a group of biosynthetic and semisynthetic antibiotics that are closely related to one another in chemical structure. The nucleus of a penicillin molecule is 6-aminopenicil-lanic acid (6-APA), a heterocyclic compound that consists of a four-member β-lactam ring (A) and a five-member thiazolidine ring (B):

Penicillins differ in their side chain (R).

Biosynthetic penicillins are formed by several ascomycetous fungi, including many mold fungi of the genus Penicillium. The biosynthetic penicillin complex was first isolated in 1929 by the British microbiologist A. Fleming from P. notatum, but it was not sufficiently purified for clinical use until 1941. In the USSR, penicillin was obtained in 1942 by Z. V. Ermol’eva and T. I. Balezina from P. crustosum. Benzylpenicillin, or penicillin G, has proved most valuable in medical practice; its structural formula is R—CH₂—C₆ H₅. When growing the producer fungus (in the USSR a strain of P. chrysogenum is used), the process of biosynthesis is usually directed toward predominant formation of penicillin G. Salts of penicillin G with the metals Na and K or the organic bases Novocain and dibenzylethylenediamine are also used in medicine. The trademark for the dibenzylethylenediamine salt is Bicillin. Penicillin G salts are white, crystalline powders that are readily degraded by acids, alkalies, and the enzyme penicillinase. Sodium and potassium salts of penicillin G are readily soluble in water, while the Novocain salt is only slightly water-soluble.

Biosynthetic penicillins are highly active against some pathogenic fungi, staphylococci, streptococci, pneumococci, gono-cocci, and meningococci, as well as the causative agents of gaseous gangrene, tetanus, botulism, anthrax, diphtheria, and syphilis. However, they are only slightly active or completely inactive against typhoid and paratyphoid enterobacteriaceae, members of the genus Proteus, and the causative agents of brucellosis, plague, and tuberculosis, as well as rickettsiae, viruses, protozoans, and almost all fungi.

Penicillins operate by suppressing the formation of cell walls in microbes. Thus, resting cells—those not engaged in growth and the formation of new structures—are not affected by penicillins. L-phase variants are bacteria in which synthesis of the cell membrane has been disrupted by penicillin. Some microbes, for example, staphylococci, form the enzyme penicillinase, which inactivates penicillins by rupturing the β-lactam chain. The number of such microbes, which are resistant to penicillins, is increasing because of the widespread use of penicillins; thus, about 80 percent of strains of pathogenic staphylococci that are isolated from patients are resistant to penicillin G.

After 6-APA was isolated from P. chrysogenum in 1959, the chemical synthesis of new penicillins became possible by adding various radicals to the free amino group. More than 15,000 semisynthetic penicillins are known; however, only a few are superior to biosynthetic penicillins in biological properties. Some, for example, methicillin and oxacillin, are not destroyed by penicillinase and therefore act on staphylococci that are resistant to penicillin G. Other semisynthetic penicillins are resistant to penicillinase in an acid medium and (in contrast to the majority of biosynthetic penicillins) may be used internally; these include phenethicillin and propicillin. Ampicillin and carbenicil-lin are semisynthetic penicillins with a broader range of antimicrobial activity than penicillin G. Furthermore, ampicillin and oxacillin are acid-resistant and are readily absorbed in the gastrointestinal tract. All penicillins are only slightly toxic; however, some patients have an increased sensitivity to penicillin that may produce allergic reactions, for example, hives, edema of the face, and pains in the joints.

Biosynthetic penicillins are used in treating pneumonias, sepsis, and purulent infections of the skin, soft tissues, and mucosa. They are also used to treat many other diseases, including purulent pleuritis, peritonitis, cystitis, osteomyelitis, diphtheria, scarlet fever, epidemic meningitis, gonorrhea, and syphilis. Bicillin and Bicillin preparations that contain other salts of penicillin G are used principally for the prevention and treatment of rheumatism and for the treatment of syphilis. Methicillin and oxacillin are used with diseases that are caused by penicillin G-resistant strains of staphylococci. Ampicillin is used to treat infections of the soft tissues and of the respiratory, gastrointestinal, and urinary tracts.

Sodium and potassium salts of penicillin G are rapidly absorbed in the blood when injected intramuscularly and are quickly excreted from the body with the urine. The action of the Novocain salt of penicillin G is long lasting, and that of Bicillin is even longer lasting.