Lactic acid bacteria

Lactic Acid Bacteria (LAB): 

1.Introduction :

Lactic Acid Bacteria (LAB) are a group of Gram-positive, non-spore forming, aerotolerant anaerobes, typically rod- or cocci-shaped.

They produce lactic acid as the major end product of carbohydrate fermentation.

Found in dairy products, fermented foods, plant surfaces, and in the gastrointestinal tracts of humans and animals.

2. Characteristics :

Gram-positive
Catalase-negative (do not produce catalase enzyme)
Non-motile 
Facultative anaerobes or microaerophilic
Tolerate acidic environments (pH 4.0–6.5)
Ferment sugars primarily into lactic acid (either homofermentative or heterofermentative)

3. Classification :

Based on Fermentation Pathways:

Homofermentative LAB: produce mainly lactic acid (e.g., Lactococcus, Streptococcus)

Heterofermentative LAB: produce lactic acid, ethanol/acetic acid, and CO₂ (e.g., Leuconostoc, Weissella)


Genera in LAB Group:

Lactobacillus (now split into several genera)

Lactococcus
Leuconostoc
Pediococcus
Streptococcus
Enterococcus
Oenococcus
Weissella
Carnobacterium

4. Habitat :

Milk and dairy products

Fermented foods (vegetables, meat, cereals)

Human and animal mucosal surfaces (oral, vaginal, intestinal)

Silage and plant materials.

5. Industrial and Health Importance :

A. Food Industry Applications

Dairy fermentation: yogurt, cheese, kefir (e.g., Lactobacillus, Streptococcus thermophilus)

Meat fermentation: sausages (e.g., Pediococcus acidilactici)

Vegetable fermentation: sauerkraut, kimchi (e.g., Leuconostoc mesenteroides)

Beverage fermentation: wine, beer, kefir


B. Probiotic Properties

Improve gut health

Enhance immune response

Compete with pathogens (competitive exclusion)

Produce bacteriocins (antimicrobial peptides)


C. Health Benefits

Lower cholesterol

Antioxidant activity

Anti-inflammatory effects

Prevention of diarrhea and IBS

6.  Metabolism :

Homofermentative Pathway:

Glucose → 2 lactic acid + 2 ATP


Heterofermentative Pathway:

Glucose → lactic acid + ethanol/acetic acid + CO₂ + 1 AT

7. Bacteriocins :

Ribosomally synthesized antimicrobial peptides

Examples: Nisin (produced by Lactococcus lactis), Pediocin

8. Genetic and Molecular Aspects :

LAB can be genetically engineered to enhance their probiotic or fermentation capabilities.

Many LAB genomes have been sequenced for understanding their metabolic capabilities and probiotic potential.

9. Preservation and Culturing :

Stored by freeze-drying (lyophilization) or cryopreservation

Cultured in MRS agar/broth (de Man, Rogosa and Sharpe medium) optimal for LAB growth.

10. Challenges and Limitations :

Sensitivity to oxygen

Bacteriophage attacks during fermentation

Strain-specific functionality

Survival in gastric conditions for probiotics

LAC foods :

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