Generality
Gram-positive bacteria are - as can be deduced from their own name - positive for the Gram stain by retaining a purple color after being subjected to such laboratory analysis.
Bacterial cell wall
The bacterial cell wall can be defined as a sort of rigid "container" that encloses the bacterial cell giving it a certain robustness and conditioning its shape.
The fundamental component of the cell wall is peptidoglycan (otherwise known as bacterial or murein mucopeptide ).
Peptidoglycan is a polymer consisting of long linear polysaccharide chains, joined together by cross-links between amino acid residues.
The polysaccharide chains are composed by the repetition of a disaccharide, consisting in turn of two monosaccharides, the N-acetylglucosamine (or NAG ) and the N-acetylmuramic acid (or NAM ), bound together by glycosidic bonds of the β-1 type, 6.
The disaccharides are then bonded to each other with β-1, 4 type glycosidic bonds.
Linked to each molecule of NAM we find a pentapeptide (ie a "tail" of five amino acids) which ends with two equal amino acids, in particular, with two molecules of D-Alanine .
It is precisely these terminal D-Alanines which - following the action of the enzyme transpeptidase - allow the formation of cross-links within the peptidoglycan.
More precisely, the transpeptidase originates a peptide bond between the third amino acid of a polysaccharide chain and the fourth amino acid of the parallel polysaccharide chain.
Functions
The cell wall does not only play a protective role against the bacterial cell but also regulates the transport of substances within it.
Therefore, it can be stated that the main functions of the cell wall are:
- Prevent the breakdown of bacterial cells due to osmotic pressure. In fact, very often, bacteria live in hypotonic environments, that is in environments in which large quantities of water are present and which are "more diluted" than the internal environment of the bacterial cell. This difference in concentration causes the water to pass from the external environment (less concentrated) into the bacterial cell (more concentrated) in an attempt to equalize the concentration between the two environments. The uncontrolled entry of water would cause the bacterial cell to swell until it burst (osmotic lysis).
The function of the cell wall is precisely to resist the external pressure of the water, thus preventing swelling and bacterial lysis.
- Protect the plasma membrane and the cellular environment from molecules or substances harmful to the same beat.
- Adjust the entry of nutrients into the bacterial cell.
Everything described so far can be found both in the Gram-positive and Gram-negative walls.
However, since the purpose of this article is to clarify the peculiarities of Gram-positive bacteria, only the characteristics of the cell wall of the latter will be described below and the Gram-negative wall will not be considered.
Gram-positive cell wall
In the Gram-positive wall the peptide bond between the polysaccharide chains of the peptidoglycan is formed through a pentaglycine bridge, that is a bridge consisting of five glycine molecules (an amino acid).
The Gram-positive cell wall is uniform and relatively thick (20-80 nm). It consists of several layers of peptidoglycan that are intersected by teicoic acids (polymers of alcohols and phosphates).
The Gram-positive wall is very polar and allows the permeation of hydrophilic molecules (such as those used in the Gram stain that will be described below) but not of hydrophobic compounds.
Gram stain
The Gram stain is a procedure conceived and developed in 1884 by a Danish bacteriologist, Hans Christian Gram.
The first stage of this procedure involves the preparation of a hot smear (ie a thin film of the material to be analyzed). In other words, a sample of the bacteria to be analyzed is placed on a slide and - through the use of heat - the micro-organisms are killed and blocked on the slide itself (hot fixation). After preparing the smear, you can proceed with the actual coloring.
The Gram staining technique involves four main phases.
Phase 1
The hot-fixed smear must be covered with the crystal violet dye (also known as gentian violet) for three minutes. In this way all the bacterial cells will turn purple.
Step 2
At this point, Lugol's solution (an aqueous solution of iodine and potassium iodide, defined as an etching agent as it is able to fix the color) is poured onto the slide and it is left to act for about a minute.
Lugol's solution is polar and penetrates the bacterial cell where it meets the crystal violet with which it forms a hydrophobic complex.
Since the Gram-positive cell wall is polar, the crystal-iodine hydrophobic complex cannot cross it thus remaining blocked within the bacterial cell itself.
Step 3
The slide is washed with a bleach (usually, alcohol or acetone) for about twenty seconds. Then wash it with water to stop the action of the bleach.
At the end of this phase, the Gram-positive bacteria cells will have retained the purple color.
Gram-negative cells, on the other hand, will be discolored. This happens because the alcohol attacks the lipopolysaccharidic structure of the external membrane typical of Gram-negative and absent in the Gram-positive, thus facilitating the loss of the previously absorbed dye.
Step 4
A second dye (usually, acid fuchsin or safranin ) is added to the slide and a couple of minutes is left to act.
At the end of this phase, the cells of the Gram-negative bacteria that have undergone discoloration in the previous phase will take on a color ranging from pink to red.
Types of Gram-positive bacteria
The large Gram-positive group includes many bacterial species.
Below, some of the bacteria belonging to this great category will be briefly listed.
Staphylococci (or Staphylococcus)
Staphylococci are cocci (ie spherical bacteria) belonging to the Staphylococcaceae family. Staphylococci grow in clusters.
Among the various types of pathogenic staphylococci, we recall:
- Staphylococcus aureus, responsible for various infections of the genito-urinary tract, of the snow system, of the skin, of bones, of joints, of the cardiovascular system, of the respiratory tract and of the eye. Furthermore, this bacteria is also responsible for opportunistic infections associated with impairment of the host's immune system, nosocomial opportunistic infections (ie infections contracted within health facilities) and food-borne diseases.
- Staphylococcus epidermidis, responsible for infections of the cardiovascular system, opportunistic infections associated with compromise of the host's immune system and nosocomial opportunistic infections.
- Staphylococcus saprophyticus, responsible for infections of the urinary tract.
Generally, antibiotics are used against this type of bacteria, such as penicillins, vancomycin, daptomycin, cephalosporins or fluoroquinolones.
Streptococci (or Streptococcus)
Streptococci are cocci that grow in pairs or form chains. Streptococci are capable of producing toxins capable of destroying red blood cells, ie they have haemolytic activity.
Streptococci can in turn be subdivided according to the degree of hemolysis they induce. We can therefore distinguish:
- Alpha-hemolytic (or α-haemolytic) streptococci which cause partial haemolysis;
- Beta-hemolytic (or β-haemolytic) streptococci which cause total hemolysis;
- Gamma-hemolytic (or γ-hemolytic) streptococci that do not cause hemolysis.
Among the pathogenic streptococci, we recall:
- Streptococcus pyogenes, responsible for infections of the respiratory tract, skin, bones, joints, cardiovascular system, digestive glands and peritoneal cavity. Furthermore, it is also responsible for opportunistic infections in guests with compromised immune systems.
- Streptococcus agalactiae, responsible for infections in the fetus and newborn, infections of the nervous system and respiratory tract.
- Streptococcus pneumoniae, responsible for infections of the respiratory tract, nervous system, cardiovascular system, digestive glands, peritoneal cavity and opportunistic infections associated with impairment of the host's immune system.
Usually, beta-lactam and macrolide antibiotics are used against streptococci.
Clostridium (or Clostridium)
Clostridia are bacilli (ie cylindrical bacteria) that - in adverse environmental conditions - are able to generate spores in order to survive.
Among the various existing pathogenic clostridia, we recall:
- Clostridium difficile, this bacteria can be part of the normal human bacterial flora and is responsible for opportunistic infections of the gastrointestinal tract. It is the main cause of pseudomembranous colitis which can occur in patients treated for a long time and at high doses with various types of antibiotic drugs. Antibiotics such as metronidazole, chloramphenicol, vancomycin or erythromycin are usually used against Clostridium difficile infections.
- Clostridium tetani, responsible for tetanus (or spastic paralysis). Generally, metronidazole or benzylpenicillin are used against this bacteria. A vaccine is also available to prevent infection.
- Clostridium botulinum, responsible for botulism (or flaccid paralysis).
Corinebacteria (or Corynebacterium)
The chorinebacteria are bacilli belonging to the Corynenacteriaceae family.
Among the numerous exponents of this genus, we recall the Corynebacterium diphteriae responsible for cutaneous diphtheria and respiratory diphtheria.
The antibiotics usually used to treat diphtheria are penicillins, cephalosporins, clindamycin and erythromycin.
A vaccine is also available to prevent infection.
Other Gram-positive
Other Gram-positive bacteria are:
- Bacillus anthracis (known as anthrax), responsible for the onset of cutaneous, pulmonary and gastrointestinal anthrax;
- Listeria monocytogenes, a bacillus responsible for the onset of meningitis, encephalitis, meningoencephalitis and brain abscesses;
- Enterococcus faecium and Enterococcus faecalis, two cocci that normally populate the human intestinal bacterial flora, but which may be responsible for nosocomial opportunistic infections of the urinary tract, septicemia or endocarditis.
