Bacteria use carbohydrates as a source of energy for growth, replication and metabolic processes. They can use many different sources of carbohydrates, some of the most common being; glucose, fructose, pentose and lactose. Probiotic bacteria are often referred to as lactic acid bacteria, which includes; Lactobacilli, Bifidobacteria and Streptococci. Most of these lactic acid bacteria use glucose as their primary energy source, an exception being Streptococcus thermophillus which uses lactose preferentially. However, although most lactic acid bacteria primarily ferment glucose, many can also utilise lactose.
There are two properties which allow these bacteria to metabolise lactose, firstly they must have an intracellular transport system allowing them to take up the lactose and secondly they have to be able to produce the enzyme β-galactosidase (also known as lactase). This enzyme breaks down lactose into β-galactose and glucose, which can then be metabolised to produce energy.
The exact method of lactose metabolism varies between the different species of lactic acid bacteria. In order for the bacteria to utilise lactose as an energy source they must be able to ingest the sugar and move it to the part of the cell which produces lactase. It is then, once inside the bacteria that lactose is broken into its composite parts by the enzyme. β-galactose and glucose are then metabolised via different pathways to produce energy. These different metabolic processes will produce a number of energetic molecules, in the form of ATP and a number of other molecules, which can include; lactate, ethanol and carbon dioxide.
Lactobacillus acidophilus, Lactobacillus bulgaricus and Lactobacillus rhamnosus use the same intracellular transport mechanism as the Bifidobacterium species to take up lactose but they produce different molecules during it’s metabolism to produce energy. In contrast Lactobacillus casei and Streptococcus thermophilus use a different method to move lactose into the right part of the cell, they actively transports the sugar as lactose phosphate. These bacteria also use a different metabolic method to break down the sugar and produce energy; however, this produces the same metabolic products as that used by the other Lactobacillus species.
Lactose Utilisation Test
The method used on the bacteria tested here was based on the detection of lactic acid produced from lactose utilisation. Bacteria were grown in a broth where the only carbohydrate source was lactose. Subsequent growth was dependent upon the ability to ferment lactose and thus was dependent upon the presence of β-galactosidase (lactase). Fermentation of lactose would result in the production of lactic acid. The presence of lactic acid would result in a drop in pH which would subsequently cause the phenol red indicator to change to yellow. In the test preformed for the bacteria bellow a positive result would be identified via a colour change to yellow whereas a negative result would be indicated by the media staying red.
Tests carried out at the Veterinary Laboratories Agency (Weybridge), Addlestone, UK on behalf of Probiotics International Ltd.
The results indicate that the bacterial strains tested (as above) are capable of lactase (β-galactosidase) production and are thus able to utilise lactose (milk sugar) as a source of energy.
The ability of these probiotic bacteria to produce lactase and utilise lactose, makes them ideal for use in the diets of lactose intolerance sufferers. People with lactose intolerance are unable to produce enough lactase to enable the digestion of foods containing lactose. Lactose therefore remains within the gut, causing symptoms such as diarrhoea, bloating and abdominal pain. If probiotic bacteria, which produce lactase, are given to sufferers then they may help to enable the re-introduction of foods containing lactose into their diets. Probiotic bacteria are known to both improve lactose digestion and the symptoms of lactose intolerance.