The bacterium is a single-celled organism, without a nucleus and microscopic in size. It is the most primitive microorganism in the evolution of life on Earth.
Bacteria can be of different types:
- Gram-positive cocci: round-shaped bacteria that stain purple with Gram’s stain, such as staphylococci and streptococci.
- Gram-negative cocci: spherical bacteria that stain pink with Gram’s stain, such as meningococci and gonococci.
- Gram-positive bacilli: rod-shaped bacteria that stain purple/purple, such as mycobacteria and la Listeria monocytogenes.
- Gram-negative bacilli: filamentous bacteria that stain pink with Gram’s stain, such as Escherichia colipresent in the intestine.
- vibes: bacteria with a curved shape, such as Vibrio cholerae.
- Spirochetes: spiral-shaped bacteria such as leptospires and Helicobacter pylori.
- Mycoplasma: bacteria that do not have a cell wall, such as Mycoplasma and the Ureaplasma.
Below we present each of these types of bacteria.
Gram-positive cocci are bacteria that are seen under the light microscope as small circular dots. When stained with a special stain known as Gram’s stain (after the bacteriologist Hans Christian Gram), they appear purple or purple. This is because they have a thick cell wall, unlike Gram-negative bacteria.
Among the Gram-positive cocci there are pathogenic bacteria, that is to say, that cause disease, and commensal bacteria, which are part of the normal microbiota of humans.
Examples of Gram-positive cocci are streptococci such as Streptococcus pneumoniaewhich can cause pneumonia, staphylococci such as Staphylococcus aureus and the Gram-positive cocci of the intestine, the enterococci.
Gram-negative cocci are so named because they look like tiny dots when viewed under an optical microscope. They are Gram-staining negative, as they stain light red or pink instead of purple or purple like Gram-positive bacteria. This is because their cell wall is much thinner and less resistant to the staining procedure.
Gram-negative cocci can be found singly, in pairs (diplococci) or in long chains.
Examples of pathogenic Gram-negative cocci are the meningococcus Neisseria meningitidiscausative agent of bacterial meningitis, and gonococcus Neisseria gonorrhoeae which produces gonorrhea.
Gram-positive bacilli are bacteria that look like short threads or rods under the optical microscope. They stain purple-purple with Gram’s stain. The cell wall of Gram-positive bacilli is thick, made up of multiple layers of peptidoglycan.
Examples of Gram-positive pathogenic bacilli are the Corynebacterium diphtheriaeagent producing diphtheria, the Listeria monocytogenes and the Clostridium difficile.
Gram-negative bacilli are rod-shaped or filamentous bacteria that stain pink with the Gram stain. They can possess flagella, with which they mobilize.
Gram-negative bacilli can be pathogenic or form part of the normal microbiota of humans.
Examples of Gram-negative bacilli are found in the microbiota of the skin with the Acinetobacter. Bacilli also proliferate in the intestine, such as Escherichia coli and the Salmonella sp., which produces an intestinal condition known as salmonellosis. Other bacilli are found in the environment, such as Pseudomonas aeruginosa and the Actinobacillus.
Vibres are Gram-negative, rod-shaped organisms.
Examples of curved bacteria are the Vibrio cholerae, pathogen responsible for cholera, a gastrointestinal disease that can be fatal.
Spirochetes are spiral bacteria, generally Gram-negative.
We have examples of these bacteria in the Leptospira sp., causing a zoonosis known as leptospirosis, the Helicobacter piloand, which colonizes the stomach and produces gastritis, and the Treponema pallidumproducer of syphilis.
They are bacteria that don’t have a cell wall, so they don’t stain with Gram’s stain either. They can be cocci or bacilli and are very small in size.
Mycoplasmas are extremely difficult to observe under the optical microscope, so their discovery was only possible through the electron microscope.
We have examples of this type of bacteria in Mycoplasma pneumoniaewhich causes lung infections, and the ureaplasma, which produces urinary infections.
You may also be interested in seeing:
Gillespie, SH, Hawkey, PM (eds.)(2006). Principles and Practice of Clinical Bacteriology Second Edition. John Wiley & Sons.
Hollar, S. (Editor) (2012). A closer look at Bacteria, Algae, and Protozoa. Britannica Educational Publishing. New York