Penicillin Group. Penicillin G, the most popularly used antibiotic because it is the cheapest, safest, and most effective antibacterial treatments available. Although penicillins are extremely safe, patients can occasionally develop an immune reaction to them and become allergic. In such cases the availability of alternative antibiotics is critical.

• Penicillin G and V remain the drugs of choice for treating many gram-positive bacterial infections. Penicillin G and V are used to treat infections caused by gram-positive Staphylococcus pyogenes (strep throat), and Streptococcus pneumoniae (respiratory tract infections). These are also used to treat the sexually transmitted diseases gonorrhea and syphilis. Penicillin V is similar in its activity to penicillin G, but because it is not destroyed by stomach acid it is a better drug for oral dose.

Methicillin was the first penicillin to have activity against the staphylococcus strains that were resistant to penicillin G.

• Ampicillin and amoxycillin are similar antibiotics with a broader spectrum of activity than earlier penicillins, it is active against common gram-negative bacteria as well as gram positive bacteria. But they are not active against penicillin G-resistant staphylococci. Both are effective on oral administration and are active against the gram-negative bacterium Escherichia coli (a common cause of urinary tract infections), Haemophilus influenzae (an important cause of ear infections and meningitis in infants), and Salmonella typhi (responsible for typhoid fever).
• Carbenicillin was the first penicillin synthesized to possess useful activity against Pseudomonas aeruginosa. This bacterium is normally only responsible for infections in hospitalized patients and had proved particularly difficult to treat.

Cephalosporin Group - injectable forms of this group are generally broad-spectrum, bactericidal agents that are restricted to hospital use for the treatment of serious infections.

Tetracycline Group - are broad-spectrum antibiotics that have been used to treat a wide range of infections, are orally active, and are bacteriostatic. It is used in the treatment of gonorrhea (caused by Neisseria gonorrhea) and urinary tract infections caused by Escherichia coli. Drugs belonging to this group can cause pigmentation of the teeth in children, they are not usually prescribed for patients less than 12 years old.

Macrolide Group - the most important member of this group is Erythromycin, a very safe antibiotic, it is effective orally, bacteriostatic, and active against gram-positive infections, especially those of the respiratory tract caused by streptococci. For certain patients unable to tolerate penicillins, erythromycin has provided a valuable alternative. This antibiotic has also been useful in the treatment of lung infections caused by mycoplasma. Erythromycin has the unusual characteristic of being able to penetrate human cells and because of this has become important in the treatment of Legionnaires' disease, a type of pneumonia caused byLegionella pneumophila.

Aminoglycoside Group - this antibiotic family is highly active against gram-negative bacteria, it is only effective by injection, and are bactericidal. Generally they are used to treat serious gram-negative infections in hospitalized patients. Streptomycin was the first member of this group to be used widely, but it has now been largely replaced by newer aminoglycosides, such as gentamicin. Aminoglycosides group has a potential to damage the kidneys and cause hearing impairment.

Chloramphenicol - this is a broad-spectrum , orally effective, bacteriostatic antibiotic. Chloramphenicol is an important alternative for treating typhoid fever and bacterial meningitis because of it ability to penetrate the central nervous system efficiently. The use of these antibiotics in most countries has declined because of concerns about its ability to cause a very rare but fatal anemia and because of the availability of other safer drugs.