The terms “gram-positive” and “gram-negative” are used to describe the nature of bacterial cell walls. The cell wall of a particular kind of bacteria is determined at the laboratory through an experiment called gram-staining. Determining whether a particular bacteria is gram-positive or gram-negative is important in the identification of bacterial species especially in identifying pathogenic bacteria that cause diseases to humans and animals.
There are structural differences between gram-positive and gram-negative bacterial cell walls. These differences are discussed below.
Difference in the Number of Peptidoglycan Layers that Constitutes the Cell Wall
Gram-positive bacteria have more peptidoglycan layers than gram-negative bacteria. As a result, the cell wall of gram-positive bacteria is thicker than the cell wall of gram-negative bacteria. Moreover, the gram-negative bacterial cell wall is more prone to mechanical breakage by having only few layers of peptidoglycan.
Peptidoglycan is a structural molecule that constitutes bacterial cell wall. Peptidoglycan molecules are joined together to form a peptidoglycan layer and several layers of peptidoglycan are joined together to form a thick and rigid cell wall that protects the internal structures of bacteria from damages brought about by external forces. Bacterial cell wall prevents the entry of molecules from the outer environment that can cause harm to the bacteria.
Gram-Negative Bacterium has an Outer Membrane, Gram-Positive Bacterium Hasn’t
Gram-negative bacterial cell wall is different from gram-negative bacterial cell wall by having an outer membrane that covers the peptidoglycan layer. The peptidoglycan layers are attached to the outer membrane by lipoproteins.
The outer membrane of gram-negative bacteria is composed of lipoproteins, lipopolysaccharides (LPS), and phospholipids. The membrane helps the bacteria to survive considering the presence of external elements and forces that can harm the bacteria. The outer membrane is negatively charged, and this helps prevent the bacteria from being phagocytosed (by macrophages for example). The outer membrane also acts as a barrier for the disastrous effects of antibiotics, digestive enzymes, detergents, heavy metals, and among others.
The lipopolysaccharides (LPS) of gram-negative bacteria act as bacterial antigens. These antigens are used in the laboratory to identify a bacterial species. This is possible since each bacterium has different LPS antigen to other bacteria. Today, there are now laboratory tests that detect the antigen specific for a single bacterial species. It is now possible to identify what particular bacterium is causing an infection to an individual or even to an animal.
Difference in the Transport of Nutrients and Other Compounds towards the Bacterial Cytoplasm
Gram-positive bacteria have molecules called techoic and lipotechoic acids that transport important nutrients from the external environment towards the bacterial cytoplasm. The molecules are embedded in the peptidoglycan layer in the cell wall and regulate the entry of substances from the external environment. The molecules also act as bacterial antigens that can be detected through laboratory tests and they are utilized in the identification of bacterial species.
Gram-negative bacteria don’t have techoic and lipotechoic acids but do have molecules called porins. Porins are doughnut-shaped proteins that traverse the bacterial cell wall and create a channel for the passage of nutrients and other compounds needed by the bacteria to survive.
Labeled Diagram of Gram-Positive and Gram-Negative Bacterial Cell Walls