Bacterial spores and the sporulation process

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Original article: Spore batteriche e processo di sporulazione, by Marianna Minniti

Bacteria are single-celled microorganisms that hold many surprises. Among these, some can produce spores using the sporulation process. Bacteria, in particular, produce spores that have cellular characteristics of resistance and quiescence. Starting from these considerations, a study conducted by some researchers is working on evaluating the longevity of the spores: it is The 500-year Microbiology Experiment”.

Bacterial spores: general characteristics

Bacterial spores are specialized cell forms for survival, even in adverse environmental conditions. They are always produced within a vegetative cell, which is why they are called endospores.

Morphologically, the spores of the different species have different shapes: elongated, oval, with a smooth or wrinkled surface, with or without appendages. As well as the shape, the position they take – in the center or one of the distal parts of the cell – is also important: they can be found in central or para-central, terminal or sub-terminal position and, more rarely, laterally.

From a structural point of view, bacterial spores consist of a central part (core) containing cytoplasm with ribosomes, DNA, calcium dipicolinate, and enzymes. To this is added a rudimentary cell wall. The core is then covered with a series of “sporal envelopes”. From the inside out, there is the cortex (consisting of a thick layer of modified peptidoglycan typical of bacterial spores), a second membrane (similar to the first), a protein tunic formed by an alternation of layers of lamellae and fibrils that generate an element similar to keratin and, finally, the exosporium (tunic) (Fig.1). The latter is present only in some groups of bacteria and is a lipoprotein membrane containing carbohydrates (teichoic acids, glucosamine).

 Bacillus anthracis endospore
Figure 1 – Structure of the Bacillus anthracis endospore. From the image, you can see the exosporium (EX), the outer membrane (SC), the cortex (CX), the inner membrane (CW), and the core (Source: ASM Digital Image Collection, Stahly)

Spores are metabolically inactive cells in a state of cryptobiosis (or latent life) comparable, in some way, to plant seeds.

Physiology of bacterial spores

A spore is an alternative form of life that aims to keep the species in a period of adverse conditions, waiting for favorable conditions. The spore has low water content, is resistant to dehydration, UV rays, and many other harmful agents. Specifically, in terms of resistance to UV rays, it is known that bacterial spores contain low molecular weight proteins called SASP (Small Acid Soluble Proteins): some of them interact with DNA and protect it from UV rays’ effects.

In any case, the spores can infect, particularly humans, with spores coming from the external environment and not necessarily by direct contact with an infected subject. This is due to the spore’s antigenic characters, which are identical to those of the vegetative form, in addition to some specific antigens.

Sporulation process

Sporulation is a mechanism that allows the bacterium to change its shape when there are environmental conditions that do not let it survive. Therefore, it is the morphogenetic process that helps the transition from vegetative form to spore. For the bacterium, the spore formation allows it to maintain a state of “dormancy”. It will “wake up” and resume its metabolic functions if the environment returns suitable for its survival.

The sporulation mechanism (Fig. 2) has several stages:

  • Thickening of the chromonema;
  • DNA duplication;
  • Separation of the nucleoid and formation of the asymmetrical dividing septum;
  • Formation of the prespore cell;
  • Incorporation phase: the peptidoglycan in the septum is degraded, and the mother cell integrally incorporates the prespore cell;
  • Formation phase of the cortex and the tunic: the first is deposited between the two membranes that envelop the prespore cell and is a specialization of the peptidoglycan. The tunic, composed of the acid dipicolinic (DPA), is crucial because it protects and stabilizes the hereditary material, in addition, to confer resistance to the cell;
  • Maturation and release phase: the spore is released by autolysis of the sporangium. Free in the environment, it will remain so until its germination.
sporulation phases
Figure 2 – Phases of sporulation (Source: www.biopills.net)

Spore-forming species of medical significance

The spore is an element of resistance mainly known in the medical field. Specifically, sporogenesis has a significant diagnostic value because it is a feature of some pathogenic bacterial species – for example, Bacillus anthracis, the causative agent of anthrax (anthrax). Others belong to the Clostridium genus: among these, it is essential to remember Clostridium botulinum, the causative agent of botulism, and Clostridium tetani, the etiological agent of tetanus (Fig. 3).

Spores of Clostridium tetani
Figure 3 – Spores of Clostridium tetani (source: www.slideplayer.it)

To detect bacterial spores, different types of staining can be used. Under the optical microscope, they are easily visible as highly refractive bodies, making visualization of the bacterial spore difficult. The most significant difficulty is the complexity of its wraps that makes it hardly permeable to the most common dyes. While it is possible to observe the spores also through “classic” colors such as that of Gram or Ziehl-Neelsen, it is good to consider colors that ensure a sharper and more contrasted image as the Schaeffer-Fulton coloring.

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