Types of Bacteria

Bacteria are virtually omnipresent in our environment and play a vital role in everything from digestion to decomposition. Our bodies are filled with and surrounded by bacteria at all times. Many bacteria are helpful to other living organisms, while some can be harmful or even deadly. There are different ways to categorize bacteria and their colonies into "types of bacteria", based on their shape and composition, as well as the diseases they may cause. 

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Table of contents
    • Types of bacteria
    • Bacteria colonies
    • Types of bacterial infection
    • Types of bacteria in food
    • Types of food poisoning due to bacteria

    Different types of bacteria

    Bacteria can be classified into four separate types according to their shape, though there can be considerable variation within these shape classes and there are some bacteria that do not conform to any of these four types. The four primary types of bacterial shapes are:

    • Bacilli (rods)

    • Cocci (spherical)

    • Spirilla (spirals)

    • Vibrio (comma-shaped)

    Cocci (spheres)

    Cocci bacteria are any species that has a round or spherical shape.

    Cocci bacteria are typically arranged either individually, in chains, or in clusters. While some cocci bacteria are pathogens, some are also harmless or beneficial. The word “cocci” is derived from the Greek word for “berry”, coccos. Cocci can be both Gram-positive or Gram-negative.

    Cocci classificationExampleDescription
    Diplococcus (paired cocci)Neisseria gonorrhoeaeA Gram-negative species which can cause the sexually transmitted genitourinary infection gonorrhoea
    Streptococcus (chained cocci)Streptococcus pyogenesGram-positive species which can cause group A streptococcus (GAS) infections
    Tetrad (cocci present in four squares)Microccocus antarticusGram-positive psychrophile species which lives in the extreme cold temperatures of Antarctica
    Sarcina (cocci present in eight cubes)PeptostreptococcusGram-positive genus which can cause fatal endocarditis, paravalvular abscesses, and pericarditis
    Staphylococcus (irregularly arranged cocci)Staphylococcus aureusGram-positive species, which can cause severe infections in humans, including methicillin-resistant S. aureus (MRSA).

    Table 1. Examples of cocci bacteria

    Bacilli (rods)

    Bacilli are bacteria species that are shaped like a rod. Bacilli can be both Gram-positive or Gram-negative.

    Bacilli classificationExampleDescription
    Bacillus (individual bacillus)Escherichia coliGram-negative species which can cause severe gastrointestinal illness in humans
    Streptobacillus (chained bacilli)Streptobacillus moniliformisGram-negative species which causes Haverhill Fever, a type of rat-bite fever
    Coccobacillus (oval bacilli)Chlamydia trachomatisGram-negative species which causes the sexually transmitted disease chlamydia

    Table 2. Examples of bacilli bacteria shapes

    Bacilli can also appear grouped together as pairs (diplobacilli) or as a fence-like structure (palisades).

    Spirilla (spirals)

    Spirilla are spiral- or helical-shaped bacteria species, which are stereotypically Gram-negative. These bacteria typically have flagella, which are long structures used for motility.

    Spirilla classificationExampleDescription
    Vibrio (comma-shaped)Vibrio choleraeGram-negative species which causes the potentially fatal gastrointestinal disease cholera in humans
    Spirillum (spiral-shaped and thick) - flagella are externalHelicobacter pyloriGram-negative species that can cause peptic ulcer disease in humans
    Spirochete (spiral-shaped and thin) - flagella are internalTreponema pallidumGram-negative species which can cause syphilis

    Table 3. Examples of spirilla bacteria shapes

    Some other bacteria can have shapes that do not conform to the above types of shapes, such as pleomorphic, spindles, squares, and stars.

    Types of Bacterial Colonies

    Bacterial colonies are classified by their morphology, which includes the bacteria’s elevation, form, and margin. The form of these colonies can be classified as:

    • circular,
    • filamentous,
    • irregular, or
    • rhizoid.

    These different morphologies allow bacteria to adapt to and survive the external and internal conditions they may encounter. Bacterial morphology contributes to its survival rate against "primary" and "secondary" selective pressures.

    Selective pressures are external factors that condition the capacity of an organism to survive in a given environment.

    There are generally considered to be three "primary" selective pressures and four "secondary" selective pressures. The "primary" selective pressures include:

    1. The ability to obtain nutrients
    2. Cellular division
    3. Predation.

    The "secondary" selective pressures include:

    1. Surface attachment
    2. Dispersion
    3. Motility
    4. Differentiation.

    Bacterial colonies are also classified by elevation. Bacterial colonies can be:

    • raised,
    • crateriform,
    • convex,
    • flat, and
    • umbonate.

    Lastly, bacterial colonies are also classified by their margin, which can be:

    • curled,
    • entire,
    • filiform,
    • lobate, or
    • undulate.

    Types of Bacterial Infection

    There are numerous different types of bacterial infection, depending on the type of bacteria involved and the location of the infection. Unlike viral infections, bacterial infections involve living organisms (bacteria are alive, while viruses are not) and are usually treated with antibiotics.

    Some common examples of bacterial infections include many forms of gastroenteritis/food poisoning, abscesses, urinary tract infections, mycobacterial infections, and strep throat.

    In the following sections, we will go over several bacterial species and the illnesses that may result from being infected with them.

    Types of food poisoning bacteria

    Food poisoning happens when a person eats food contaminated with microorganisms, many of which can be bacteria. There are many different strains of bacteria that can cause food poisoning. Although the symptoms can be quite dramatic (diarrhoea, nausea, stomach pain or cramps, vomiting), food poisoning is usually not very serious and passes on its own. However, the sick person should make sure to stay hydrated and replenish enough nutrients and minerals while going through the illness.

    Escherichia coli

    While you may associate its name exclusively with food poisoning, most strains of Escherichia coli are actually harmless and already live inside humans and other mammals. The few strains that are pathogenic can produce the typical symptoms of food-borne illness: abdominal cramping and diarrhoea.

    E. coli is the most common cause of traveller's diarrhoea and is typically acquired through contaminated food or drink. In severe cases, E. coli can cause colitis and bloody diarrhoea. While E. coli infections are usually self-limiting, sometimes antibiotics are used in order to shorten the duration of the illness.

    Helicobacter pylori

    Helicobacter pylori is a stomach-inhabiting bacteria species that can cause gastritis, duodenitis, and ulcers in some infected individuals. It is important to note that the vast majority of those infected with H. pylori will not develop the disease, and roughly 50% of the human population (mostly within the developing world) is believed to be infected with the bacteria. When the organism does cause disease, symptoms may include heartburn, tarry faeces, nausea, vomiting, and pain. The disease can eventually progress to gastric cancer or even perforation into the abdominal cavity.

    Prior to the discovery of H. pylori in the 1980s, it was believed that these gastric ulcers were primarily caused by stress and an acidic diet. Initially, there was much resistance in the medical community to the idea that bacteria could cause ulcers, as it went against the traditional views of the time. In order to prove the ability for H. pylori to cause disease, Australian doctor Barry Marshall ingested a broth containing the bacteria, quickly developed symptomatic gastritis, and cured himself with an antibiotic cocktail.

    Vibrio cholerae

    Vibrio cholerae is the causative agent in cholera, a gastrointestinal disease that is currently only known to occur in humans. Infection with V. cholerae causes severe, life-threatening diarrheal illness in about 10% of those infected while the remainder will experience only mild diarrhoea or a lack of symptoms entirely. The most common feature distinguishing cholera from other common diarrheal diseases is the "rice water" appearance of diarrhoea produced by the infected individual. This is in contrast to other bacterial diseases, such as dysentery, which may produce bloody diarrhoea.

    V. cholerae is a highly contagious species that is commonly spread through contaminated food or water. This has resulted in devastating outbreaks throughout history, such as the deadly outbreak that took place in Haiti following the 2010 earthquake. While antibiotics may shorten the duration of the illness, supportive rehydration therapy is usually the most effective treatment until the self-limiting infection passes.

    Some other bacteria that cause food poisoning are Salmonella, transmitted by the fecal-oral route (including the consumption of contaminated food and water and by direct animal contact) and Clostridium botulinum. C botulinum causes botulism, which is currently a farely rare but serious infection. Botulism is caused by the toxin released by C botulinum which affects the nerves and causes paralysis of the muscles, including those used to breathe. Therefore, botulism can be deadly.

    Types of bacterial pneumonia

    Pneumonia involves inflammation of the lungs and may be caused by bacteria, viruses, fungi, or other conditions. Symptoms usually consist of coughing, breathing difficulties, and chest pain, but may also include more generalized symptoms such as fever, nausea, and vomiting.

    Bacterial pneumonia is caused by a variety of different species of bacteria, most commonly S. pneumoniae and Klebsiella pneumoniae. Bacterial pneumonia can be placed into four types:

    • community-acquired,
    • healthcare-associated,
    • hospital-acquired, and
    • ventilator-associated.
    Type of pneumoniaDescription
    Community-acquired pneumonia (CAP)CAP is bacterial pneumonia that is acquired within the individual's community and not within a hospital or healthcare setting.
    Healthcare-associated pneumonia (HCAP)HCAP is bacterial pneumonia that is acquired in locations such as retirement communities, nursing homes and outpatient facilities.
    Hospital-acquired pneumonia (HAP)HAP is bacterial pneumonia that is acquired in a hospital setting, except in situations where the patient has been intubated.
    Ventilator-associated pneumonia (VAP)VAP is bacterial pneumonia that is acquired while the patient is intubated.
    Table 4. Bacterial pneumonia classification

    Types of bacteria in urine

    Urinary tract infections (UTIs) are infections that may involve any part of the urinary tract and typically involve symptoms such as increased urination, increased urinary urgency even when the bladder is empty, painful urination, and, in some cases, fever.

    UTIs occur when bacteria enter the urinary tract, which can occur in a number of ways, including through sexual intercourse and poor hygiene. For anatomical reasons, females are at a greater risk of developing UTIs than males. The bacteria that is usually associated with UTIs is E. coli (around 80% of cases), though some other bacteria species and even fungi may occasionally be involved.

    Types of Bacteria Urinary tract infection symptoms. Study SmarterFig.1 Symptoms of a urinary tract infection

    Types of bacteria in food

    Bacteria in food are not always damaging to the humans that consume it. In fact, they can be extremely beneficial, helping restore and keep a healthy microbiota (gut flora) and digest difficult foods, among the most obvious functions.

    There are many damaging food bacteria, as we mentioned above, like Salmonella, Vibrio cholerae, Clostridium botulinum and Escherichia coli, among others. However, there are two main types of beneficial gut bacteria that you have probably heard of: Lactobacillus and Bifidobacterium.

    Bacteria genusDescription
    LactobacillusLactobacillus is a genus of Gram-positive bacteria, which inhabit the human gut and other body parts, like the female reproductive system. In those locations, they help guard off other bacteria that would cause harm to the host.Additiobally, Lactobacillus are used in the food industry to ferment a number of products, like yogurt, cheese, wine, kefir, etc. Products containing Lactobacillus can be used as probiotics.
    BifidobacteriumAs the Lactobacillus genus, Bifidobacterium are Gram-positive bacteria that mostly inhabit the human (and other animals') gut.They help fight off other damaging bacteria that try to colonise the gut, treat ulcerative colitis, modulate the immune response, produce of vitamins and other functions.They are the most common bacteria in the gut of infants, who ingest these bacteria through their mother's milk.
    Table 5. Examples of helpful gut bacteria.

    Overall, bacteria come in all shapes and sizes and have widely differing functions with regard to humans: they can make us sick, or even kill us, but they can also protect us and help our bodies function to their best ability.

    Types of Bacteria - Key takeaways

    • The main three types of bacteria shapes are bacilli (rods), cocci (spherical), and spirilla (spirals).
    • Bacterial colonies are classified by their morphology, which includes the bacteria’s elevation, form, and margin.
    • Some common examples of bacterial infections include many forms of gastroenteritis/food poisoning, abscesses, urinary tract infections, mycobacterial infections, and strep throat.
    • Bacterial pneumonia can be placed into four types: community-acquired, healthcare-associated, hospital-acquired, and ventilator-associated.
    • The bacteria that is usually associated with UTIs is E. coli (around 80% of cases).


    1. Depiction of a lady who has a Urinary Tract Infection (UTI). (n.d.). [Online image]. In Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Depiction_of_a_lady_who_has_a_Urinary_Tract_Infection_(UTI).png
    Frequently Asked Questions about Types of Bacteria

    What type of cell are bacteria?

    Bacteria are the prokaryote cell type.

    Which type of bacteria causes Lyme disease?

    Lyme disease is caused by the bacteria Borrelia burgdorferi and infrequently by Borrelia mayonii.

    What are 4 types of bacteria?

    There are four types of bacteria: bacilli (rods), cocci (spherical), spirilla (spirals), vibrio (comma-shaped).

    Which type of bacteria causes blood poisoning?

    Blood poisoning or sepsis is usually caused by bacteria. The bacteria that most commonly cause blood poisoning are Staphylococcus aureus, Escherichia coli and some strains of Streptococcus.

    What types of foods support rapid bacterial growth?

    The food that most supports rapid bacterial growth is protein-rich, moist food.

    Test your knowledge with multiple choice flashcards

    What color does safranin turn gram negative bacteria to?

    What kind of substance can be used as a decolorizer?

    Which dye is used first, crystal violet or safranin


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