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Gram staining is the first method in identifying and classifying bacteria into two major groups, that is; gram-positive and gram-negative. This is mainly done through basing their chemical and physical cell walls. Gram-positive bacteria have a thick compacted outer cell wall due to the existence of peptidoglycan making up about 90% of its composition. On the other hand, a gram-negative bacterium is chemically complex; its peptidoglycan is about 5-20% and doesn’t compose the outermost layer but lies in the outer and the inner membrane. According to Bergey (2001), “the outer membrane of the gram negative bacteria also has a plasma membrane though less permeable and is composed of lipopolysaccharides”. The purpose of this paper is to identify the nature of an organism using Bergey’s Manual of Systematic Bacteriology.
- Should the cell wall appear pink after staining; then it is a gram-negative bacterium due to its nature of retaining counter stain after the removal of decolorizer and gram-positive if it appears purple,
- Should the colony appear shiny on the agar plate; then it is gram-positive bacterium since that of gram-negative bacterium usually appears to be cloudy,
- Should the optical property of the colony tend to appear opaque; then it is a gram-positive colony since that of a gram-negative colony has a translucent optical structure.
The organism was a gram-positive bacterium of the mycobacterium genus. This was because; after the gram test was carried out, the organism had a purple stain which is a characteristic of the gram-positive bacterium. Purple stain differentiates the gram-positive from the gram-negative bacterium which has a pink stain. This is brought about by the chemical reaction of the cell wall and the stain (Madigan et al, 2004). Gram-positive bacterium normally retains the primary stain due to the presence of a high percentage of peptidoglycan (close to 90%) in its cell wall (Gladwin & Trattler, 2007).
The bacteria’s colony had an opaque optical structure due to the presence of a high amount of peptidoglycan, also known as murein in the cell walls. These peptidoglycans were made up of heteropolymers of glycan strands, polysaccharides and teichoic acid which form short peptides. Bergey, et al (1994) argued that “murein/peptidoglycan are chains of alternating residues of N-acetylglucosamine and N-acetyl muramic acid which are Beta -1, 4-linked”. This differentiates the gram-positive bacterium from gram-negative since the peptidoglycan of the latter is simple and uniformed throughout.
The colony shape structure being circular suggested that the unknown organism was a gram-positive bacterium. Gram-positive bacterium normally has a circular structure due to the presence of closely linked short polypeptides of both L- and D- amino acids. Since these polypeptides are curved, they join together in circular strands that eventually form a circular structure (Bergey et al, 1994).
Polysaccharides are normally shiny. There was the presence of two types of polysaccharides which is a feature in gram-positive. According to Madigan et al (2004), “these polysaccharides are: arabinogalactans and arabinomannans and are linked to mycolic acids”. Their presence in the cell wall of gram-positive bacteria makes it look shiny in appearance. The presence of these features differentiates the organism from gram-negative bacterium whose cell walls have lipopolysaccharide, phospholipids, protein, lipoprotein and some peptidoglycan (Madigan et al, 2004).
The colony has a pigmentation that is cream and colored with a yellow tint which suggested that the organism was a gram-positive bacterium. This type of pigmentation is found in gram-positive bacteria of mycobactiriaceae family. Bergey (2001) notes that “many species form white or creamy colored colonies but some form bright yellow or orange colonies based on carotenoid pigments”.
By using the Gram reaction; the chemical and physical characteristics of the unknown organism suggested that it was gram-positive bacteria from mycobactriaceae family and the mycobacterium genus. This was because the organism had a creamed-yellow pigmentation. There were also groups forming circular colonies which were shiny and opaque in nature. Lastly, there were remains of purple stains when passed through a gram stain reaction which is a characteristic of the gram-positive bacterium.
- Bergey, D., et al, (1994). Bergey’s Manual of Determinative Bacteriology. (Ninth Edition). New York: Lippincott Williams & Wilkins publishers.
- Gladwin, M., & Trattler, B. (2007). Clinical Microbiology made ridiculously simple. Miami: MedMasterI Press.
- Madigan, M., et al, (2004). Brock Biology of Microorganisms. (10th Edition). New York: Lippincott Williams & Wilkins publishers.