Under this particular course, following bacterial groups have been selected to study. Here we discuss the characteristics & importance of the main generas of this lactobacillales order such as Streptococcus and Lactobacilluus.
- Order Lactobacillales
- Order Enterobacteriales
- Order Bacillales
- Order Clostridiales
- Order Pseudomonales
- Order Rhodospirillales
- Order Spirochaetales
The above bacterial groups are studied concentrating on the following criteria,
- General characteristics (Morphological features, Habitats, Colony characteristics etc.).
- Sugar fermentation pattern.
- Special characteristics of the economically important genera of the order.
- Economic importance of the Order.
Phylum Firmicutes - Order Lactobacillales
General characteristics of Lactobacillales
Cell Shape – Rod or Cocci Shaped
Cell wall type – Gram positive
Respiratory pattern – Facultative anaerobic
Endospore production – Non-spore forming
Non-pigmented tiny colonies
Fastidious organisms – Members of this Lactobacillales, need lots of nutrients to grow. They cannot be grown on nutrient agar. Due to their special nutrition requirements, they need special growth media to grow in a laboratory.
Ex: MRS Agar, Tomato Juice Agar
These medias are rich of nutrients (Especially Amino acids). pH of these medias is very low. Therefore other competitive bacteria cannot grow.
Extra-cellular Polymeric Substances (EPS) – Some of them are produced EPS.
Production of these substances can be observed as Gel-like Substances around the colony.
Habitat – Body cavities of Humans and other Mammals (Vaginal, Gastro-intestinal and etc.) Specially grow on food products (Especially milk and dairy products).
Sugar Fermentation Pattern of Lactobacillales
Members of this Order show two distinct Fermentation Patterns/Pathways;
1. Homofermentative pathway
2. Heterofermentative pathway
Lactic acid bacteria (L.A.B) , as a group exhibits enormous capacity to degrade different kinds of sugars and related compounds. Generally, the main end product is Lactic Acid. As a result of L.A.B. adapting to various environmental conditions, there may be significantly different end-product patterns.
Under conditions of; Excess glucose Less Oxygen
Homalactic (Homofermentative) L.A.B. catabolize one mole of Glucose in Embden- Mayerhoff Pathway to yield two moles of Pyruvate. Intracellular Redox balance is maintained through the oxidation of NADH, concomitant (Parallel) with Pyruvate reduction to Lactic acid. This process yields two ATP molecules per one Glucose molecule consumed.
Transport and Phosphorylation of sugars occur by;
- Transport of free glucose and Phosphorylation by an ATP-dependent hexose kinase enzyme. (Other sugars likes Mannose and Fructose enter the major pathways at the level of Glucose-6-phosphate or Fructose-6-phosphate after isomerization or Phosphorylation or both.)
- The PEP (phosphoenolpyruvate) group translocation, also known as the phosphotransferase system (PTS), in which PEP is the Phosphoryl donor for the uptake of sugar. Some species of L.A.B use the PTS for transport of Galactose only; others use the PTS for all sugars.
Heterofermentative L.A.B. use the Phosphoketolase Pathway (Pentose phosphate pathway) to dissimilate sugars. In this pathway, Glucose-6-phosphate is initially dehydrogenated to 6-phosphogluconate and subsequently decarboxylated to yield one mole of carbondioxide. The resulting Pentose-5-phosphate is cleaved in to one mole of Glyceraldehyde phosphate (GAP) and one mole of Acetyl Phosphate. GAP is further metabolize and cleaved in to Lactate as in the Homofermentation pathway, with the Acetyl phosphate reduced to Ethanol via Acetyl-CoA and Acetaldehyde intermediates. Theoretically, end products (Lactic acid, CO2 & Ethanol) are produced in equimolar quantities from the metabolism of one mole of Glucose.
The above two fermentation pathways can be differentiated using Gibson’s semi solid medium.
Optical configuration of the Lactic acid
In the fermentation pathway of these bacteria, pyruvic acid is converted to lactic acid by the action of the enzyme which is called lactic dehydrogenase. Optical configuration of the lactic acid is depend on the action of the above enzyme. Some L.A.B. contain L-lactic dehydrogenase enzyme, which result in producing L-lactic acid where as others have D- lactic dehydrogenase which produces D-lactic acid as their end product. Some species have both forms of the enzymes thus result in giving a racemic mixture of lactic acid. (equimolar mixture of L-lactic and D-lactic acid).
Lactic acid bacteria have a limited capacity to synthesize amino acids using inorganic nitrogen sources. Therefore, they are depended on preformed amino acids being present in the growth medium as a source of nitrogen. The requirement for amino acids differs among species and strains within species. Some strains are prototrophic for most amino acids, whereas others may require 13-15 amino acids. Since quantities of free amino acids present in their environment are not sufficient to support the growth of bacteria to high cell density, they require a proteolytic system capable of hydrolyzing peptides and proteins in order to obtain essential amino acids. All dairy Lactococci used for acidification of milk (Ex- Cheese manufacture) have proteolytic activity. The lactococcal proteolytic system consists of enzymes outside the cytoplasmic membrane, transport systems and intracellular peptidases. The proteolytic activity of L.A.B contribute to the development of the Flavor, Aroma, and texture of fermented products.
Special characteristics of the economically important genera of the Lactobacillales
- Spherical, Gram positive bacteria. Cell division occurs along a single axis in these bacteria, and thus they grow in pairs or chains.
- Some are pathogenic. Causes;
Streptococcal pharyngitis (Strep throat), Pink-eye, Meningitis, Bacterial pneumonia etc. However, many are non-pathogenic. Forms a part of commensal human microbiome of mouth, skin, intestine and upper respiratory tract.
- Hemolytic organisms;
These organisms produce hamolysin enzyme to lysis Red Blood Cells (R.B.C) . Species of the genus Streptococcus are classified based on their Hemolytic properties. Ability or the Inability to produce Hemolysin can be tested by growing the specific organism on Blood Agar. Possible results of growing Streptococcus spp. On Blood Agar as follows;
a.Clear Zone around the colony – Complete Hydrolysis of R.B.C
b.Greenish Zone around the colony – Partial hydrolysis of R.B.C
c.No green or Clear Zone – No Hydrolysis
Streptocoocal species are classified according to their hemolytic properties. Species which perform oxidation of Iron in hemoglobin within red blood cells and thus result in forming greenish zone around the colony when grown on Sheep Blood Agar; are known as α-Hemolytic organisms. Unlike α- Hemolytic Streptococci, Some species perform complete rupture of R.B.C. producing a clear zone around the colony on Blood Agar. These are known as β- Hemolytic Streptococci. Other streptococci species, do not perform any hemolytic activity are known as ɤ-Hemolytic Streptococci.
Hemolyses of Streptococcus spp. (left) α-hemolysis (S. mitis); (middle) β-hemolysis (S. pyogenes); (right) γ-hemolysis (= non-hemolytic, S. salivarius)
To cause complete rupture of R.B.C. β-Hemolytic organisms produce different enzymes; Fibrinolysin (Lysis Fibrin), Leucolysin (Lysis White Blood Cells) and Hyaluronidase (Lysis hyaluronic acid).
- Gram positive, Rod shaped bacteria. Facultative anaerobic. Occasionally some are Microaerophillic.
- Major part of the lactic acid Bacteria group. Can convert Lactose and other sugars to Lactic acid through fermentation.
- In humans, these organisms are mainly found in Vagina forming a mjor part of the Vaginal Flora. And in gastro-intestinal tract where they form a small portion of the gut flora.
- Some species are associated with dental cavities and tooth-decay (dental caries).
- Many species are prominent in decaying plant material.
- Due to the production Lactic acid, its environment becomes acidic which eventually inhibits the growing of some harmful microbes.
- Many Lactobacilli are Homofermentative. But there are exceptions. Depending on the metabolism, these Lactobacilli species can be divided in to three groups;
Economic Importance of the Lactobacillales
Lactic acid bacteria are extremely rich in economic value. Their importance can be studied under several fields.
a) Food Industry : Due to their ability to produce acid in their fermentation, they are of immense importance in fermented food industry such as Yogurt, Cheese, Sauerkraut, and in fermented vegetable. They are used as starter cultures particularly in production of dairy products such as Yogurt. As they lower the pH of the medium, they also create the signature “sourness” associated with Yogurt. These organisms change the texture of the food and add flavors and aroma.
Yogurt – Lactobacillus bulgaricus and Lactobacillus thermophiles
Fermented vegetables – Lactobacillus planetarium and Lactobacillus fermentum
Curd – Lactobacillus bulgaricus and Lactobacillus fermentum
b) Probiotics and bio therapeutics : Lactobacillus spp. are used as probiotics to increase the quality of the human gastro-intestinal tract.
Ability of Lactobacillus species to acidify its environment through lactic acid fermentation, allows it to inhibit the growth of harmful microbes (Such as Salmonella, Shigella etc.) when the intestine is colonized using them. Ingesting some strains of L.A.B as probiotics, have several other health benefits. Few are mentioned below;
> As treatments for diarrhea and antibiotic-associated diarrhea.
> Some strains have proven to help Lactose intolerance patients by enabling them to tolerate more lactose.
> Reduces serum cholesterol levels: presumably by breaking down the bile in the gut.
Some strains of Lactobacillus spp. and other L.A.B possess potential therapeutic properties including anti-inflammatory and anti-cancer activities. Lactobaciilus can also be used to restore particular physiological balance such as in the vaginal eco-system. Their role is;
(1) to physically protect the vaginal epithelium by building a thick layer separating the epithelium from pathogens,
(2) to physiologically keep the balance of the vaginal ecosystem in maintaining the pH at ~4.5, and
(3) generating hydrogen peroxide against pathogens. Lactobacilli are highly tolerant to low pH and can easily maintain low pH and protect the vaginal eco-system from Gram-negative and Gram-positive bacteria.
c) Pathogenicity: Dental Caries: Species of the genus Streptococcus (Ex- Streptococcus mutans, Streptococcus salivarius) are the main cause of bacterial tooth decay. Although considered as beneficial, some lactobacilli, to a lesser extent are associated with dental caries.
Lactobacilli characteristically cause existing carious lesions to progress, especially those in coronal caries. However, recent studies show probiotics can allow beneficial lactobacilli strains to populate sites on teeth, preventing streptococci pathogens from taking hold and inducing dental decay.
- Gram positive cocci, often occur in pairs (diplococci) or short chains.
- Difficult to distinguish from Streptococci from Physical characteristics alone.
- Facultative anaerobes.
- Tolerant to various harsh environmental conditions.
- Their Optimum temperature and pH are 10-45 °C and 4.5-10 pH. They can tolerate high NaCl concentration.
- Typically exhibit ɤ-Hemolytic Properties on Sheep’s blood Agar.
- Enterococcus feacalis is use as the indicator organism in several procedures to test fecal pollution in water (Enterococcus fecalis and Enterococcus fecium)
Lives in the intestinal tract of the humans ( Enterococcus feacalis).
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Gerard J.Tortora, Berdell R.Funke, C. L. C. (2010). MICROBIOLOGY AN INTRODUCTION (10th ed.). Pearson Education,Inc.
Michael T. Madigan, John M. Martinko, Kelly S. Bender, Daniel H. Buckley, D. A. S. (2015). BROCK BIOLOGY OF MICROORGANISMS (14th ed.). Pearson Education,Inc
Pasindu Chamikara – Microbiologist