Antibiotic

V

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 membrane. When bacteria are subjected to a Gram's stain, these differences in structure affect the differential staining of the bacteria with a dye called gentian violet and other solutions. The differences in staining coloration (gram-positive bacteria appear purple and gram-negative bacteria appear colourless 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 fight many gram-positive bacteria. Aminoglycosides, also narrow-spectrum, work against gram-negative bacteria. The tetracyclines and chloramphenicols are both broad-spectrum drugs, effective against both gram-positive and gram-negative bacteria.

VI

Extermination and Growth Suppression

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

VII

Types of Antibiotics

Penicillins are the oldest group of antibiotics. They are bactericidal, inhibiting formation of the cell wall by mechanical action. There are four types of penicillins: the narrow-spectrum penicillin-G types, ampicillin and its relatives, the penicillinase-resistants, and the antipseudomonal penicillins. Penicillin-G types are effective against gram-positive strains of streptococci, staphylococci, enterococci, and meningococci and are used to treat such diseases as syphilis, gonorrhoea, meningitis, anthrax, and yaws. The related penicillin V is used for respiratory infections.

Ampicillin and amoxycillin have a range of effectiveness similar to that of penicillin-G types, with a slightly broader spectrum, including the gram-negative bacteria. Ampicillin and its relatives are effective against typhoid fever, bronchitis, urinary tract infections, pneumonia, meningitis, and bacteraemia. The penicillinase-resistants are penicillins that combat bacteria that have developed resistance to penicillin G. The aminoglycosides are penicillins that fight infections caused by gram-negative Pseudomonas bacteria, a particular problem in hospitals. These penicillins are administered as a prophylactic to patients with compromised immune systems, who are at risk from gram-negative infections.

Side-effects of the penicillins, while relatively rare, can include immediate and delayed hypersensitivity—specifically, skin rashes, fever, and anaphylactic shock (abnormal reaction to the drug). Ampicillin can produce more side-effects than the penicillins; these primarily include nausea, vomiting, and diarrhoea. Amoxycillin has fewer adverse reactions.

VIII

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 are used to treat strains of meningitis and as a prophylactic for orthopaedic, abdominal, and pelvic surgery. They tend to be more expensive than the penicillins, although their low risk factor recommends their usage. Because of their greater effectiveness against gram-negative bacteria, the cephalosporins are favoured prophylactics. Rare hypersensitive reactions from the cephalosporins include skin rash and, less frequently, anaphylactic shock.

IX

Aminoglycosides

Streptomycin is the oldest of the aminoglycosides and, after penicillin, the most commonly administered antibiotic. The aminoglycosides are narrow-spectrum antibiotics, inhibiting bacterial protein synthesis in gram-negative bacilli and staphylococci. They are sometimes used in combination with penicillin. All of the members of this group—particularly neomycin—tend to be more toxic than other antibiotics. Rare adverse effects associated with prolonged use of aminoglycosides include damage to the vestibular region of the ear, hearing loss, and kidney damage.