Tag: living world and classification of microbes

The Gram-negative cell wall is characterized by presence of outer membrane, a thinner peptidoglycan layer and extensive periplasmic space surrounding the peptidoglycan. The uppermost layer of the outer membrane has lipopolysaccharide, while the innermost layer is a typical lipid layer. Presence of membrane channels formed by porin proteins in outer membrane impart make them less penetrable as compared to Gram positive cells. Gram positive cells are characterized by presence of single thick layer of cell wall which is made up of peptidogycan, teichoic acid and lipoteichoic acid. They lack outer membrane and porin and have narrow periplasmic space. Plasma membrane is the unit membrane in both, Gram positive and Gram negative bacteria, present beneath cell wall with primarily phospholipids (making up about 30–40% of the membrane mass) and proteins (60–70%). The dense gelatinous solution surrounded by plasma membrane is cytoplasm which has same composition in both, Gram positive and Gram negative cells. Cilia are the eukaryotic structures which are absent in bacteria as they are prokaryotic. 

Outer membrane and periplasmic space are absent in Gram positive bacteria but present in Gram negative bacteria. The outer membrane in Gram negative bacteria is made up of lipopolysaccharides.

Cell wall of Gram +ve bacteria is made up of peptidoglycans (mucopeptide). Mucopeptide form about 50% of component of cell wall. Cell wall of Gram -ve bacteria is made up of single layer of peptidoglycans.

When a decolourizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane. A Gram-negative cell loses its outer lipopolysaccharide membrane and the inner peptidoglycan layer is left exposed. The crystal violet complexes are washed from the Gram-negative cell along with the outer membrane.

Pyrogens can be the so called endotoxins or lipopolysaccharides, cell wall components of Gram negative bacteria such as E. coli, Salmonella or Bordetella, but also of the cell walls from Gram positive bacteria, e.g., so called lipoteichoic acids (LTA). When bacteria or viruses invade the body and cause tissue injury, one of the immune systems responses is to produce pyrogens. These chemicals are carried by the blood to the brain, where they disturb the functioning of the hypothalamus, the part of the brain, that regulates body temperature.

Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in a thick layer in gram-positive bacteria. In a gram stain test, gram-positive bacteria retain the crystal violet dye, while a counterstain (safranin or fuchsine) added after the crystal violet gives all gram-negative bacteria a red or pink colouring.

Gram - negative bacteria display the following characteristics: outer membrane containing lipopolysaccharide (which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet; porins exist in the outer membrane, which act like pores for particular molecules; lipoproteins are attached to the polysaccharide membrane.

Gram positive cells are characterized by presence of single thick layer of cell wall which is made up of peptidogycan, teichoic acid and lipoteichoic acid. Teichoic acid is acidic polymer of ribitol phosphate /glycerol phosphate and is covalently linked to peptidoglycan layer. Acetic acid is organic compound and is the main component of vinegar. Glutamic acid is one of the 20 standard amino acids and also serves as neurotransmitter. Malic acid is organic compound and serves as important intermediate in number of biochemical pathways; Kreb cycle and Glyoxalate cycle, etc. Thus, the correct answer is option B.

Eosin is an acidic dye with negatively charged chromophore and stains positively charged molecules such as certain WBCs; presence of acidic substances in large quantity with negatively charged surfaces on bacterial cells inhibit their staining with eosin. Option A is incorrect. Haematoxylin is positive/basic dye which stains acidic substances carrying negative charge and is used with eosin in histology; option C is incorrect. Carmine is a natural dye produced by insect Dactylopius coccus and is used food and cosmetics; option D is incorrect. Gram staining includes staining of both Gram positive and Gram negative bacterial cell purple with crystal violet followed by stabilization of dye in peptidoglyacan layer with mordant—Gram’s iodine and application of alcohol to dissolve outer membrane and dye from peptidoglycan layer. Presence of thick peptidogycan layer in Gram positive cells make them retain the dye after alcohol treatment while Gram negative bacteria become colourless. Thus, option B is the correct answer. 

The archaebacteria are the primitive, single-celled, prokaryotic organisms. These are the ancient and oldest living organisms. These were the first organism which evolved on the Earth. These organisms have evolved to survive in the extreme conditions like the halophiles, thermophiles, acidophiles, methanogens, etc.