Lactobacilli Basics, Testing & Identification
Jvo Siegrist
Microbiology Focus Edition 1.1
Structure and Metabolism of Lactobacillus
Lactobacilli [sing: lactobacillus] are rod-shaped, gram-positive, fermentative, facultative anaerobic, or microaerophilic organotrophs that play an important and varied role in human health. Normally, they form straight rods but under certain conditions spiral or coccobacillary forms have been observed. In most cases, they form chains of varying lengths. Lactobacilli belong to the lactic acid bacteria and comprise a major part of this group. As their name implies, they produce lactic acid and derive energy from the fermentation of lactose, glucose, and other sugars to lactate via homofermentative metabolism.
Rod-shaped Lactobacillus bacteria
About 85-90% of the sugar utilized in the fermentative process is converted to lactic acid. Also, some heterofermentative lactobacilli produce alcohol in addition to lactic acid from sugars. This acid-producing mechanism inhibits the growth of other organisms and favors the growth of lactobacilli that thrive in low-pH environments. ATP is generated during the process by non-oxidative substrate-level phosphorylation.
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Lactobacillus are divided into three groups based on their metabolism (According to Kandler and Weiss):
- Obligately homofermentative (Group I) L. acidophilus, L. delbrueckii, L. salivarius, L. helveticus
- Facultatively heterofermentative (Group II) L. casei, L. plantarum, L. curvatus, L. sakei
- Obligately heterofermentative (Group III) L. brevis, L. buchneri, L. fermentum, L. reuteri
Lactobacilli have a generation time ranging from 25 to several hundred minutes. The optimal growth temperature ranges from 30 to 40 °C, although some thermophilic strains grow well and have highly activated metabolism at temperatures around 45 °C.
NATURAL SOURCE
Lactobacilli are ubiquitous and normally harmless. In humans and animals, they are found in the intestinal tract and perform many beneficial functions, including immunomodulation, suppression of enteric pathogens, and maintenance of intestinal flora. On the contrary, Lactobacilli decompose plant material and are responsible for spoiling vegetables, fruits, beverages, and other nutrients. L. casei and L. brevis are two of the most common beer-spoilage organisms. Certain lactobacilli like L. delbrueckii are known to cause urinary tract infections.1
ADVANTAGES OF LACTOBACILLI
Life without Lactobacilli is unimaginable: They are used in the production of yogurt, cheese, chocolate, sourdough bread, sauerkraut, pickles, kimchi (a traditional Korean pickled dish), beer, wine, cider, and many other fermented foods. Lactobacilli are also important in animal feed (silage) production. They produce lactic acid, lower the pH and thereby inhibit the growth of bacteria.
Many studies have shown the beneficial effect of a healthy intestinal Lactobacilli-containing microflora. Their potential therapeutic roles include anti-inflammatory, anti-cancer, and boosting the immune system, among other benefits. As a result, Lactobacilli are often added to foods as a probiotic supplement.
IDENTIFICATION OF LACTOBACILLI AND BIFIDOBACTERIA
Selective media is widely used to differentiate lactobacilli based on their phenotype. Classical phenotypic tests for the identification of lactobacilli are based on physiological characteristics, like motility, growth temperature, respiratory type, and growth in sodium chloride, as well as on diverse biochemical characteristics, such as fermentation type, metabolism of carbohydrate substrates, production of lactic acid isomers, coagulation of milk and presence of specific enzymes like arginine dihydrolase. In Bergey’s Manual, Lactobacillus is described as a Gram-positive rod, non-spore forming, acid-fast negative, and catalase negative. The colony morphology on certain media is taken for presumptive identification. There are also three API tests (API 50 CH, LRA Zym, and API Zym) for identification, but the reliability of these tests has been questioned 2. Another interesting method is the protein fingerprint, where an SDS gel electrophoresis is made of the whole bacteria cell.
As a modern alternative, molecular biology-based methods, like PCR, can be consulted. However, they are often quite expensive. We provide a revolutionary molecular biology method – HybriScan® Rapid Microbial Test System that is rapid, easy, and cost-effective. Based on the detection of rRNA, this method completely avoids the need for PCR amplification. The sandwich hybridization test, called HybriScan®, is performed on a microtiter plate. The range of Lactobacilli detected by HybriScan® tests is listed in Table below.
References
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