I've just finished a course of double antibiotics for Helicobacter pylori in my stomach. I looked up H. pylori and found that it was a gram-negative bacterium. I looked up gram-negative and didn't understand the definition. I assume that if there are gram-negative bacteria then there are also gram-positive ones. Can someone explain the terms gram-negative and gram-positive in basic, layman's terms?



  • 2
    $\begingroup$ Just to make @yamad's answer more layman friendly, essentially Gram-positive bacteria have thicker cell walls than Gram-negative bacteria. $\endgroup$
    – terdon
    Commented Feb 7, 2013 at 18:41
  • $\begingroup$ Why is this distinction important anyway? $\endgroup$ Commented Feb 21, 2013 at 17:52
  • $\begingroup$ It is simply one of the major ways that bacteria are categorised. The cell wall will affect what drugs can kill them for example. $\endgroup$
    – terdon
    Commented Feb 21, 2013 at 18:38

2 Answers 2


@yamad is right on - Gram stain is a major way of categorizing bacteria. Its worth mentioning what the Gram stain color means.

While not failproof, the Gram stain test reflects a major division of the sorts of cells bacteria, helping to type the bacteria of interest

Gram stain reacts with the peptidogycan to create the purple color. The peptidoglycan is a mesh of carbohydrate (sugar) chains with short peptides attached to them that gives the bacterial wall mechanical strength against osmotic changes in the environment (e.g. large concentrations changes in sugars or salts) and mechanical stresses such as shearing from current.


Gram Negative bacteria have two lipid membranes with peptidoglycan sandwiched in between them while Gram positive bacteria have a single lipid membrane with the peptidoglycan mesh surrounding the membrane, forming an outer coat to the bacteria. There is typically 5-10 times more peptidoglycan by weight in Gram Positive bacteria. As such these two distinct categories of Bacteria is often the first determination used to characterize a bacterium. Again while not without some error, Gram stains reflect a significant cell physiological division in bacteria.

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The Gram stain is one of a large number of techniques used to characterize bacteria. In particular, a bacterial species is usually either Gram-positive (purple when stained) or Gram-negative (pink when stained). In Gram-postive bacteria, the cell wall is thicker and has much more peptidoglycan compared to Gram-negative bacteria. The chemicals in the stain react with these molecules in the cell wall and result in the purple color. All of this information is well covered in the Wikipedia article on the subject.


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