Lactobacillus thermophilus is a bacterial species that belongs to the genus ‘Bacillus’. It is predominantly found in milk.

The genus ‘Bacillus’:

Bacterial species that belong to this genus are known for their ability to produce biologically and industrially important enzymes and proteins. For example, alpha amylase is used for the hydrolysis of starch. These bacteria are also commonly used for the production of antibiotics (B. subtilis). Apart from the benefits they provide, bacterial species belonging to this genus can also be pathogenic, the most commonly known species being B. anthrax which causes anthracis and B. cereus which causes common food poisoning. B. thuringiensis is a biotechnologically significant bacterial species as it is used to make crops insect-resistant.

Structural and physiological characteristics:

  • These bacterial species are gram-positive but can become gram-negative during their life.
  • They are rod-shaped, hence, the name bacillus.
  • They can be both non-spore and spore forming. Endospore forming species possess the ability to form a capsule around the cell wall, enabling them to remain dormant under unfavourable environmental conditions for several years at a time.
  • These bacteria are categorised as obligate aerobes and facultative anaerobes, meaning, some species cannot survive in the absence of oxygen while some can survive even in the absence of oxygen.
  • They are also important endophytes and play an important role in nitrogen cycling and nutrient absorption in plants.

Isolation and culturing:

Bacteria belonging to this genus are usually found in soil, hence the following steps are followed for isolation, culturing and identification of Bacillus species-

  • Collected soil samples are first suspended in distilled water.
  • After filtration, the collected supernatant is subjected to high temperatures. This is done to preserve only the viable spores while killing off any vegetative cells present.
  • The spores collected are then cultured on a nutrient agar plate.
  • The bacterial colonies are then observed under the microscope.
  • Specific species can be selected using differential and selective nutrient media.

Bacillus cereus

Gram-staining of Bacillus species

Characteristics of the bacterial cultures:

  • The cultures observed are usually round in shape, with irregular edges.
  • The colour of the colony may vary from brown to red or even white.
  • The colonies are umbonate, meaning a protrusion is present at one end of the colony which is made up of vegetative cells.

Bacillus subtilis

SEM image of a species of Bacillus

Lactobacillus thermophilus:

This species of bacteria are present in high numbers in unpasteurized milk, however its occurrence in milk is not all that common. The growth of this bacteria increases in proportion to the increase in the surrounding temperature. They produce high amounts of lactic acid and render milk unpalatable when present in excess.

Acidophilus milk, dairy and non-dairy

Curdling of milk due to Lactobacillus bacteria

Morphological and physiological characteristics:

Upon culturing this bacterial species, it was noted that it does not grow well on normal nutrient media used to plate other bacterial species present in milk, hence certain alterations to the nutrient medium had to be made. Upon growth of the colonies, the following characteristics were observed:

  • These bacteria are present as straight or slightly curved rods.
  • They are non-motile and non-spore forming.
  • They are gram-positive in terms of staining, but when cultured in milk culture, they are observed to be gram-negative. 
  • They are present as single cells on solid media and occur as chains of cells in a liquid broth medium.
  • They are non-pathogenic and do not cause any harm when consumed.
  • These bacteria are present in raw milk in small amounts but possess the ability to proliferate under suitable conditions.
  • A culture of Lactobacillus thermophilus is usually translucent and iridescent in appearance. The colonies are irregular in shape and have a rough texture. The profile of the culture is slanted and umbonate.

It is important to note that unlike most other lactobacillus species, Lactobacillus thermophilus is heat-resistant and does not grow when kept at room temperatures. It requires high temperatures for its optimum growth. This bacterial species is acidic and grows well when kept in a low pH condition.


Lactobacillus thermophilus is commonly found in raw milk and as of now does not provide any health benefits to humans unlike other Lactic Acid Bacteria. This bacterial species grow well when kept in high temperatures and is hence being studied for its ability to produce heat-resistant primers that can be used for DNA annealing and polymerisation. Lactobacillus thermophilus is a major focus of recombinant DNA technology and genetic engineering.

Deepika Thilakan


Tags: Lactobacillus thermophilus

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