1,454
edits
Changes
Jump to navigation
Jump to search
Gram Negative Bacteria - Overview (view source)
Revision as of 07:37, 16 August 2011
, 07:37, 16 August 2011Created page with "'''Gram negative bacteria stain pink''' following a Gram stain, whereas Gram positive bacteria stain purple. The Gram stain is an empirical procedure, but this reaction is a man..."
'''Gram negative bacteria stain pink''' following a Gram stain, whereas Gram positive bacteria stain purple.
The Gram stain is an empirical procedure, but this reaction is a manifestation of '''fundamental differences''' in the properties of the bacterial species. Gram positives and negatives have a different cell wall structure, different nutritional requirements, different types of toxins and different resistance to chemotherapeutic agents, disinfectants and enzymes.
==Gram Stain==
This is the most widely used differential stain. The primary stain (crystal violet) is applied. This is then washed away and a '''mordant''' (iodine solution) is applied to complex the crystal violet to the bacterial cell. This is removed and a '''decolourising agent''', usually acetone, is aplied. This '''removes the purple stain''' only from the Gram negative bacteria, while Gram positive organisms retain it. The unstained Gram negatives are then visualised with a '''red counterstain''', usually Carbol Fuchsin, which will not alter the purple appearance of Gram positives.
Dead bacteria, cells and organic debris will stain pink. In older cultures of Gram positive bacteria there are likely to be dead cells which stain pink. Since Gram negative bacteria do not give false positive reaction it is usually safe to assume that bacteria giving a doubtful reaction are Gram positive.
==Cell Wall Structure==
The '''cell wall''' of Gram negative bacteria has some special features:
:'''thin peptidoglycan layer''' just outside the cytoplasmic membrane
:'''Outer membrane''', similar to the cytoplasmic membrane but containing different proteins, specifically '''lipopolysaccharide''', located in the outer leaflet of the outer membrane
==Lipopolysaccharide (LPS)==
There are '''three regions''' to this molecule:
The lipid region, '''lipid A''', contains 3-hydroxy fatty acids linked to a diglucosamine backbone. These fatty acids are hydrophobic and intercalate into the phospholipid bilayer of the outer membrane.
The '''LPS core region''' is linked to lipid A and is a short oligosaccharide which is variable between bacterial type and contains unusual sugar residues. Bacteria with a LPS which stop at this point are called R-form, or rough bacteria, and they will autoagglutinate in saline and are '''of low virulence'''.
The third region is the '''O-side chain''', which is a repeating oligosaccharide with 50 or 80 repeat units extending into the external environment of the bacterium. This makes the surface of the bacterium hydrophilic and the O-side chain is highly antigenic.
Bacteria with an O-side chain are termed smooth and are usually '''virulent'''.
The LPS also has '''biological properties'''. The lipid A part disrupts mammalian cell membranes, and causes '''pyrexia'''. Gram negative septicaemia leads to circulatory collapse. The LPS is also known as '''endotoxin''' because of these properties.
==Gram Negative Bacteria==
Individual bacteria have '''characteristic shapes'''. The cells may be spherial (coccus), rod shaped (bacillus), comma shaped (curved rod), spiral (spirochaete) or filamentous. The shape can differ to some degree with the growth conditions, but usually the shape of most bacteria is an important clue to their identity.
See a list of the [[:Category:Gram negative bacteria|Gram negative bacteria]].
==References==
Rycroft, A. (2008) '''Fundamentals of veterinary bacteriology''' ''RVC student notes''
Ryan, K. (2010) '''Sherris medical microbiology''' ''McGraw Hill Professional''
Black, J. (2008) '''Microbiology: principles and explorations''' ''John Wiley and Sons''
[[Category:To Do - Helen]]
[[Category:To Do - Review]]
The Gram stain is an empirical procedure, but this reaction is a manifestation of '''fundamental differences''' in the properties of the bacterial species. Gram positives and negatives have a different cell wall structure, different nutritional requirements, different types of toxins and different resistance to chemotherapeutic agents, disinfectants and enzymes.
==Gram Stain==
This is the most widely used differential stain. The primary stain (crystal violet) is applied. This is then washed away and a '''mordant''' (iodine solution) is applied to complex the crystal violet to the bacterial cell. This is removed and a '''decolourising agent''', usually acetone, is aplied. This '''removes the purple stain''' only from the Gram negative bacteria, while Gram positive organisms retain it. The unstained Gram negatives are then visualised with a '''red counterstain''', usually Carbol Fuchsin, which will not alter the purple appearance of Gram positives.
Dead bacteria, cells and organic debris will stain pink. In older cultures of Gram positive bacteria there are likely to be dead cells which stain pink. Since Gram negative bacteria do not give false positive reaction it is usually safe to assume that bacteria giving a doubtful reaction are Gram positive.
==Cell Wall Structure==
The '''cell wall''' of Gram negative bacteria has some special features:
:'''thin peptidoglycan layer''' just outside the cytoplasmic membrane
:'''Outer membrane''', similar to the cytoplasmic membrane but containing different proteins, specifically '''lipopolysaccharide''', located in the outer leaflet of the outer membrane
==Lipopolysaccharide (LPS)==
There are '''three regions''' to this molecule:
The lipid region, '''lipid A''', contains 3-hydroxy fatty acids linked to a diglucosamine backbone. These fatty acids are hydrophobic and intercalate into the phospholipid bilayer of the outer membrane.
The '''LPS core region''' is linked to lipid A and is a short oligosaccharide which is variable between bacterial type and contains unusual sugar residues. Bacteria with a LPS which stop at this point are called R-form, or rough bacteria, and they will autoagglutinate in saline and are '''of low virulence'''.
The third region is the '''O-side chain''', which is a repeating oligosaccharide with 50 or 80 repeat units extending into the external environment of the bacterium. This makes the surface of the bacterium hydrophilic and the O-side chain is highly antigenic.
Bacteria with an O-side chain are termed smooth and are usually '''virulent'''.
The LPS also has '''biological properties'''. The lipid A part disrupts mammalian cell membranes, and causes '''pyrexia'''. Gram negative septicaemia leads to circulatory collapse. The LPS is also known as '''endotoxin''' because of these properties.
==Gram Negative Bacteria==
Individual bacteria have '''characteristic shapes'''. The cells may be spherial (coccus), rod shaped (bacillus), comma shaped (curved rod), spiral (spirochaete) or filamentous. The shape can differ to some degree with the growth conditions, but usually the shape of most bacteria is an important clue to their identity.
See a list of the [[:Category:Gram negative bacteria|Gram negative bacteria]].
==References==
Rycroft, A. (2008) '''Fundamentals of veterinary bacteriology''' ''RVC student notes''
Ryan, K. (2010) '''Sherris medical microbiology''' ''McGraw Hill Professional''
Black, J. (2008) '''Microbiology: principles and explorations''' ''John Wiley and Sons''
[[Category:To Do - Helen]]
[[Category:To Do - Review]]