GRAM POSITIVE COCCI: MORPHOLOGY, STRUCTURE AND PATHOGENS - BIOLOGY - 2025

The gram positive cocci are bacteria with spherical shapes that show positive results when subjected to Gram staining. This group is made up of 21 genera of bacteria capable of colonizing humans.

In addition to the spherical shape and the positive reaction to crystal violet, this group of prokaryotes does not possess spores. The rest of its characteristics are quite heterogeneous.

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The gram-positive cocci belong to the phylum called Actinobacteria, together with the aerobic and anaerobic actinomycetes, and other genera of facultative gram-positive bacilli.

They can be catalase positive aerobes such as Staphylococcus, Micrococcus, Stomatocuccus and Alloicoccus); catalase negative aerobes (such as Enterococcus) and anaerobes such as the genus Peptostreptococcus.

The habitat of these organisms includes the human body, such as mucous membranes and the normal flora of the skin, although it can also be found in other mammals and even birds.

Morphology

Bacteria have four basic morphology models: rods or bacilli, spiral or spirilla-shaped, comma or vibrios, and spherical cells or cocci.

Cocci are characterized by having rounded shapes and grouping in various ways, depending on the tendency of the cells to hold together and the planes of cell division. The diameter of these prokaryotes is variable, ranging between 0.8 and 10 um.

Some species, such as Enterococcus and Vagococcus, have flagella and are responsible for cell mobility.

Classification according to morphology

Depending on the grouping pattern, cocci can be classified into diplococci, tetrads, sarcins, streptococci, and staphylococci.

Bacteria that remain united in pairs after the process of cell division belong to the first group. An example of this group is the species Streptococcus pneumoniae.

In contrast, sarcines undergo divisions in three perpendicular directions, resulting in a cubic grouping. As an example of this group we have the genus Sarcina. The tetrads are divided in two perpendicular directions, generating a square arrangement.

Streptococci are characterized by a plane of division in a single plane, generating chains of four or more units. Finally, staphylococci resemble a bunch of grapes in shape, since the arrangement of the cells is irregular.

The cocci may not present any visible pattern or special grouping and their size is smaller than those mentioned above, in this case they are called micrococci.

However, the morphological classification can present intermediate variations with lanceolate or flattened cells, which are called coccobacilli.

By Mariana Ruiz (LadyofHats); translation to Portuguese by PatríciaR (File: Bacterial morphology diagram.svg), via Wikimedia Commons

Cell wall structure

The bacterial wall is of utmost importance, as it provides the necessary rigidity and gives the cell its shape. Bacteria belonging to the gram-positive category have a defined and complex organization in their cell wall, characterized by a thick layer (approximately 80 nm) of peptidoglycan.

Likewise, many of the gram-positive bacteria are distinguished by having a large amount of teichoic acids anchored to the wall. Said acids are polymers of ribitol or glycerol linked by a phosphodiester bond. In addition to teichoic acids, various proteins and polysaccharides can be found inside the wall.

The structure may have certain peculiarities depending on the species of study. In the case of Staphylococcus aureus, the wall has a series of amino acid residues crossed with the strands of the wall by five glycine residues. Thanks to this arrangement the wall is more rigid and tighter.

In the case of pathogenic bacteria, the cell wall is an element that plays an important role in virulence. One of the best known virulence factors is the M protein of streptococci, and polysaccharide C has been found in the species Streptococcus pneumoniae.

Gram stain

The Gram stain is one of the most used methodologies in microbiological studies, mainly because of its simplicity, speed and power. This technique makes it possible to differentiate between two large groups of bacteria, taking the structure of the cell wall as a criterion.

To carry out this staining, the bacteria are fixed by heat and crystal violet (a dye that precipitates with iodine) is applied. The next step is the removal of excess colorant. Then a second "contrast" dye called safranin is applied.

Gram positive bacteria retain their purple color, as their cell wall is composed of a thick layer of peptidoglycan that is organized like a mesh and surrounds the cell. As a reminder, the "positive purple" rule of thumb is used.

Peptidoglycan is present in all bacteria (except mycoplasma and ureoplasmas) and is composed of alternating carbohydrates of N-acetylglucosamine and N-acetylmuramic acid, which are linked by means of a β-1,4 bond.

Medically important pathogens

There are several genera of gram positive cocci that stand out in the field of medicine: Staphylococcus, Streptococcus, Enterococcus, Alloicoccus, among others.

In terms of metabolism, these genera are facultative aerobic and anaerobic. For their culture they can grow in usual medium, with the exception of streptococci, which necessarily require blood agar.

However, not all species of the genera mentioned are pathogenic, some can live as harmless hosts in humans.

Staphylococcus

The Staphylococcus aureus species, also known as golden staphylococcus, is found in the nasal mucosa and is the causative agent of various skin infections, be it folliculitis, boils, among others.

Organisms belonging to the species S. epidemidis are found mainly on the skin. This bacteria is capable of contaminating the prostheses when they are placed in the patient, causing infections in the material.

This genus is characterized by causing difficult-to-treat therapeutic problems, mainly by the rapidity with which they develop resistance to antibiotics.

Streptococcus

The Streptococcus pyogenes species can be found in the pharynx and is the cause of the condition called purulent pharyngitis, in addition to various infections. The species S. agalactiae is found in the digestive system, in the female reproductive tract and can have serious consequences in newborn children.

Lastly, the well-known Streptococcus pneumoniae can colonize the oropharynx, causing typical pneumonia and otitis.

References

1. Jiménez, JR (2012). Clinical infectology. Editorial El Manual Moderno.
2. Koneman, EW, & Allen, S. (2008). Koneman. Microbiological diagnosis: Text and Color Atlas. Panamerican Medical Ed.
3. Murray, P., Rosenthal, KS, & Pfaller, MA (2015). Medical microbiology. Elsevier Brazil.
4. Negroni, M. (2009). Stomatological microbiology. Panamerican Medical Ed.
5. Prats, G. (2006). Clinical microbiology. Panamerican Medical Ed.
6. Rincón, S., Panesso, D., Díaz, L., Carvajal, LP, Reyes, J., Munita, JM, & Arias, CA (2014). Last-line antibiotic resistance in Gram positive cocci: the post-vancomycin era. Biomedica: journal of the National Institute of Health, 34 (0 1), 191.