Carbohydrates

Introduction

Carbohydratesis a very important source of energy for bacteria and the name carbohydrate indicates that they are substances that contain carbon (C), as well as hydrogen (H) and oxygen (O) in the same proportions as in water (there are, however, some exceptions). Bacteria, however, cannot utilize di-, tri- or polysaccharides for glycolysis unless they have enzyme systems that can degrade them into monosaccharides. Examples of carbohydrates, which bacteria can metabolize by fermentation to produce an acid, is provided below.

Identification of bacteria

Fermentation may be detected by adding a pH indicator to the culture medium (liquid or solid) and this can be utilized for the differentiation of bacteria.
Different laboratories are using different combinations of carbohydrates in liquid media and, and the selection of combination. is depending on the suspected bacterium.  Carbohydrates that are used for identification of bacteria are listed below. Sometimes you want to perform fermentation tests on agar plates, since it gives more reliable results in some cases.

• A.1. Aldotriose monosaccharides:
  The aldotriose monosaccharide has thee carbon atoms and one aldehyde group in position 1.
• Glyceraldehyde, which is the simpliest one of all aldoses (formula: C₃H₆O₃).


• A.2. Triose monosaccharide alcohol:
  In a triose monosaccharide alcohol, the carbonyl group (=O) has been reduced to a hydroxyl  group (-OH).
• Glycerol, which is the simpliest one of all saccharide alkohols (formula: C₃H₅(OH)₃).


• B.1. Aldopentose monosaccharides:
  Aldopentose monosaccharides have five carbon atoms and one aldehyde group in position 1.
• L-arabinose
• D-ribose
• D-xylose
  
  B.2. Ketopentose monosaccharides:
  Ketopentose monosaccharides have five carbon atoms and one carbonyl group in position 2.
• D-ribulose


• B.3. Pentose monosaccharide alcohols:
  In a pentose monosaccharide alcohol, the carbonyl group (=O) has been reduced to a hydroxyl group (-OH).
• adonitol (= ribitol)
• xylitol


• C.1. Aldohexose monosaccharides:
  Aldohexose monosaccharides have six carbon atoms and one aldehyde group in position 1.
• galactose
• glucose
• mannose
• L-rhamnose (= 6-deoxy-L-mannose)
• fucose, is an aldodeoxyhexose, which lacks the hydroxyl group in position 6.


• C.2. Ketohexose monosaccharides:
  Ketohexose monosaccharides have six carbon atoms and one carbonyl group in position 2.
• fructose
• sorbose


• C.3. Hexose monosaccharide alcohols:
  In hexose monosaccharide alcohols has the carbonyl group (O=) been reduced to a hydroxyl group (OH-).
• dulcitol
• inositol, is not a classical sugar, but a carbohydrate in the form of a av sixfold alcohol of cyklohexan (formula: C₆H₁₂O₆)
• mannitol
• sorbitol


• D. Hexose disaccharides:
  Hexose disaccharides are composed of two hexose monosaccharides linked to each other by a glycosidic bond.
• cellobiose, is composed of two D-glucose moieties linked to each other by a β-(1→4) glycosidic bond
• lactose, is composed of one D-galactose and one D-glucose moiety linked to each other by a β-(1→4) glycosidic bond
• maltose, is composed of two D-glucose moieties  linked to each other by a α-(1→4) glycosidic bond
• melibiose, is composed of one D-galactose and one D-glucose moiety linked to each other by a α(1→6) glycosidic bond
• sucrose (= saccharose or cane sugar), is composed of one D-glucose and one D-fructose moiety linked to each other by a (2→1) glycosidic bond
• trehalos, is composed of two D-glucose moieties linked to each other by a (1→1) glycosidic bond


• E. Hexose trisaccharides:
  Hexose trisaccharides are composed of three hexose monosaccharides linked to each other by glycosidic bonds.
• raffinose, is composed of one D-galactose, one D-fructose and one D-glucose moiety
• melezitos, is composed of av two D-glucose and one D-fructose moiety


• F. Polysaccharides:
  Polysackarider are composed of long linear or branched carbohydrate chains, where the monosaccharide moieties are linked together by glycosidic bonds.
• glycogen, is primarily composed of D-glucose moieties
• inulin, is composed of D-fructose and a terminal D-glucose moiety
• starch, is composed of linear and branched D-glucose chains
• dextrin, is a mixture of  polymers  of D-glucose moieties linked to each other by α-(1,4) or α-(1,6) glycosidic bonds


• G. Glycosides:
  Glycosides are composed of a carbohydrate(s), which is linked to another molecule of non-carbohydrate nature, by a glycosidic bond.
• salicin, is an alcoholic β-glycoside of o-hydroxy benzyl alcohol containing one glucose moiety
• amygdalin, is a disaccharide, which is composed of two glucose moieties, one benzene ring and a cyano group (-CN).