Most bacteria are characterized into two major categories called gram negative and gram positive. It is divided by special lab technique staining introduced by Christian Gram 1884. Gram stains are weakly alkaline solution of crystal violet or gentian violet. The ability of an organism to retain the color of the stain due to its cell wall structure is the major factor that differentiates between a Gram positive and Gram negative organism. The bacteria that retain the color of the stains (blue) are called Gram positives while the bacteria which loose the color of the stain (red) are called gram negative bacteria.
Gram Positive Bacteria
Bacteria that stain dark purple or blue are known as gram positive bacteria. They have thick peptidoglycan layer outside the cell membrane which absorbs huge amount of violet dye to stain itself as purple. Examples of gram positives are streptococci species, staphlococci, b.cereus etc.
Gram Negative Bacteria
Bacteria that stain red or orange are known as gram negative bacteria. It happens because it loses the color after a step called decolorization step. Alcohol used in this stage degrades the outer membrane of gram negative cells making the cell wall more porous and incapable of retaining the dye. They have think proteoglycan layer sandwiched between outer and inner membrane. This layer is responsible to take the counter dye color. The examples of gram negatives are, serratia marcescens, shigella, E.Coli and klabsiella etc.
- Gram positives retain crystal violet dye while gram negative decolorizes to accept the counter stain and turn red.
- In gram positive the cell wall is 20 to 30 nanometer thick while in gram negative cell wall is 8 to 12 nanometer thick.
- The cell wall of gram positives is smooth while in gram negatives it is wavy.
- Peptidoglycan layer of gram positive is thick and multi-layered, while it is think and single in case of gram negative.
- Teichoic acid is present in many gram positive bacteria while it is completely absent in gram negative bacteria.
- Periplasmic space is absent in gram positive while it is present in gram negative.
- Outer membrane is absent in gram positive while it is present in gram negative.
- Porins occur in outer membrane thus they are absent in gram positives while present in gram negatives.
- Lipopolysacherides (LPS) content are virtually absent in gram positive bacteria while they are in high quantity in gram negative bacteria.
- Lipid and lipoprotein content is low in gram positive while it is high in gram negative due to presence of outer membrane.
- Mesosomes are quite prominent in gram positive while it is less prominent in gram negatives.
- In flagellar structure of gram positive bacteria there are two rings at the basal body while in gram negative there are four rings.
- Exotoxins are produced by gram positive while gram negatives can produce Exotoxins or endotoxins.
- Resistance to physical disruption is high in gram positive while it is low in gram negatives.
- In gram positive the cells wall his highly disrupted by lysozymes while it is low in gram negatives.
- Susceptibility to penicillin, sulphonamides and anionic detergent is high in gram positives while it is low in gram negatives.
- Susceptibility to chloramphenicol and tetracycline is low in gram positives while high in gram negatives.
- Gram positives are highly resistant to drying and sodium azide while gram negatives are of low resistance.
- Gram positives are highly inhibited by basic dye while gram negative are low.