Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue. It is used mainly to make a preliminary morphologic identification or to establish that there are significant numbers of bacteria in a clinical specimen. It cannot identify bacteria to the species level, and for most medical conditions, it should not be used as the sole method of bacterial identification.
Electron micrograph of a thin section of Neisseria gonorrhoeae showing the organizational features of prokaryotic cells. Note the electron-transparent nuclear region n packed with DNA fibrils, the dense distribution of ribosomal particles in the cytoplasm, more The Nucleoid Prokaryotic and eukaryotic cells were initially distinguished on the basis of structure: As the electron micrograph in Fig.
Under the light microscope, the nucleoid of the bacterial cell can be visualized with the aid of Feulgen staining, which stains DNA. Gentle lysis can be used to isolate the nucleoid of most bacterial cells. The DNA is then seen to be a single, continuous, "giant" circular molecule with a molecular weight of approximately 3 X see Ch.
The bacterial nucleoid, then, is a structure containing a single chromosome. The number of copies of this chromosome in a cell depends on the stage of the cell cycle chromosome replication, cell enlargement, chromosome segregation, etc. Although the mechanism of segregation of the two sister chromosomes following replication is not fully understood, all of the models proposed require that the chromosome be permanently attached to the cell membrane throughout the various stages of the cell cycle.
Bacterial chromatin does not contain basic histone proteins, but low-molecular-weight polyamines and magnesium ions may fulfill a function similar to that of eukaryotic histones. Surface Appendages Two types of surface appendage can be recognized on certain bacterial species: Flagella occur on both Gram-positive and Gram-negative bacteria, and their presence can be useful in identification.
For example, they are found on many species of bacilli but rarely on cocci. In contrast, pili occur almost exclusively on Gram-negative bacteria and are found on only a few Gram-positive organisms e. Some bacteria have both flagella and pili. The electron micrograph in Fig.
Figure A Electron micrograph of negatively stained E. B The long sex pilus can be distinguished from the shorter common pili by mixing E.
The protein subunits of a flagellum are assembled to form a cylindrical structure with a hollow core. A flagellum consists of three parts: The basal body traverses the outer wall and membrane structures. It consists of a rod and one or two pairs of discs.Its rigid structure gives the bacterial cell shape, surrounds the plasma membrane and provides prokaryotes with protection from the environment.
Article Summary: Amount and location of the peptidoglycan molecule in the prokaryotic cell wall determines whether a bacterium is Gram-positive or Gram .
In a Gram stain test, bacteria are washed with a decolorizing solution after being dyed with crystal srmvision.com adding a counterstain such as safranin or fuchsine after washing, Gram-negative bacteria are stained red or pink while Gram-positive bacteria retain their crystal violet dye.. This is due to the difference in the structure of their bacterial cell wall.
Bacterial Cell Structure Objectives: 1) List the three parts of bacterial taxonomy.
2) Compare and contrast gram-positive, gram-negative and Acid Fast cell envelopes with regards to structure and chemical composition 3) Describe the Gram stain procedure 4) List the 3 components of bacterial cell envelopes that are highly toxic to mammalian cells.
A chemical that helps retain the stain in certain cells, such as iodine added to crystal violet stain. Differential Stain Makes use of two or more dyes and distinguishes between two kinds of organisms or between two different parts of an organism.
Gram-negative bacteria cannot retain the violet stain after the decolorization step; alcohol used in this stage degrades the outer membrane of gram-negative cells, making the cell wall more porous and incapable of retaining the crystal violet stain.
The Gram stain is a technique devised to categorise most bacteria into two sub-categories; gram positive and gram negative, based on the properties of the cell wall.
Cell wall The cell wall’s characteristics determine Gram staining.