Structure of Prokaryotes

As a phospholipid bilayer the lipid portion of the outer membrane is largely impermeable to all charged molecules. However channels called porins are present in the outer membrane that allow for passive transport of many ions, sugars and amino acids across the outer membrane. Three porin molecules cluster together and span the outer membrane to form a narrow channel through which molecules smaller than about 600 to 700 daltons can pass.

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.

A chemoreceptor also known as chemosensor is a sensory receptor that transduces a chemical signal into an action potential. A chemosensor detects certain chemical stimuli in the environment. Attractants and repellents are detected by chemoreceptors. These chemoreceptor proteins may be located in the periplasmic space or the plasma membrane.

Bacteria that are naturally having no cell walls include Mycoplasma and Ureaplasma. Ureaplasma is a type of bacteria in the family of bacteria known as Mycoplasma. These bacteria are incredibly tiny and are unique in that they lack a cell wall. Because the cell wall is the part of the bacteria attacked by a number of antibiotics, some common antibiotics such as those in the beta lactam family are not effective against these bacteria.

Spirochetes have two or more specialized flagella (endoflagella) arising from opposite poles of the cell, which together constitute the so called axial filament that is located within the periplasmic space between the flexible cell wall and an outer sheath. The rotation of the axial filament relative to the cell body causes the entire bacterium to move forward in a corkscrew-like motion even through material viscous enough to prevent the passage of normally flagellated bacteria. Each axial filament is composed of fibrils extending toward each other between two layers of the cell wall.

Motile bacteria move about with structures called flagella. Many species in this family are highly motile with numerous flagella that allow several different modes of locomotion. Most of these movements are directly tied to the differential expression of flagella and other factors. When in liquid environments normal movement is facilitated by swimming.

Howard Berg and Douglas Brown at the University of Colorado used a novel three dimensional tracking microscope they had developed to follow the swimming of individual Escherichia coli bacteria. Swimming was found to consist of smooth runs interrupted roughly every second by transient tumbles or twiddles. Chemotaxis (movement toward chemical attractants and away from repellents) was based on the suppression of tumbles in cells that happened by chance to be moving up the gradient.

Many genera in the high GC gram positive bacterial group such as Mycobacterium and Nocardia contain mycolic acids embedded in the peptidoglycan. Mycolic acids are a class of waxy extremely hydrophobic lipids. Certain genera contain very large amounts of this lipid and are difficult to gram stain. These genera may be identified by the acid fast staining technique.

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.

They are polymers of glycerol or ribitol joined by phosphate groups and are not present in gram negative bacteria like Escherichia. They give the gram positive cell wall of Bacillus an overall negative charge due to the presence of phosphodiester bonds between teichoic acid monomers.

Some bacteria make glycocalyx material that forms capsules around their cell walls. This property increases the virulence of the species. The capsule resists the host’s defenses by impairing phagocytosis (the process by which certain cells of the body engulf and destroy microbes) by evading the complement and antibody body defense responses. The chemical nature of the capsule appears to prevent the phagocytic cell from adhering to the bacterium.

It is happening during stage 3 of endospore formation. In second stage an inward folding of the cell membrane to enclose part of the DNA occurs and produces the forespore septum which is followed by the next stage in which the membrane continues to grow and engulfs the immature spore in a second membrane.

 In many species including B. subtilis, the coat is the outermost spore structure and in other species such as the pathogenic organisms B. anthracis and B. cereus, the spore is encased in an additional layer called the exosporium. Both the coat and the exosporium have roles in protection of the spore and in its environmental interactions. The spore coat also protects the spore from some chemicals such as hydrogen peroxide (H₂O₂) but not others such as organic solvents.

Some bacteria also use flagella to move over the tops of solid surfaces in a form of movement called swarming. Swarming motility is operationally defined as a rapid multicellular bacterial surface movement powered by rotating flagella. Swarming motility was first reported by Jorgen Henrichsen and has been mostly studied in genus Serratia, Salmonella, Aeromonas, Bacillus, Yersinia, Pseudomonas, Proteus, Vibrio, and Escherichia.

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. Examples of bacteria having subterminal endospore include Bacillus subtilis.