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Bacteria are either spherical (coccal), rod-like (bacillary), or spiral/helical (spirochetal) in shape. The exact shape is dictated by the bacterium's cell wall, a rigid, complex, layered structure.
Although bacteria are classified in part by their distinctive shape, most bacteria also are classified as either gram-positive or gram-negative depending on their color after a stain, called the Gram's stain, is applied during laboratory testing.
Bacteria that stain blue are gram-positive, whereas those that stain pink are gram-negative.
However, the classification of bacteria as gram-negative or gram-positive also corresponds to certain characteristics of the bacteria's outer wall, the kind of infections produced by the bacteria, and the types of antibiotics that are likely to kill the bacteria.
The distinctive feature of gram-negative bacteria
is the presence of a double membrane surrounding each bacterial cell.
Although all bacteria have an inner cell membrane, gram-negative bacteria have a unique outer membrane.
This outer membrane excludes certain drugs and antibiotics from penetrating the cell, partially accounting for why gram-negative bacteria are generally more resistant to antibiotics than are gram-positive bacteria.
The outer membrane of gram-negative bacteria is rich in a molecule called lipopolysaccharide.
If gram-negative bacteria enter the bloodstream, lipopolysaccharide can trigger a cascade of events, including high fever and a drop in blood pressure.
For this reason, lipopolysaccharide is often referred to as an endotoxin.
Gram-negative bacteria have a great facility for exchanging genetic material (DNA) among strains of the same species and even among different species.
This means that if a gram-negative bacterium either undergoes a genetic change (mutation) or acquires genetic material that confers resistance to an antibiotic, the bacterium may later share its DNA with another strain of bacteria and the second strain can become resistant as well.
Source: The Merck Manual: Home Edition. Section 17, Chapter 177: Bacillary Infections (online version).
http://www.merck.com/mrkshared/mmanual_home/contents.jsp
Gram-positive bacteria are a minority.
They are generally susceptible to penicillin and are usually slow to develop resistance to this antibiotic.
Some gram-positive bacteria (such as certain streptococci) can penetrate deep into the tissue, whereas others inflict harm by producing extremely poisonous substances (for example, the toxins produced by Clostridium botulinum).
Although bacteria are classified in part by their distinctive shape, most bacteria also are classified as either gram-positive or gram-negative depending on their color after a stain, called the Gram's stain, is applied during laboratory testing.
Bacteria that stain blue are gram-positive, whereas those that stain pink are gram-negative.
However, the classification of bacteria as gram-negative or gram-positive also corresponds to certain characteristics of the bacteria's outer wall, the kind of infections produced by the bacteria, and the types of antibiotics that are likely to kill the bacteria.
The distinctive feature of gram-negative bacteria
is the presence of a double membrane surrounding each bacterial cell.
Although all bacteria have an inner cell membrane, gram-negative bacteria have a unique outer membrane.
This outer membrane excludes certain drugs and antibiotics from penetrating the cell, partially accounting for why gram-negative bacteria are generally more resistant to antibiotics than are gram-positive bacteria.
The outer membrane of gram-negative bacteria is rich in a molecule called lipopolysaccharide.
If gram-negative bacteria enter the bloodstream, lipopolysaccharide can trigger a cascade of events, including high fever and a drop in blood pressure.
For this reason, lipopolysaccharide is often referred to as an endotoxin.
Gram-negative bacteria have a great facility for exchanging genetic material (DNA) among strains of the same species and even among different species.
This means that if a gram-negative bacterium either undergoes a genetic change (mutation) or acquires genetic material that confers resistance to an antibiotic, the bacterium may later share its DNA with another strain of bacteria and the second strain can become resistant as well.
Source: The Merck Manual: Home Edition. Section 17, Chapter 177: Bacillary Infections (online version).
http://www.merck.com/mrkshared/mmanual_home/contents.jsp
Gram-positive bacteria are a minority.
They are generally susceptible to penicillin and are usually slow to develop resistance to this antibiotic.
Some gram-positive bacteria (such as certain streptococci) can penetrate deep into the tissue, whereas others inflict harm by producing extremely poisonous substances (for example, the toxins produced by Clostridium botulinum).
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