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Helpful and Harmful Types of Bacteria

Bacteria are microscopic organisms that form a huge invisible world around us, and within us. They are infamous for their harmful effects, whereas the benefits they provide are seldom known. Get a brief overview of the good and bad bacteria.
Prabhakar Pillai
Last Updated: Apr 20, 2018
For the first half of geological time our ancestors were bacteria. Most creatures still are bacteria, and each one of our trillions of cells is a colony of bacteria. - Richard Dawkins

Bacteria - the oldest living organisms on earth - are omnipresent. The human body, the air we breathe, the surfaces we touch, the food we eat, the plants that surround us, the environment we live in, etc., are all replete with bacteria.

Almost 99% of these bacteria are helpful, where the remaining are notorious. In fact, some are essential for the proper growth of other living beings. They are either free-living or form a symbiotic relationship with animals or plants.

The list of helpful and harmful bacteria contain some of the most commonly known beneficial and deadly bacteria.

Helpful Bacteria

Lactobacillus/Döderlein's bacillus

Characteristics: Gram-positive, rod-shaped

Presence: Lactobacilli species are present in milk and dairy products, fermented foods and also form part of our oral, intestinal and vaginal microflora. L. acidophilus, L. reuteri, L. plantarum, etc., are some of the most predominant species.

Benefit: Lactobacilli are known for their ability to utilize lactose and produce lactic acid, as a metabolic byproduct. This ability to ferment lactose makes lactobacilli an important ingredient for preparing fermented foods. It is also an important part of the pickling process since lactic acid serves as a preservative. The formation of yogurt from milk is done through what is called, fermentation. Certain strains are even used commercially for the production of yogurt. In mammals, lactobacilli aid the breakdown of lactose during digestion. The resulting acidic environment prevents the growth of other microbes in the body tissues. Being so, lactobacilli are an important part of probiotic formulations.


Characteristics: Gram-positive, branched, rod-shaped

Presence: Bifidobacteria are present in the gastrointestinal tract of humans.

Benefit: Similar to lactobacilli these are also known for lactic acid production. In addition, it also produces acetic acid. This inhibits the growth of pathogenic bacteria by controlling pH levels in the intestines. B. longum helps in the breakdown of non-digestible plant polymers. B. longum and B. infantis help prevent diarrhea, candidiasis, and other yeast infections in infants and children. Owing to these benefits, this particular species are also included in commercially available probiotics.

Escherichia coli (E. coli)

Characteristics: Gram-negative, rod-shaped

Presence: E. coli is a part of the normal microflora of small and large intestines.

Benefit: E. coli helps in the breakdown of undigested monosaccharide sugars and thus aid digestion. These bacteria produce vitamin K and biotin which are essential for a variety of cellular processes.

Note.- Certain strains of E. coli can cause severe toxicity, diarrhea, anemia, and kidney failure.


Characteristics: Gram-positive, filamentous

Presence: They are widely found in soil, water, and decaying matter.

Benefit: Streptomyces spp. play an important role in soil ecology by bringing about the decomposition of organic matter present in soil. As a result, they are being explored as agents for bioremediation. S. aureofaciens, S. rimosus, S. griseus, S. erythraeus and S. venezuelae are some of the commercially important species used for the production of antibacterial and antifungal compounds.


Characteristics: Gram-negative, rod-shaped

Presence: Rhizobia are present in soil or form a symbiotic association with the root nodules of leguminous plants.

Benefit: Rhizobium etli, Bradyrhizobium spp., Azorhizobium spp.., and many other species, are useful for fixing atmospheric nitrogen, including ammonia, thus making it available for plants. Plants do not possess the ability to utilize atmospheric nitrogen and are dependent on nitrogen-fixing bacteria, that is present in soil.


Characteristics: Gram-negative, rod-shaped

Presence: Cyanobacteria are mainly aquatic bacteria but are also found on bare rocks and in soil.

Benefit: Also known as blue-green algae and blue-green bacteria, they are a group of environmentally significant bacteria. They bring about nitrogen fixation in aquatic habitats. Their calcification and decalcification abilities make them essential for maintaining coral reef ecosystem balance.

Harmful Bacteria


Characteristics: Neither Gram-positive or Gram-negative (due to high lipid content), rod-shaped

Presence: Mycobacteria are generally found in water and food. However, M. tuberculosis and M. leprae (aka, Hansen's coccus spirilly), are obligate parasites and cannot survive in their free form.

Disease: The bacteria under the genus Mycobacterium are pathogens with long doubling times. M. tuberculosis and M. leprae, the most notorious species, are the causative agents for tuberculosis and leprosy, respectively. M. ulcerans causes ulcerated and non-ulcerated nodules in the skin. M. bovis causes tuberculosis in cattle.

Clostridium tetani

Characteristics: Gram-positive, box-shaped

Presence: C. tetani spores are found in soil, skin, and the gastrointestinal tract.

Disease: C. tetani is the etiologic agent of tetanus. It enters the body through a wound, replicates there and releases toxins, namely tetanospasmin (aka, spasmogenic toxin) and tetanolysin. These lead to muscular spasms and respiratory failure.

Yersinia pestis

Characteristics: Gram-negative, rod-shaped

Presence: Y. pestis can only survive within the host, namely rodents (fleas) and mammals.

Disease: Y. pestis causes bubonic and pneumonic plague. A skin infection with Y. pestis leads to the bubonic form characterized by malaise, fever, chills, and even seizures. An infection in the lungs caused by Y. pestis leads to pneumonic plague causing coughs, difficulty in breathing, and fever. According to WHO, around 1000 to 3000 cases of plague are reported worldwide every year. This microbe is being recognized and explored as a potential bioweapon.

Helicobacter pylori

Characteristics: Gram-negative, rod-shaped

Presence: H. pylori colonizes the mucosal lining of the human stomach.

Disease: It is the leading cause for gastritis and peptic ulcers. It produces cytotoxins and ammonia which damage the stomach epithelium leading to abdominal pain, nausea, vomiting, and bloating. H. pylori is present in half of the world's population, however, most of them are asymptomatic while only a few develop gastritis and ulcers.

Bacillus anthracis

Characteristics: Gram-positive, rod-shaped

Presence: B. anthracis is widely present in soil.

Disease: The deadly disease called anthrax is a result of a B. anthracis infection, where the inhalation of B. anthracis endospores is what causes this illness. Anthrax mainly occurs in sheep, goats, cattle, etc. However, the transmission of bacteria from domestic cattle to humans, occurs in rare cases. The formation of sores, fever, headache, abdominal pain, nausea, diarrhea etc., are the most common symptoms (of anthrax).

We are surrounded by bacteria, some friendly and some deadly. It is up to us to make the most from these tiny living beings. Gain from the helpful ones by avoiding excessive or unnecessary use of antibiotics, and keep the harmful ones at bay by taking appropriate preventive measures, like maintaining hygiene and visiting the doctor from regular checkups.