Penicillin

Penicillin acts both by killing bacteria and by inhibiting their growth. It does not kill organisms in the resting stage but only those growing and reproducing. Penicillin is effective against a wide range of disease-bearing microorganisms, including pneumococci, streptococci, gonococci, meningococci, the clostridium that cause tetanus, and the syphilis spirochete. The drug has been successfully used to treat such deadly diseases as endocarditis, septicemia, gas gangrene, gonorrhea, and scarlet fever.

SEMISYNTHETIC PENICILLIN

Despite the effectiveness of penicillin in curing a wide range of diseases, infections caused by certain strains of staphylococci cannot be cured by the antibiotic because the organism produces an enzyme, penicillinase, capable of destroying the antibiotic. In addition, enterococci and other bacteria known to cause respiratory and urinary tract infections were found intrinsically resistant to the action of penicillin. Appropriate chemical treatment of a biological precursor to penicillin, isolated from bacterial cultures, resulted in the formation of a number of so-called semisynthetic penicillins. The most important of these are methicillin and ampicillin—the former is remarkably effective against penicillinase-producing staphylococci and the latter is not only active against all organisms normally killed by penicillin, but also inhibits enterococci and many other bacteria.

DOSAGES

The strength and dosage of penicillin are measured in terms of international units. Each of these units is equal to 0.0006 g of the crystalline fraction of penicillin called penicillin G. In the early days of penicillin therapy, the drug was administered every three hours in small doses. More recently, a preparation called benzathine penicillin G has been produced that provides detectable levels of antibiotic for as long as four weeks after a single intramuscular injection; it is useful for treatment of syphilis and strep throat. Bacterial resistance to some penicillins has increased over the years, creating a need for alternative therapies.

Streptomycin

Streptomycin, antibiotic agent produced by a filamentous (thready) soil bacterium, Streptomyces griseus. Microorganisms that cause many serious diseases, such as tuberculosis, leprosy, and cholera, are susceptible to streptomycin. The first drug effective against tuberculosis, it is still used for this condition, although it has been replaced for most purposes by newer, more effective antibiotics of the same chemical family, the aminoglycosides. These include gentamicin, tobramycin, amikacin, and kanamycin.

An important drawback to the use of streptomycin is the development of resistance to the drug by certain bacteria. Resistance may be delayed by supplementing streptomycin with para-aminosalicylic acid or isoniazid. Streptomycin occasionally causes damage to the vestibular or auditory division of the eighth cranial nerve. Aminoglycosides may also damage the kidney.

Antibiotics

Antibiotics (Greek anti, “against”; bios, “life”) are chemical compounds used to kill or inhibit the growth of infectious organisms. Originally the term antibiotic referred only to organic compounds, produced by bacteria or molds, that are toxic to other microorganisms. The term is now used loosely to include synthetic and semisynthetic organic compounds. Antibiotic refers generally to antibacterials; however, because the term is loosely defined, it is preferable to specify compounds as being antimalarials, antivirals, or antiprotozoals. All antibiotics share the property of selective toxicity: They are more toxic to an invading organism than they are to an animal or human host. Penicillin is the most well-known antibiotic and has been used to fight many infectious diseases, including syphilis, gonorrhea, tetanus, and scarlet fever. Another antibiotic, streptomycin, has been used to combat tuberculosis.