacterium, measuring between 100-750 micrometers (or 0.1 to 0.75 millimeters). Epulopiscium fishelsoni (also not microscopic) measures between 200-700 micrometers (or 0.2 to 0.7 millimeters).

Bacteria according to metabolism (life sustaining activities) can be divided into several categories:
1) Aerobic or anaerobic or facultative.
Aerobic bacteria use oxygen to produce energy, while anaerobic bacteria do not require it. Bacteria can be obligate aerobe organisms, meaning they rely on oxygen to grow (example: Mycobacterium tuberculosis). Some are facultative anaerobe, meaning they use oxygen if available to produce energy for the cell to use, but are also capable to survive and grow without it. Obligate anaerobic bacteria are damaged by the presence of oxygen (example: all Clostridium). Microaerophile bacteria require low oxygen concentrations (up to 10% atmospheric oxygen concentration), but are damaged by higher values (example: Helicobacter pylori). Aerotolerant anaerobes do not require or use oxygen if present, but are not damaged by it (example: Streptococcus mutans).

2) Fermenting bacteria (example: Lactobacillus) rely on fermentation to produce energy for the bacterial cell and support its metabolism. They can be facultative anaerobic or microareophilic. These are the bacteria that convert sugar or starch into lactic acid.

2) Motile or non-motile. How do bacteria move? For example, Escherichia coli have a flagellum, a tail-like projection that allows them to move. Helicobacter pylori have multiple such projections that allow them to eat their way into the stomach lining. Neisseria meningitidis has shorter, hair-like protuberances that allows it to attach to cells. Myxococcus xanthus moves by propelling itself in a gliding movement. Pseudomonas aeruginosa moves by crawling over surfaces with the help of hair-like projections called pili. Klebsiella pneumoniae and others are not motile.

3) Spore forming or non-spore forming. Gram positive bacteria like Bacillus and Clostridium form spores. In simple words, spores are inactive seed-like productions that carry the genetic material of a bacterium. They do no grow or multiply and are resilient to external factors that would normally cause the active bacterium to cease to be viable. They can however produce infection once the right conditions are met, ensuring the propagation of the bacterium strain. Most gram negative bacteria do not form spores.

Bacteria types in relation to host with examples

Some bacteria thrive in the environment, others need to be in a relationship with a host to live, grow and multiply. This relationship bacteria-host is called symbiosis and is a form of relatively long-term biological partnership. According to it, the following types arise:

1) Commensal bacteria: they benefit from their host, but don’t help or harm it. Some strains of Staphylococcus aureus are commensal bacteria. They are normally part of the human skin micro-flora or present in the mucous membranes of the nose or mouth, where they thrive thanks to conditions provided by the host. However, they are not pathogenic and do not cause infection and disease. At the same time, they do not provide any benefits for their host.

2) Mutualistic bacteria: they benefit from the association with a host and, at the same time, produce beneficial outcomes for said host. Examples: probiotic human gut bacteria (also referred to as gut flora or microbiota). For the most part, they produce various metabolites that inhibit the growth of other pathogenic bacteria or fungi and contribute to normal gut flora populations in exchange for nutrients. This often results in better immunity for the host, especially at the level of the gastrointestinal tract.

For example, Lactobacillus is known for converting carbohydrates (sugars, starch) into lactic acid in the digestive tract through a process called fermentation. Other Lactobacillus species help synthesize vitamin K or B vitamins like vitamin B12 or vitamin B9 (folic acid) or inhibit Candida growth. Some mutualistic strains help prevent tooth decay.

3) Pathogenic bacteria: they infect a host and produce disease. Some infect specific tissues, some can only produce disease inside a cell, others are opportunistic and require specific conditions to cause disease (like lowered immune system defenses from other diseases). Some strains will feed off other microorganisms. Examples of bacteria pathogenic to humans and the diseases they may cause:

– Escherichia coli: diarrhea, traveler’s diarrhea, meningitis.
– Haemophilus influenzae: bacterial meningitis, pneumonia, bronchitis, infections of the upper respiratory tract.
– Helicobacter pylori: gastritis, gastric ulcers, stomach cancer.
– Mycobacterium: tuberculosis.
– Pseudomonas aeruginosa: pneumonia, endocarditis (type of heart inflammation), sepsis.

What do bacteria do?

Bacteria are living organisms and their purpose is to essentially survive, grow and multiply. In order to do so, they may need a host to get food from and support life sustaining activities (metabolism). Depending on the type of association with a host, they may produce infection and disease, benefits for the host or neither of the two.

How do bacteria make you sick?

Bacteria can make us ill in several ways. One way is by attaching to host cells and using them for their own nutrition. So basically they feed off host cells to support their own life sustaining processes (metabolism) which allows the bacteria to grow, multiply and make you sick. Some bacteria steal nutrients such as iron to support their metabolism. Others trick host cells to swallow them up. This alone affects the cell. Some move on from one cell to another. Most divide inside our cells until they rupture and release the multiplied bacteria.

When Gram negative bacteria come into contact with immune system cells, they release toxins from their outer cell wall which generate inflammation and cause symptoms such as fever, diarrhea, aches or even sepsis in severe cases. Only pathogenic bacteria make you ill and not all of them all the time. Some require very specific conditions to produce disease and are called opportunistic pathogens.

How do bacteria feed to grow and multiply?

Different bacteria have different metabolisms, meaning they use different elements to grow and multiply as well as obtain them through various processes. Bacteria may feed on organic elements such as carbohydrates, protein or fats or use water. Different species may rely on carbon, hydrogen, sulfur, iron or ammonia oxidation to produce energy for the cell and sustain metabolic processes. Some can produce carbon themselves, for example, while others ingest it from their environment. There are those that secrete enzymes that digest elements around them and turn them into food. Aerobic, anaerobic respiration, fermentation are three common processes through which bacteria produce energy to fuel themselves.

How do bacteria grow and multiply?

Once they are in the right environment, bacteria need necessary food or fuel to use for life-sustaining processes. These processes allow them to grow. Bacteria essentially multiply by creating clones of themselves, a process called fission. One bacterial cell will grow, then replicate its DNA which will then move to the opposite part of the cell. Finally, the cell becomes longer and divides, forming two identical copies or clones (binary fission). Sometimes, one cell can create several copies (multiple fission). It can take anywhere from less than 10 minutes to 30 minutes and up to more than 3 days for bacteria to multiply.