Cell wall in bacteria

[Lot3ch]

Is the difference between gram positive and negative bacteria defined as one
have a cell wall and the other not? Or is it simply the compositions of their cell walls
regarding peptidoglycan and liposaccharides? I'm asking because I've read somewhere
that there are gram positive bacteria without cell walls (classified as such only because
they are derived from gram positives not because they actually retain Gram crystals).

[MrMistery]

Stricly speaking, Gram negative refers to when a cell is colored red by the Gram staining method, and Gram positive refers to when a cell is colored in a sort of purple. Therefore, the vast majority of eukaryotic cells for example will be Gram negative.
However, in bacteria this staining can be correlated with a particular structure in the cell wall, which can be best represented by the following picture.
The Gram-positive bacteria are considered a systematic group, and quite a large one - they rival the proteobacteria in diversity. Among others they include the Mycoplasmas, the only bacteria known that lack cell walls(do not confuse them with some Archea that also lack walls). I don't know exactly what color a Mycoplasma would yield in the Gram stain, but my best guess is negative, like any other cell that lacks a wall. However,you need to understand that the Gram-positive bacteria is a systematic group derived mainly from molecular systematics(rRNA similarities) - the Mycoplasmas simply lost their walls because they do not need them, as they are intracellular parasites.
In conclusion I can give you this advice: learn the steps of the Gram staining(you can easily find it on the net) and try to ask yourself why exactly a cell turns out Gram+ or Gram-
Don't forget to study the picture

[Lot3ch]

Ah I see, thanks for the answer and will do on the Gram staining method.

[mysticshadow]

My teacher explained it simply, Gram positive cells have much more peptidoglycan in their cell wall, gram negative bacteria are multilayered with a thin layer of peptidoglycan in the middle. This explained both the gram staining and the effect of antibiotics.

[MrMistery]

Fairly decent explanation. Too simple for my taste though...