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Lactobacillus: a Friendly Bacteria

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A photo of cheese, yoghurt and sourdough bread, all aided by lactobacillus microbes.

Who cannot have heard of Lactobacillus, a genus of bacteria which as the television adverts for yoghurts would have us believe, should be termed 'friendly bacteria'.

Lactobacillus is a genus of gram-positive bacteria, a rod-shaped member of the lacto-acid bacterial group. There are many species of Lactobacillus (over 180), which live in human digestive, urinary and genital systems without causing disease, and as the probiotic yoghurt adverts would wish us to accept, actually having quite beneficial and advantageous effects on human health. Lactobacillus acidophilus is a particular species or strain which 'probiotic yoghurts' advertise themselves as containing. Another strain, marketed in probiotic drinks, is Lactobacillus casei, which promotes the growth of L. acidophilus in the human gut.

Lactobacillus are facultative anaerobic1, non-motile2 and non-spore-forming3, rod-shaped bacteria, with a size range of 0.7 to 1.1 micrometres wide by 2.0 to 4.0 micrometres long.

The bacterium is acid tolerant with a strictly fermentative metabolism, lactic acid being the most dominant end-product of such.

Lactose, the main sugar found in milk, as well as glucose and other sugars, form the carbon sources for Lactobacillus. The bacteria uses energy to transport lactose and other sugars across the cell membrane, where it is metabolized to lactic acid4. Lactobacillus bacteria which produce only lactic acid are termed homofermentative; those which also produce acetic acid, ethanol and carbon dioxide are termed heterofermentive. The purpose of this reaction, for the bacterium, is to produce energy to allow the bacterium to grow and multiply.

Etymology

The etymology of Lactobacillus is extremely straightforward: lac - lactis - milk; bacillus - a small rod; Lactobacillus - milk rodlet.

Taxonomy

Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Lactobacillaceae
Genus: Lactobacillus

Industrial and Commercial Importance

Various different Lactobacillus species have roles in the production of various human foods and beverages, including:

as well as in silage production for animal feed.

However, some Lactobacillus species, especially L. casei and L. brevis, have the rather unfortunate effect of leading to spoilage of beer,

Examples of Species from the Genus

With over 180 species and many subspecies and strains of Lactobacillus , we'll concentrate on a few examples with particular roles in either the human body and/or commercial uses.

L. acidophilus is the most commonly used member of the genus in probiotic supplements and drinks. It is thought to be protective of overpopulation by other micro-organisms, as its byproducts of metabolism, including lactic acid and hydrogen peroxide, can inhibit growth and multiplication of other bacteria. It grows in anaerobic conditions of pH less than 3.5, and metabolically only undergoes fermentation to produce lactic acid. It is thought that L. acidophilus produces bacteriocins - various proteins which actively work against and prevent (in the lab at least) the growth of other bacteria, and that this may have an important role in its beneficial effects when ingested by humans in probiotic supplements and drinks.

L. acidophilus can grow with or without the presence of oxygen, and is homofermentative as it only produces lactic acid.

There's a colossal list of health claims attributed to taking L. acidophilus in supplement or 'health' drink forms. However, for the most part, these are unverified claims made by the manufacturers of the products. As yet most 'health' claims must carry the warning 'thought to', or 'not yet proven to'. Such effects as:

  • Relieves constipation
  • Reduces flatulence
  • Relieves urinary tract infection
  • Reduces/prevents heartburn
  • Protects against indigestion
  • Prevents stomach ulcers
  • Kills deadly strains of E coli and Salmonella
  • Protects against cancer
  • Prevents/protects against cardiovascular disease
  • Helps reduce symptoms of irritable bowel syndrome
  • Relieves bloating
  • Prevents thrush
  • Reduces risk and symptoms of ulcerative colitis
  • Improves hypersensitivity reactions among infants with food allergies
  • Prevents diarrhea
  • Reduces abdominal/gastrointestinal cramping
  • Combats vaginal yeast infections
  • Reduces bad breath
  • Encourages production of immune system cells to fight viral infections
  • Protects against colon cancer
  • Prevents development of diverticulitis
  • Protects against and relieves symptoms of Crohn's disease

L. acidophilus appears to have a role in healthy women in maintaining the vaginal pH, but studies have not shown the effectiveness of orally-ingested supplements or tablets on altering the vaginal microflora.

It is also claimed that L. acidophilus can help boost an individuals immune system, however studies on this seem to produce mixed results and are inconclusive. There does, however, seem to be some validity (supported by research) suggesting that L. acidophilus can have an effect on reducing the presence of undesirable micro-organisms within the intestine, as a result of its lowering of the intestinal pH. This effect is facilitated because of the bacterium's ability to survive very low pHs, such as are present in the stomach, and so be able to pass to lower regions of the human gastrointestinal tract.

L. brevis has a few commercial functions, particularly in pickle and Sauerkraut production. In humans, ingestion has been potentially linked with assisting the functioning of the human immune system, though again, this species sadly moves away from the title of 'friendly bacteria', as it is thought to be one of the major species of micro-organisms involved in the spoilage of beer.

L. casei is considered safe for human consumption, and can be found in such probiotics as Yakult. It is found widely in the human gastrointestinal tract and mouth, where it compliments the growth of L. acidophilus, which is a useful bacteria as it produces the enzyme amylase, involved in enzymatic digestion of carbohydrates. Commercially it has a role in maturation of cheddar, and has also been shown to help in the fermentation of certain beans.

Lactobacillus sanfranciscensis, formerly known as Lactobacillus sanfrancisco, is one important species of Lactobacillus, used in helping ferment and give San Francisco sourdough bread its typical flavour. Dried sourdough starters, containing a mixture of yeast and particular strains of L. sanfranciscensis for producing San Francisco sourdough, are produced and sold worldwide.

1Capable of producing energy via either anaerobic or aerobic respiration, depending on the oxygen and fermentative products, present in its environment.2Incapable of self-propelled movement.3Unable, unlike many similar micro-organisms, of forming spores as part of their lifecycle.4Some species also produce acetic acid, ethanol and carbon dioxide.5Korean pickle.

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