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8.8.3: 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.