الفهرس 
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Abstract
Since its accidental discovery in 1928, penicillin is the oldest known and still one of the most widely used antibiotic agents. It is derived from the mold or fungi Penicillium chrysogenum. The penicillin group is characterized by the presence of the 4-membered b-lactam ring fused to a 5-membered thiazolidine ring and an acyl side chain to the b-lactam ring. The penicillins are also called beta-lactam antibiotics. The presence of the lactam ring is essential for biological activity and is usually the site of cleavage by bacterial penicillinase or by an acid. The nature of the R (variable) group determines the drug’s stability to enzymatic and acidic hydrolysis and its activity against different bacteria. Alteration of the R group, such as the addition of bulky side chains or functional groups, gives different variants of penicillins that are known to enhance activity, show resistance to penicillinase, and are more resistant to acid. Although penicillins have structural differences that dictate their antimicrobial activity, stability against penicillinase and acids, and behavior within the body, they uniformly have low toxicity. Unless used in high dosage, the absence of direct toxicity is one of the most remarkable properties of penicillin as antibiotics. Penicillin acts as a bactericidal agent since the mode of action is the inhibition of the bacterial cell wall synthesis. They act on the peptidoglycan layer of gram-positive and gram-negative bacteria. Since penicillins are dipeptide analogs of D-ala-D-ala, it does prevent the assembly of the peptidoglycan by binding to and competitively inhibiting the transpeptidase enzyme used by bacteria to cross-link the peptide (D-alanyl-alanine). This destroys the ability of the peptidoglycan to bear the stress of osmotic pressure that acts on a bacterium. The resultant weakening of the peptidoglycan fabric of the growing bacterial cell results in osmotic lysis. In
gram-positive organisms, the antibacterial effect of b-lactam antibiotics depends on the affinity of the antibiotic for the essential penicillin binding proteins (PBPs) of the organism because the cell wall peptidoglycan offers no resistance to the diffusion of small molecules. In gram-negative bacteria, antibiotic efficacy depends on the ability of the antibiotic to diffuse through the pores in the outer membrane of the bacteria and reach the target PBPs. Penicillin is the first antibiotic manufactured and used from the types of antibiotics known in the medical history of mankind. Penicillin was discovered by Alexander Fleming in the early twentieth century, where Fleming observed that a certain type ofmold descended from the Penicillium family has the ability to inhibit the growth of certain types of bacteria. But penicillin was not manufactured and developed and penicillin did not enter into use as an effective antibiotic until several years after Fleming’s discovery, where a group of researchers from the University of Oxford then developed penicillin and made it available as an effective drug for humanity, giving birth to penicillin, which is one of the most important types of drugs credited with saving large numbers of people during World War II. The penicillin family includes several different types of antibiotics, and penicillin works to fight bacteria by disrupting the walls of bacterial cells. Currently, the penicillin family includesseveral types of antibiotics, which play an important role in the treatment and resistance of many pathogenic bacterial strains. Penicillin-based antibiotics are used to resist many types of bacterial strains responsible for various health problems, such as: ear infections, urinary tract infections, oral and gum infections, and infections of the nose, throat or lungs. Skin infections and meningitis. and endocarditis (endocarditis).These are some of the bacterial strains that penicillin is effective against: Streptococci, Clostridium, Listeria, and Salmonella