Gram-Positive Bacteria and Actinobacteria

Overview of Gram-Positive Bacteria and Actinobacteria

Actinobacteria are Gram-positive bacteria with high guanine and cytosine content in their DNA and can be terrestrial or aquatic.

Learning Objectives

Discuss the characteristics associated with Actinobacteria

Key Takeaways

Key Points

  • Actinobacteria include some of the most common soil life, freshwater life, and marine life, playing an important role in the decomposition of organic materials, such as cellulose and chitin, and thereby playing a vital part in organic matter turnover and carbon cycle.
  • Actinobacteria are well-known as secondary metabolite producers and are hence of high pharmacological and commercial interest, since they can produce antibiotics like actinomycin.
  • Actinobacteria are responsible for the peculiar odor emanating from the soil after rain (petrichor), mainly in warmer climates.

Key Terms

  • actinomycin: Any of a class of toxic polypeptide antibiotics found in soil bacteria of genus Streptomyces.
  • actinobacteria: A group of Gram-positive bacteria with high guanine and cytosine content in their DNA

Actinobacteria is one of the dominant phyla of bacteria. They are Gram-positive bacteria with high guanine and cytosine content in their DNA and can be terrestrial or aquatic. Analysis of glutamine synthetase sequence has been suggested for their phylogenetic analysis.

Actinobacteria include some of the most common soil life, freshwater life, and marine life, playing an important role in the decomposition of organic materials, such as cellulose and chitin; thereby playing a vital part in organic matter turnover and carbon cycle. This replenishes the supply of nutrients in the soil and is an important part of humus formation.

Other Actinobacteria inhabit plants and animals, including a few pathogens, such as Mycobacterium, Corynebacterium, Nocardia, Rhodococcus, and a few species of Streptomyces.

image

Actinomyces israelii: Scanning electron micrograph of Actinomyces israelii.

Actinobacteria are well-known as secondary metabolite producers and are hence of high pharmacological and commercial interest. In 1940 Selman Waksman discovered that the soil bacteria he was studying made actinomycin, a discovery for which he received a Nobel Prize. Since then, hundreds of naturally-occurring antibiotics have been discovered in these terrestrial microorganisms, especially from the genus Streptomyces.

Some Actinobacteria form branching filaments, which somewhat resemble the mycelia of the unrelated fungi, among which they were originally classified under the older name Actinomycetes. Most members are aerobic, but a few, such as Actinomyces israelii, can grow under anaerobic conditions. Unlike the Firmicutes, the other main group of Gram-positive bacteria, they have DNA with a high GC-content, and some Actinomycetes species produce external spores.

Some types of Actinobacteria are responsible for the peculiar odor emanating from the soil after rain (petrichor), mainly in warmer climates. The chemical that produces this odor is known as Geosmin. Most Actinobacteria of medical or economic significance are in subclass Actinobacteridae, order Actinomycetales. While many of these cause disease in humans, Streptomyces is notable as a source of antibiotics.

Non-Spore-Forming Firmicutes

The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure and some of which do not produce spores.

Learning Objectives

Discuss the role of non-spore forming Firmicutes in industrial applications, specifically lactic acid bacteria (LAB)

Key Takeaways

Key Points

  • Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions.
  • The lactic acid bacteria (LAB) comprise a class of Firmicutes that are Gram-positive, low-GC, acid-tolerant, generally non-sporulating, and non-respiring.
  • The lactic acid bacteria (LAB) are rod-shaped bacilli or cocci characterized by an increased tolerance to a lower pH range.
  • LAB are amongst the most important groups of microorganisms used in the food industry and are the most common microbes employed as probiotics.

Key Terms

  • endospore: A dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum.
  • probiotic: Describing any dietary supplement that contains live bacteria for therapeutic purposes.
  • organoleptic: Of or pertaining to the sensory properties of a particular food or chemical: its taste, color, odor and feel.

Firmicutes

From Latin: firmus, strong; cutis, skin; referring to the cell wall. These are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative.

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Humans use of prokaryotes: This is a microscopic image of Bacillus subtilis (ATCC 6633) with a gram staining of magnification: 1,000. The oval, unstained structures are spores.

Scientists once classified the Firmicutes to include all Gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria.

They have round cells, called cocci (singular, coccus), or rod-like forms (bacillus). Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage.The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are obligate or facultative aerobes, and the Mollicutes.

LACTIC ACID BACTERIA (LAB)

These comprise a class of Firmicutes and are Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics.

These bacteria, usually found in decomposing plants and lactic products, produce lactic acid as the major metabolic end-product of carbohydrate fermentation. This trait has, throughout history, linked LAB with food fermentations as acidification inhibits the growth of spoilage agents. Proteinaceous bacteriocins are produced by several LAB strains and provide an additional hurdle for spoilage and pathogenic microorganisms.

Furthermore, lactic acid and other metabolic products contribute to the organoleptic and textural profile of a food item. The industrial importance of the LAB is further evinced by their generally recognized as safe (GRAS) status, due to their ubiquitous appearance in food and their contribution to the healthy microflora of human mucosal surfaces, particularly the gastrointestinal tract.

The lactic acid bacteria (LAB) are rod-shaped bacilli or cocci, characterized by an increased tolerance to a lower pH range. This aspect partially enables LAB to outcompete other bacteria in natural fermentation, as they can withstand the increased acidity from organic acid production (e.g., lactic acid).

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Streptococci: Light microscopy view of streptococci, a non-sporulating lactic acid bacteria.

LAB PATHWAYS

LAB are amongst the most important groups of microorganisms used in the food industry. Two main hexose fermentation pathways are used to classify LAB genera. Under conditions of excess glucose and limited oxygen, homolactic LAB catabolize one mole of glucose in the Embden-Meyerhof-Parnas pathway to yield two moles of pyruvate. Intracellular redox balance is maintained through the oxidation of NADH, concomitant with pyruvate reduction to lactic acid. This process yields two moles of ATP per mole of glucose consumed.

Representative homolactic LAB genera include Lactococcus, Enterococcus and Streptococcus. Heterofermentative LAB in turn use the pentose phosphate pathway, alternatively referred to as the pentose phosphoketolase pathway. One mole of glucose-6-phosphate is initially dehydrogenated to 6-phosphogluconate and subsequently decarboxylated to yield one mole of CO2. The resulting pentose-5-phosphate is cleaved into one mole glyceraldehyde phosphate (GAP) and one mole acetyl phosphate. GAP is further metabolized to lactate as in homofermentation, with the acetyl phosphate reduced to ethanol via acetyl-CoA and acetaldehyde intermediates.

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Gram staining of Bacillus Subtilis: A Gram-positive, catalase-positive bacterium which is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate extreme environmental conditions.

In theory, end-products (including ATP) are produced in equimolar quantities from the catabolism of one mole of glucose. Obligate heterofermentative LAB include Leuconostoc, Oenococcus and Weissella.

PROBIOTICS

Strains of LAB are the most common microbes employed as probiotics. Most strains belong to the genus Lactobacillus.

Probiotics have been evaluated in research studies in animals and humans with respect to antibiotic-associated diarrhea, travelers’ diarrhea, pediatric diarrhea, inflammatory bowel disease, and irritable bowel syndrome (IBS).

In the future, probiotics will possibly be used for different gastrointestinal diseases, vaginosis, or as delivery systems for vaccines, immunoglobulins, and other therapies.

Firmicutes

The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure and some of which can form endospores.

Learning Objectives

Describe the characteristics associated with endospores found in Firmicutes

Key Takeaways

Key Points

  • Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions.
  • An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum.
  • The endospore consists of the bacterium’s DNA and part of its cytoplasm, surrounded by a very tough outer coating.
  • Endospores can survive without nutrients and they are resistant to ultraviolet radiation, desiccation, high temperature, extreme freezing and chemical disinfectants.

Key Terms

  • firmicutes: A phylum of bacteria, most of which have Gram-positive cell wall structure.
  • endospore: A dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum.

Firmicutes

The Firmicutes (Latin: firmus = strong, and cutis = skin, referring to the cell wall ) are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer- membrane that causes them to stain Gram-negative.

Scientists once classified the Firmicutes to include all Gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus; ).

ENDOSPORES

Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage. The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are obligate or facultative aerobes, and the Mollicutes. On phylogenetic trees, the first two groups show up as paraphyletic or polyphyletic, as do their main genera, Clostridium and Bacillus.

An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name “endospore” is suggestive of a spore or seed-like form (endo means within), but it is not a true spore (i.e. not an offspring). It is a stripped-down, dormant form to which the bacterium can reduce itself.

ENDOSPORE FORMATION

This is usually triggered by a lack of nutrients, and normally occurs in Gram-positive bacteria. It occurs when the bacterium divides within its cell wall. One side then engulfs the other. Endospores enable bacteria to lie dormant for extended periods, even centuries. When the environment becomes more favorable, it can reactivate itself to the vegetative state.

The endospore consists of the bacterium’s DNA and part of its cytoplasm, surrounded by a very tough outer coating. They can survive without nutrients and are resistant to ultraviolet radiation, desiccation, high temperature, extreme freezing and chemical disinfectants. They are commonly found in soil and water, where they may survive for long periods of time. Bacteria produce a single endospore internally.

The spore is sometimes surrounded by a thin covering known as the exosporium, which overlies the spore coat, which acts like a sieve that excludes large toxic molecules like lysozyme, is resistant to many toxic molecules and may also contain enzymes that are involved in germination. The cortex lies beneath the spore coat and consists of peptidoglycan.

The core wall lies beneath the cortex and surrounds the protoplast or core of the endospore. The core contains the spore chromosomal DNA which is encased in chromatin-like proteins known as SASPs (small acid-soluble spore proteins), that protect the spore DNA from UV radiation and heat. The core also contains normal cell structures, such as ribosomes and other enzymes, but is not metabolically active. Up to 20% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA. Dipicolinic acid could be responsible for the heat-resistance of the spore, and calcium may aid in resistance to heat and oxidizing agents.

ENDOSPORE POSITIONING

The position of the endospore differs among bacterial species and is useful in identification. The main types within the cell are terminal, subterminal, and centrally-placed endospores (]).

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Endospore morphology: Variations in endospore morphology: (1, 4) central endospore; (2, 3, 5) terminal endospore; (6) lateral endospore.

Terminal endospores are seen at the poles of cells, whereas central endospores are more or less in the middle. Subterminal endospores are those between these two extremes, usually seen far enough towards the poles but close enough to the center so as not to be considered either terminal or central. Lateral endospores are seen occasionally.

When a bacterium detects environmental conditions are becoming unfavorable it may start the process of endosporulation, which takes about eight hours. The DNA is replicated and a membrane wall, known as a spore septum, begins to form between it and the rest of the cell. The plasma membrane of the cell surrounds this wall and pinches off to leave a double membrane around the DNA, and the developing structure is now known as a forespore. Calcium dipicolinate is incorporated into the forespore during this time.

Next the peptidoglycan cortex forms between the two layers and the bacterium adds a spore coat to the outside of the forespore. Sporulation is now complete, and the mature endospore will be released when the surrounding vegetative cell is degraded.

Actinobacteria (High G + C Gram-Positive Bacteria)

Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content in their DNA.

Learning Objectives

Outline the characteristics associated with Actinobacteria

Key Takeaways

Key Points

  • Actinobacteria is one of the dominant phyla of the bacteria.
  • Actinobacteria include some of the most common soil life, freshwater life, and marine life, playing an important role in decomposition of organic materials, such as cellulose and chitin, and thereby playing a vital part in organic matter turnover and carbon cycle.
  • Actinobacteria are well known as secondary metabolite producers and hence of high pharmacological and commercial interest.
  • Some types of Actinobacteria are responsible for the peculiar odor emanating from the soil after rain (Petrichor), mainly in warmer climates.

Key Terms

  • actinobacteria: A group of Gram-positive bacteria with high guanine and cytosine content in their DNA
  • petrichor: The distinctive scent which accompanies the first rain after a long warm dry spell.
  • actinomycin: Any of a class of toxic polypeptide antibiotics found in soil bacteria of genus Streptomyces.

Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content in their DNA. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria. Analysis of glutamine synthetase sequence has been suggested for phylogenetic analysis of Actinobacteria.

Actinobacteria include some of the most common soil life, freshwater life, and marine life, playing an important role in decomposition of organic materials, such as cellulose and chitin, and thereby playing a vital part in organic matter turnover and carbon cycle. This replenishes the supply of nutrients in the soil and is an important part of humus formation. Other Actinobacteria inhabit plants and animals, including a few pathogens, such as Mycobacterium, Corynebacterium, Nocardia, Rhodococcus, and a few species of Streptomyces.

Actinobacteria are well known as secondary metabolite producers and hence of high pharmacological and commercial interest. In 1940 Selman Waksman discovered that the soil bacteria he was studying made actinomycin, a discovery for which he received a Nobel Prize. Since then, hundreds of naturally occurring antibiotics have been discovered in these terrestrial microorganisms, especially from the genus Streptomyces.

Some Actinobacteria form branching filaments, which somewhat resemble the mycelia of the unrelated fungi, among which they were originally classified under the older name Actinomycetes. Most members are aerobic, but a few, such as Actinomyces israelii, can grow under anaerobic conditions. Unlike the Firmicutes, the other main group of Gram-positive bacteria, they have DNA with a high GC-content, and some Actinomycetes species produce external spores. Some types of Actinobacteria are responsible for the peculiar odor emanating from the soil after rain (Petrichor), mainly in warmer climates. The chemical that produces this odour is known as Geosmin. Most Actinobacteria of medical or economic significance are in subclass Actinobacteridae, order Actinomycetales. While many of these cause disease in humans, Streptomyces is notable as a source of antibiotics. Of those Actinobacteria not in Actinomycetales, Gardnerella is one of the most researched. Classification of Gardnerella is controversial, and MeSH catalogues it as both a gram-positive and gram-negative organism.

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Actinomyces israelii: Scanning electron micrograph of Actinomyces israelii.