gram

gram

[gram]
gram, abbr. g, unit of mass equal to 0.001 kilogram, the basic unit of mass in the metric system. The gram is the unit of mass in the cgs system. It is approximately equal to 0.035 avoirdupois ounce, or 0.0022 pound; a 1-pound mass equals about 453.6 grams.

Unit of energy or heat. Various precise definitions are used for different purposes (physical chemistry measurements, engineering steam tables, and thermochemistry), but in all cases the calorie is about 4.2 joules, the amount of heat needed to raise the temperature of 1 g of water by 1 °C (1.8 °F) at normal atmospheric pressure. The calorie used by dietitians and food scientists and found on food labels is actually the kilocalorie (also called Calorie and abbreviated kcal or Cal), or 1,000 calories. It is a measure of the amount of heat energy or metabolic energy contained in the chemical bonds (see bonding) of a food.

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Unit of mass or weight used especially in the centimetre-gram-second (CGS) system of measurement. One gram is equal to 0.001 kg, about 0.035 oz, or 15.43 grains. The gram is very nearly equal to the mass of 1 cc of pure water at its maximum density. The gram of force is equal to the weight of a gram of mass under standard gravity. For greater precision, the mass may be weighed at a point at which the acceleration due to gravity is 980.655 cm/sec2. Seealso gravitation; metric system.

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Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain (safranin) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of peptidoglycan in the cell wall. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.

Characteristics

The following characteristics are generally present in a Gram-positive bacterium:

  1. cytoplasmic lipid membrane
  2. thick peptidoglycan layer
  3. capsule polysaccharides (only in some species)
  4. flagellum (only in some species)
    • if present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.

Classification

In the original bacterial phyla, the Gram-positive organisms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium. It has also been expanded to include the Mollicutes, bacteria like Mycoplasma that lack cell walls and cannot be Gram stained, but are derived from such forms. Actinobacteria are the other major group of Gram-positive bacteria, which have a high guanosine and cytosine content in their genomes (high G+C group). This contrasts with the Firmicutes, which have a low G+C content.

Both Gram-positive and Gram-negative bacteria may have a membrane called an S-layer. In Gram-negative bacteria, the S-layer is directly attached to the outer membrane. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.

Exceptions

The Deinococcus-Thermus bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria.

Pathogenesis

Most pathogenic bacteria in humans are gram-negative organisms. Classically, six gram-positive organisms are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (round bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium and Listeria, while Bacillus and Clostridium produce spores. The spore-forming bacteria can again be divided based on their respiration: Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe.

See also

References

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