Antibiotics

B

Range of Effectiveness

In some species of bacteria the cell wall consists primarily of a thick layer of peptidoglycan. Other species have a much thinner layer of peptidoglycan and an outer as well as an inner membrane. When bacteria are subjected to Gram's stain, these differences in structure affect the differential staining of the bacteria with a dye called gentian violet. The differences in staining coloration (gram-positive bacteria appear purple and gram-negative bacteria appear colorless or reddish, depending on the process used) are the basis of the classification of bacteria into gram-positive (those with thick peptidoglycan) and gram-negative (those with thin peptidoglycan and an outer membrane), because the staining properties correlate with many other bacterial properties. Antibacterials can be further subdivided into narrow-spectrum and broad-spectrum agents. The narrow-spectrum penicillins act against many gram-positive bacteria. Aminoglycosides, also narrow-spectrum, act against many gram-negative as well as some gram-positive bacteria. The tetracyclines and chloramphenicols are both broad-spectrum drugs because they are effective against both gram-positive and gram-negative bacteria.

C

Cell Death and Growth Suppression

Antibiotics may also be classed as bactericidal (killing bacteria) or bacteriostatic (stopping bacterial growth and multiplication). Bacteriostatic drugs are nonetheless effective because bacteria that are prevented from growing will die off after a time or be killed by the defense mechanisms of the host. The tetracyclines and the sulfonamides are among the bacteriostatic antiobiotics. Antibiotics that damage the cell membrane cause the cell's metabolites to leak out, thus killing the organism. Such compounds, including penicillins and cephalosporins, are therefore classed as bactericidal.

IV

Types of Antibiotics

Following is a list of some of the more common antibiotics and examples of some of their clinical uses. This section does not include all antibiotics nor all of their clinical applications.

A

Penicillins

Penicillins are bactericidal, inhibiting formation of the cell wall. There are four types of penicillins: the narrow-spectrum penicillin-G types, ampicillin and its relatives, the penicillinase-resistants, and the extended spectrum penicillins that are active against pseudomonas. Penicillin-G types are effective against gram-positive strains of streptococci, staphylococci, and some gram-negative bacteria such as meningococcus. Penicillin-G is used to treat such diseases as syphilis, gonorrhea, meningitis, anthrax, and yaws. The related penicillin V has a similar range of action but is less effective. Ampicillin and amoxicillin have a range of effectiveness similar to that of penicillin-G, with a slightly broader spectrum, including some gram-negative bacteria. The penicillinase-resistants are penicillins that combat bacteria that have developed resistance to penicillin-G. The antipseudomonal penicillins are used against infections caused by gram-negative Pseudomonas bacteria, a particular problem in hospitals. They may be administered as a prophylactic in patients with compromised immune systems, who are at risk from gram-negative infections.

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Side effects of the penicillins, while relatively rare, can include immediate and delayed allergic reactions—specifically, skin rashes, fever, and anaphylactic shock, which can be fatal.

B

Cephalosporin

Like the penicillins, cephalosporins have a Β-lactam ring structure that interferes with synthesis of the bacterial cell wall and so are bactericidal. Cephalosporins are more effective than penicillin against gram-negative bacilli and equally effective against gram-positive cocci. Cephalosporins may be used to treat strains of meningitis and as a prophylactic for orthopedic, abdominal, and pelvic surgery. Rare hypersensitive reactions from the cephalosporins include skin rash and, less frequently, anaphylactic shock.

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