Background information
Lactic acid bacteria have the property of producing lactic acid from sugars by a process called fermentation. The genera Bacillus, Leuconostoc, Pediococcus and Streptococcus are important members of this group. The taxonomy of lactic acid bacteria has been based on the gram reaction and the production of lactic acid from various fermentable carbohydrates.
Lactobacilli are gram positive and vary in morphology from long, slender rods to short coccobacilli, which frequently form chains. Their metabolism is fermentative; some species are aerotolerant and may utilize oxygen through the enzyme flavoprotein oxidase, while others are strictly anaerobic. While spore bearing Bacillus coagulans are facultative anaerobes, the rest are strictly anaerobic. The growth is optimum at pH 5.5-5.8 and the organisms have complex nutritional requirements for amino acids, peptides, nucleotide bases, vitamins, minerals, fatty acids and carbohydrates.
The genus is divided into three groups based on fermentation patterns:
- homofermentative : produce more than 85% lactic acid from glucose.
- heterofermentative : produce only 50% lactic acid and considerable amounts of ethanol, acetic acid and carbon dioxide.
- less well known heterofermentative species which produce DL-lactic acid, acetic acid and carbon dioxide.
The species which have been therapeutically used are :
- L. sporogenes*
- L. acidophilus
- L. plantarum
- L. casei
- L. brevis
- L. delbruckii
- L. lactis
The metabolic activities of lactobacilli are responsible for their therapeutic benefits.
Lactobacilli cultured in milk medium perform the following activities:
1. Proteolysis:
Proteins are broken down into easily assimilable components.
These activities of lactobacilli in the gastrointestinal tract make protein ingested by the host easily digestible, a property of great value in infant, convalescent and geriatric nutrition.
2. Lipolysis:
Complex fat is broken down into easily assimilable components.
This property is useful in the preparation of dietetic formulations for infants, geriatrics and convalescents.
Evidence from preclinical and clinical trials has revealed that lactobacilli can break down cholesterol in serum lipids. Lactobacilli also assist in the deconjugation of bile salts. Both of these findings have clinical significance.
3. Lactose metabolism:
Lactic acid bacteria have the enzymes b- galactosidase, glycolases and lactic dehydrogenase (LDH) which produce lactic acid from lactose. Lactic acid is reported to have some physiological benefits such as:
a) Enhancing the digestibility of milk proteins by precipitating them in fine curd particles.
b) Improving the utilization of calcium, phosphorus and iron.
c) Stimulating the secretion of gastric juices.
d) Accelerating the onward movement of stomach contents.
e) Serving as a source of energy in the process of respiration.
The levels of optical isomeric forms of lactic acid produced depend upon the nature of the culture. The structural configurations of these isomers are as follows :
D(-) levorotatory lactic acid L(+) dextrorotatory lactic acid
In humans, both isomers are absorbed from the intestinal tract. Whereas L(+) lactic acid is completely and rapidly metabolized in glycogen synthesis, D(-) lactic acid is metabolized at a lesser rate, and the unmetabolized acid is excreted in the urine. The presence of unmetabolized lactic acid results in metabolic acidosis in infants. L. acidophilus produces the D(-)- form and is therefore of disputable clinical benefit, although it has earlier been the probiotic of choice in various therapeutic formulations. L. sporogenes* on the other hand produces only L(+)- lactic acid and hence is preferred.
The ability of lactobacilli to convert lactose to lactic acid is used in the successful treatment of lactose intolerance. People suffering from this condition cannot metabolize lactose due to lack or dysfunction of the essential enzyme systems. Lactic acid, by lowering the pH of the intestinal environment to 4 to 5, inhibits the growth of putrefactive organisms and E. coli, which require a higher optimum pH of 6 to 7. Some of the volatile acids produced during fermentation also possess some antimicrobial activity under conditions of low oxidation-reduction potential.
Production of bacteriocins:
Bacteriocins are proteins or protein complexes with bactericidal activities directed against species which are closely related to the producer bacterium. The inhibitory activity of L. sporogenes* and lactobacilli towards putrefactive organisms is thought to be partially due to the production of bacteriocins.
Some of the bacteriocins isolated from lactobacilli are listed in Table 2.1:
Table 2.1 : Bacteriocins isolated from different Lactobacillus species.
| Substance | Producing species |
|---|---|
|
Acidolin |
L. acidophilus |
|
Acidophilin |
L. acidophilus |
|
Lactacin B |
L. acidophilus |
|
Lactacin F |
L. acidophilus |
|
Bulgarin |
L. bulgaricus |
|
Plantaricin SIK-83 |
L. plantarum |
|
Plantaricin A |
L. plantarum |
|
Lactolin |
L. plantarum |
|
Plantaricin B |
L. plantarum |
|
Lactolin 27 |
L. helveticus |
|
Helveticin J |
L. helveticus |
|
Reuterin |
L. reuteri |
|
Lactobrevin |
L. brevis |
|
Lactobacillin |
L. brevis |
Production of other antagonistic substances:
Lactic acid bacteria also inhibit the growth of harmful putrefactive microorganisms through other metabolic products such as hydrogen peroxide, carbon dioxide and diacetyl.
The metabolites of lactic acid bacteria that exert antagonistic action against putrefactive microorganisms and their mode of action are summarized in Table. 2.2:
Table 2.2 : Antagonistic activities caused by lactic acid bacteria
| Metabolic product | Mode of antagonistic action |
|---|---|
1.Carbon dioxide |
Inhibits decarboxylation? Reduces membrane permeability? |
2. Diacetyl |
Interacts with arginine-binding proteins. |
3.Hydrogen peroxide/Lactoperoxidase |
Oxidizes basic proteins. |
4. Lactic acid |
Undissociated lactic acid penetrates the membranes, lowering the intracellular pH. It also interferes with metabolic processes such as oxidative phosphorylation. |
5. Bacteriocins |
Affect membranes, DNA-synthesis and protein synthesis. |
Synthesis of B- vitamins
Experiments on fermented milk products have revealed that lactic cultures require B- vitamins for their metabolic activities. However, some lactic cultures synthesize B-vitamins16. Friend et al. reported that the B-vitamin content of fermented milk products was a function of species as well as the strain of lactic acid bacteria used in their manufacture. Similarly, vitamins are synthesized by the lactic cultures in the gut microflora, in symbiosis with other flora.
It has been observed that the diet of the host influences the nature and levels of beneficial intestinal microflora, such as lactobacilli. The presence of dietary fructooligosaccharides was found to enhance the healthful effects of intestinal lactic acid bacteria. These compounds, found naturally in foods such as onion, edible burdock and wheat, are effectively employed as non-nutritive sweeteners (Neosugar, Meiologo). They have the advantage of being indigestible by humans and farm animals, rendering them valuable in dietetic products. They are, however, selectively utilized by intestinal lactic acid bacteria, especially bifidobacteria, thereby enhancing the healthful effects of these beneficial intestinal flora.
* The taxonomical classification was revised in 1939 in the seventh edition of the Bergey’s Manual of Determinative Bacteriology to B. coagulans, although some researchers continued to use the original name.