<Table> <Tr> <Td> </Td> <Td> This section may be too technical for most readers to understand . Please help improve it to make it understandable to non-experts, without removing the technical details . (March 2014) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section may be too technical for most readers to understand . Please help improve it to make it understandable to non-experts, without removing the technical details . (March 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> Bacteria are traditionally divided into the two groups: gram - positive and gram - negative, based on their gram - staining response . Gram - positive bacteria are also referred to as monoderms having one membrane, and gram - negative bacteria are also referred to as diderms, having two membranes . These groups are often thought of as lineages, with gram - negative bacteria more closely related to one another than to gram - positive bacteria . While this is often true, the classification system breaks down in some cases . A given bacteria's staining result, bacterial membrane organization, and lineage groupings do not always match up . Thus, gram - staining cannot be reliably used to assess familial relationships of bacteria . However, staining often gives reliable information about the composition of the cell membrane, distinguishing between the presence or absence of an outer lipid membrane . </P> <P> Of these two structurally distinct groups of prokaryotic organisms, monoderm prokaryotes are indicated to be ancestral . Based upon a number of different observations including that the gram - positive bacteria are the major reactors to antibiotics and that gram - negative bacteria are, in general, resistant to them, it has been proposed that the outer cell membrane in gram - negative bacteria (diderms) evolved as a protective mechanism against antibiotic selection pressure . Some bacteria such as Deinococcus, which stain gram - positive due to the presence of a thick peptidoglycan layer, but also possess an outer cell membrane are suggested as intermediates in the transition between monoderm (gram - positive) and diderm (gram - negative) bacteria . The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide; the archetypical diderm bacteria, in which the outer cell membrane contains lipopolysaccharide; and the diderm bacteria, in which the outer cell membrane is made up of mycolic acid (e.g. Mycobacterium). </P>

How many membranes do gram negative bacteria have