1.Introduction to Lactic Acid Bacteria (LAB)
Lactic Acid Bacteria (LAB) are a diverse group of Gram-positive, rod-shaped (bacilli) or spherical (cocci) bacteria that produce lactic acid as a major metabolic byproduct of carbohydrate fermentation. They appear purple under Gram staining due to their thick peptidoglycan cell wall. These bacteria thrive in anaerobic to micro-aerophilic environments, with optimal growth temperatures ranging from 20°C to 45°C, depending on the species. They prefer slightly acidic to neutral pH (4.0–7.0) and require simple sugars like glucose, lactose, and sucrose as their primary energy sources.
2. Morphological and Biochemical Characteristics
Lactobacillus species are gram-positive, rod-shaped, non-spore-forming bacteria that differ from many pathogens in their cell wall composition. They lack lipopolysaccharides, a key virulence factor found in gram-negative bacteria (Jans et al., 2017). They are facultative anaerobes or microaerophiles and produce lactic acid through fermentation, creating an acidic environment that inhibits pathogen growth. Their colonies on agar media appear small, smooth, and often opaque. Unlike pathogenic bacteria, Lactobacillus does not produce endotoxins or hemolysins, reducing its potential to cause disease (Franz et al., 2011). While most strains are beneficial, some species, such as Lactobacillus rhamnosus and Lactobacillus casei, have been implicated in rare bloodstream infections (Bryła et al., 2022).
3. Laboratory Identification and Detection
Lactobacillus species are identified through microbial, biochemical, and molecular methods. Microbial isolation involves culturing on de Man, Rogosa, and Sharpe (MRS) agar, where they produce characteristic colonies (Ogier & Serror, 2008). Biochemical tests such as catalase-negative reactions, carbohydrate fermentation patterns, and acid production from glucose help differentiate them from pathogenic bacteria. DNA-based methods, including 16S rRNA sequencing and PCR targeting Lactobacillus-specific genes, are the most reliable for species-level identification (Ceuppens et al., 2014). Unlike pathogens that cause visible spoilage, Lactobacillus may cause changes in food texture, acidification, and souring, which are desirable in fermented foods but considered spoilage in unintended contexts such as packaged meats.
4. Dual nature of Lactobacillus Species
Unlike many pathogenic bacteria, Lactobacillus strains do not produce harmful toxins; instead, they contribute to food preservation by producing organic acids such as lactic acid, which inhibits spoilage organisms and pathogens. Some strains, like L. acidophilus, L. bulgaricus, and L. plantarum, are beneficial in fermented foods and probiotics contributing to desirable sensory characteristics such as tanginess, improved texture, and longer shelf life. Examples include Lactobacillus plantarum in sauerkraut, Lactobacillus delbrueckii in yogurt, and Lactobacillus brevis in sourdough bread (McCabe-Sellers & Beattie, 2004). Reports from the U.S. Food and Drug Administration (FDA) and European Food Safety Authority (EFSA) classify Lactobacillus as a “generally recognized as safe” (GRAS) microorganism, except in immunocompromised patients (Doyle & Erickson, 2012).
However, some species like L. brevis, L. viridescens, and L. sakei, contribute to spoilage by altering texture and flavor, particularly in dairy and meat products (Bernardeau et al., 2006) under improper storage conditions. In rare cases, some species can be pathogenic. Lactobacillus rhamnosus has been associated with sepsis in immunocompromised patients, though its virulence is significantly lower than that of true pathogens (Franz et al., 1999). Top of Form
5. Beneficial Strains of Lactobacillus in Food Production
ii. Lactobacillus acidophilus
Environment Conditions: Thrives at 37°C–42°C, prefers slightly acidic conditions (pH 5.0–6.5), and is facultatively anaerobic.
Food Sources: Yogurt, fermented milk, cheese, probiotics, and some dietary supplements.
Characteristics: Produces lactic acid, helps in digestion, enhances gut microbiota, and has probiotic properties.
ii. Lactobacillus bulgaricus
Environment Conditions: Optimum growth at 40°C–45°C, requires a pH of 4.5–5.5, and is aerotolerant.
Food Sources: Traditional yogurt fermentation and some soft cheeses.
Characteristics: Produces high levels of lactic acid, improves yogurt texture, and contributes to the tangy taste in dairy products.
iii. Lactobacillus plantarum
Environment Conditions: Grows at 15°C–45°C, pH range of 3.4–7.0, facultative anaerobe.
Food Sources: Sauerkraut, kimchi, pickles, sourdough bread, and fermented sausages.
Characteristics: A robust strain known for its high tolerance to salt and acidity, making it effective in vegetable fermentation.
iv. Lactobacillus casei
Environment Conditions: Optimal temperature at 30°C–40°C, survives in pH 4.5–6.5, facultative anaerobe.
Food Sources: Cheese, yogurt, fermented milk, and probiotic drinks.
Characteristics: Enhances gut health, extends shelf life of fermented dairy, and can survive in the digestive tract.
v. Lactobacillus reuteri
Environment Conditions: Grows at 30°C–40°C, optimal pH 5.5–6.2, microaerophilic.
Food Sources: Fermented milk, probiotic supplements, and sourdough bread.
Characteristics: Produces antimicrobial compounds like reuterin, inhibits pathogens, and aids digestion.
vi. Lactobacillus fermentum
Environment Conditions: Thrives between 30°C–42°C, survives at pH 4.0–6.5, facultative anaerobe.
Food Sources: Sourdough, fermented vegetables, dairy products, and traditional beverages.
Characteristics: Produces antimicrobial peptides, enhances immune function, and prevents spoilage in fermented products.
vii. Lactobacillus helveticus
Environment Conditions: Optimum growth at 37°C–45°C, pH 5.0–6.5, facultative anaerobe.
Food Sources: Swiss cheese, Parmigiano-Reggiano, and probiotics.
Characteristics: Involved in cheese ripening, enhances flavor, and produces bioactive peptides beneficial for health.
6. Spoilage-Causing Strains of Lactobacillus
While most Lactobacillus species are beneficial, some strains can cause spoilage in various food products, particularly under certain storage conditions.
i. Lactobacillus brevis
Spoilage Food: Beer, wine, fruit juices, and pickled products.
Storage Conditions Leading to Spoilage: Warm temperatures (above 10°C), oxygen exposure, and prolonged storage in suboptimal conditions.
Characteristics: Produces gas, off-flavors, and turbidity in beverages, leading to undesirable quality changes.
ii. Lactobacillus viridescens
Spoilage Food: Processed meats and vacuum-packed meat products.
Storage Conditions Leading to Spoilage: Poor refrigeration, vacuum-sealed environments with slight oxygen presence, and prolonged storage.
Characteristics: Causes “greening” in cured meats, produces off-odors, and leads to slime formation.
iii. Lactobacillus sakei
Spoilage Food: Chilled meats, poultry, and fish.
Storage Conditions Leading to Spoilage: Cold storage below 4°C with moderate humidity.
Characteristics: Produces acid, causing sourness in meat products, and can result in textural degradation.
iv. Lactobacillus curvatus
Spoilage Food: Sausages, ham, and other cured meats.
Storage Conditions Leading to Spoilage: Vacuum packaging, refrigerated storage above 2°C.
Characteristics: Causes slime formation and an acidic, sour taste.
v. Lactobacillus fructivorans
Spoilage Food: Fruit juices, soft drinks, wine, and pickled vegetables.
Storage Conditions Leading to Spoilage: High sugar content, anaerobic conditions, and improper refrigeration.
Characteristics: Produces gas, acidity, and off-flavors, leading to cloudy and spoiled beverages.
7. Control Measures for Preventing Spoilage Causing Lactobacillus
Preventing undesirable Lactobacillus contamination involves proper food storage, hygiene, and temperature control. Since Lactobacillus is heat-labile, pasteurization and cooking eliminate unwanted strains in dairy and meat products. Freezing slows bacterial growth but does not eliminate bacteria entirely. Good manufacturing practices (GMP), including sanitation of food processing equipment and strict temperature control, are crucial in preventing spoilage. Probiotics should be used with caution in at-risk populations, and antibiotic resistance monitoring in Lactobacillus strains is essential for food safety (Ceuppens et al., 2014).
References
- Adams, M., & Mitchell, R. (2002). Fermentation and pathogen control: A risk assessment approach. International Journal of Food Microbiology, 10(2), 123-140. Link
- Anyogu, A., Olukorede, A., Anumudu, C., & Onyeaka, H. (2021). Microorganisms and food safety risks associated with indigenous fermented foods from Africa. Food Control, 12(3), 237-255. Link
- Bernardeau, M., Guguen, M., & Vernoux, J. P. (2006). Beneficial Lactobacillus in food and feed: Long-term use and safety assessments. FEMS Microbiology Reviews, 30(4), 487-513. Link
- Bryła, M., Zapaśnik, A., & Sokołowska, B. (2022). Role of lactic acid bacteria in food preservation and safety. Foods, 11(9), 1283. Link
- Ceuppens, S., Li, D., & Uyttendaele, M. (2014). Molecular methods in food safety microbiology. Comprehensive Reviews in Food Science and Food Safety, 13(3), 551-577. Link
- Doyle, M. P., & Erickson, M. C. (2012). Opportunities for mitigating pathogen contamination during on-farm food production. International Journal of Food Microbiology, 157(2), 73-82. Link
- Franz, C. M. A. P., Huch, M., Abriouel, H., Holzapfel, W., & Gálvez, A. (2011). Enterococci as probiotics and their implications in food safety. International Journal of Food Microbiology, 151(2), 1-10. Link
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha is currently a Graduate Research Assistant at Louisiana State University, USA and holds a Master’s degree in Food Engineering and Bioprocess Technology from the Asian Institute of Technology (AIT), Thailand. She previously, she worked for the Government of Nepal at the Department of Food Technology and Quality Control (DFTQC), Kathmandu, and also served as a teaching faculty at the College of Applied Food and Dairy Technology (CAFODAT) affiliated with Purbanchal University, Nepal.