Only a tiny amount of bacteria found on Earth are actually associated with disease. The number associated with human disease is as low as 0.001%
Bacteria are extremely small when compared to humans, for example the width of an average bacterium is often no greater than 1.5μm (Around 1/25th the width of a human hair).
Being small is advantageous for the bacteria, it means that the rate of nutrient and waste transport to and from the cell remains high. Because this rate is inversely proportional to size, it means large multicellular organisms such as ourselves have had to develop organs and transport systems to ensure nutrients and waste are transported round the body efficiently.
Another advantage of being small is a rapid growth rate. Generations are very short in bacteria allowing for rapid evolution and adaption to new environments and environmental conditions.
Intracellular Bacterial Structures
Chromosomes and Plasmids
Unlike eukaryotes the DNA of bacteria is not enclosed within a nucleus. The DNA is supercoiled so tightly that it collapses on itself and forms a nucleoid which is free within the bacterial cytoplasm. The supercoiling occurs due to the absence of the packing protein histone.
Another dissimilarity to eukaryotes is the presence of DNA structures called plasmids. Plasmids are independent strands of DNA (also free in the cytoplasm) which may encode for advantageous traits such as antibiotic resistance. Plasmids are usually obtained via ‘horizontal transfer,’ which is essentially the sharing of DNA between bacteria.
Ribosomes are the organelles responsible for protein synthesis within the bacterium. They take messenger RNA (mRNA) and convert in into a protein by piecing together amino acids carried by transport RNA (tRNA).
This plasma membrane is also called the phospholipid bilayer due to the two layers of phospholipids which form a barrier around the bacterial cell. This membrane is responsible for the regulation of molecule transport in to and out of the cell. The bacterial intracellular membrane is also filled with a number of transmembrane proteins such as metabolic enzymes, signal proteins and transport proteins.
Other functions of the intracellular membrane are; energy generation, ion and molecule secretion, peptidoglycan synthesis and it houses regulatory proteins.
Essentially, everything within the bacterium is stored in the cytoplasm. DNA and RNA, can move freely around the cell in the cytoplasm as well as a number of enzymes, regulatory proteins, ribosomes and other organelles. Proteins and enzymes not found in the cytoplasm will therefore be transmembrane structures or located on the surface of the bacterium.
Bacterial Surface Structures
Fimbrae & Pili
Fimbrae are short proteins tubes that extend from the outer membranes of certain bacteria (specifically those from the Proteobacteria family, which includes a number of well known pathogens, such as Escherichia and Salmonella species). The purpose of these structures is to enable the bacteria to adhere to surfaces such as cells, which is important during pathogenesis. They act as adhesins (appendages which facilitate adhesion).
Pili are also responsible for mediating transfer of plasmids between bacteria, which is known as horizontal transfer.
Capsules & Slime Layers
Capsules are comprised primarily of polysaccharides and sometime proteins. They envelop the bacterium and are able to help protect it from phagocytosis and desiccation. A slime layer acts in a similar manner to fimbrae, in that it allows for the attachment to other surfaces and cells, however, it may also act as a ‘food’ reserve.
A large crystalline structure surrounding the bacterial cell wall. It consists of protein or glycoproteins and is believed to either; provide protection against harmful enzymes or pH extremes, protect against phagosome and complement or function as an adhesin.
Flagella are the whip-like ‘tail’ structures found on the outside of the bacterium. They are responsible for providing the bacterium with enhanced motility and comprise of multiple proteins, all of which work together to cause movement in the flagellum and thus in the bacterium.
Movement of bacteria by flagella is usually as a response to a stimulus such as nutrients, temperature, oxygen or other gases. A sensor protein will detect the stimulus resulting in a signaling cascade ultimately activating the flagella. This allows the bacterium to exploit the stimulus.
Cell Wall Structure
Bacteria can be divided in to two broad categories; gram negative and gram positive. The difference between these two groups is their cell structure. Gram staining is still used today in diagnostic microbiology to determine wether a bacterium is Gram-negative or Gram-positive.
A Gram-positive species of bacteria will retain the crystal violet-iodine complex used in the Gram stain, whereas a Gram-negative species will not. This means they can be distinguished by colour. Gram-positive will therefore appear purple under the microscope due to the retention of the crystal violet-iodine complex and Gram-negative will appear pink. A difference in the chemical and physical properties of the cell wall is responsible for whether or not the crystal violet-iodine complex is retained.
Gram-positive bacteria (such as Streptococcus, Staphylococcus, Clostridium, Listeria, etc) have a thick peptidoglycan layer at their outermost layer which retains the crystal violet-iodine complex, used during Gram-staining. The peptidoglycans form a molecular scaffold, consisting of repeating lattice of N-acetylglucosamine and N-acetylmuramic acid. The peptidoglycan scaffold is also laced with teichoic acids, the presence of these acids forms lipoteichoic acids which helps with adherence.
Beneath the thick peptidoglycan layer, the bacterial structure is ‘as expected’ i.e. a small periplasmic space before reaching the plasma membrane, the barrier around the bacterial cytoplasm.
Gram-negative bacteria (such as Escherichia, Salmonella, Pseudomonas, Legionella, Helicobacter etc.) are structurally more complex. They do not retain the crystal violet-iodine complex used during Gram-staining and instead take up the counter-stain making them appear pink under the microscope.
As with Gram-positive bacteria, there is a peptidoglycan layer around the plasma membrane, however, this layer is much thinner in comparison. There are also no teichoic or lipoteichoic acids in the peptidoglycan layer.
The most distinguishing structure of Gram-negative bacteria is the lipopolysaccahride (LPS) outer membrane, which is essentially another plasma membrane which envelops the cell. The LPS layer contains porins which span the membrane and allow passive diffusion of certain molecules into the periplasmic space (molecule diffusion varies, depending on the porin).
The LPS consists of polysaccharide chains with a lipid element known as lipid A. Lipid A is an endotoxin (i.e. not excreted) responsible for the toxicity of many Gram-negative bacteria. Lipid A is recognised by by the immune system and a very potent response is generated. A response is generated via Toll-like receptor 4, a protein predominantly responsible for detecting lipopolysaccharide which upon detection will initiate innate immune responses to remove the toxin – lipid A, and thus the bacteria.
The cell wall of Mycobacteria species (such as M. tuberculosis) contain high levels of mycolic acid. This is of benefit to the bacteria as the waxy-characteristic it bestows upon the cell wall offers protection from acids, alkalis and phagocytic digestion. However, it makes them hard to stain using the typical Gram-staining method. Instead a Ziehl-Neelsen stain is used to distinguish between Gram-positive and Gram-negative.
Mycoplasma & Haemoplasma
Mycoplasma (not be confused with mycobacteria) do not have a cell wall. Without a cell wall they are unable to reach large sizes and require cholesterol, amino acids and fatty acids to survive. The lack of a cel wall means antibiotics which target cell wall synthesis (such as penicillin) do not work as they have no target. This is of concern as many species of mycoplasma are pathogenic to humans.