The range of bacteria that an antibiotic affects can be divided into narrow spectrum and broad spectrum. Narrow spectrum antibiotics act against a limited group of bacteria, either gram positive or gram negative, for example sodium fusidate only acts against staphylococcal bacteria. Broad spectrum—antibiotics act against gram positive and gram negative bacteria, for example amoxicillin.
Gram staining (or Gram's method; is a method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative). It is based on the chemical and physical properties of their cell walls. Primarily, it detects peptidoglycan, which is present in a thick layer in Gram positive bacteria. A Gram positive results in a purple/blue color while a Gram negative results in a pink/red color. The Gram stain is almost always the first step in the identification of a bacterial organism, and is the default stain performed by laboratories over a sample when no specific culture is referred. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, thus forming Gram-variable and Gram-indeterminate groups as well.
Gram Stain
This is a microscopic image of a Gram stain of mixed Gram-positive cocci (Staphylococcus aureus, purple) and Gram-negative bacilli (Escherichia coli, red).
A broad spectrum antibiotic acts against both Gram-positive and Gram-negative bacteria, in contrast to a narrow spectrum antibiotic, which is effective against specific families of bacteria. An example of a commonly used broad-spectrum antibiotic is ampicillin. Broad spectrum antibiotics are properly used in the following medical situations: empirically (i.e., based on the experience of the practitioner), prior to the formal identification of the causative bacteria and when there is a wide range of possible illnesses and a potentially serious illness would result if treatment is delayed. This occurs, for example, in meningitis, where the patient can become fatally ill within hours if broad-spectrum antibiotics are not initiated. Broad spectrum antibiotics are also used for drug resistant bacteria that do not respond to other, more narrow spectrum antibiotics and in the case of superinfections, where there are multiple types of bacteria causing illness, thus warranting either a broad-spectrum antibiotic or combination antibiotic therapy.
Following a 40-year hiatus in discovering new classes of antibacterial compounds, three new classes of antibacterial antibiotics have been brought into clinical use: cyclic lipopeptides (such as daptomycin), glycylcyclines (such as tigecycline), and oxazolidinones (such as linezolid).