<P> The dictyostelids are another group formerly classified among the fungi . They are slime molds that feed as unicellular amoebae, but aggregate into a reproductive stalk and sporangium under certain conditions . Cells of the reproductive stalk, as well as the spores formed at the apex, possess a cellulose wall . The spore wall has been shown to possess three layers, the middle of which is composed primarily of cellulose, and the innermost is sensitive to cellulase and pronase . </P> <P> Around the outside of the cell membrane is the bacterial cell wall . Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by unusual peptides containing D - amino acids . Bacterial cell walls are different from the cell walls of plants and fungi which are made of cellulose and chitin, respectively . The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan . The cell wall is essential to the survival of many bacteria, although L - form bacteria can be produced in the laboratory that lack a cell wall . The antibiotic penicillin is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse . The lysozyme enzyme can also damage bacterial cell walls . </P> <P> There are broadly speaking two different types of cell wall in bacteria, called Gram - positive and Gram - negative . The names originate from the reaction of cells to the Gram stain, a test long - employed for the classification of bacterial species . </P> <P> Gram - positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids . In contrast, Gram - negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins . Most bacteria have the Gram - negative cell wall and only the Firmicutes and Actinobacteria (previously known as the low G + C and high G + C Gram - positive bacteria, respectively) have the alternative Gram - positive arrangement . These differences in structure can produce differences in antibiotic susceptibility, for instance vancomycin can kill only Gram - positive bacteria and is ineffective against Gram - negative pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa . </P>

Which structure accounts for most of the rigidity of a typical plant cell