Gram-positive or Gram-negative Bacteria – What Does This Mean?

Microbiology Explained

Gram-positive or Gram-negative bacteria – What Does This Mean?
When we read about bacteria in the media, we often come across the term gram-positive or gram-negative bacteria. But what does this mean?

Actually, Gram is the name given to a technique developed in 1884 by a Danish doctor named Hans Christian Joachim Gram. The popularity of this technique is due to the fact that it allows to quickly differentiate bacteria according to a characteristic of its cell wall: the amount of peptidoglycan. In fact, Gram-positive bacteria have a relatively thick layer of peptidoglycan in their cell wall, while Gram-negative bacteria have only a small layer which is dissolved during the procedure, being unable to retain the applied dye. Thus, Gram-positive bacteria are characterized by their purple color when viewed under the microscope, after applying the Gram staining, while the Gram-negative bacteria present a pink color.

microscopic image of a Gram stain of mixed Gram-positive cocci (Staphylococcus aureus ATCC 25923, purple) and Gram-negative bacilli (Escherichia coli ATCC 11775, pink) Microscopic image of a Gram…

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