The original definition of an antibiotic was a chemical substance that is produced by a microorganism and, in dilute solutions, can inhibit the growth of, and even destroy, other microorganisms. This definition has been expanded to include similar inhibitory substances that are produced by plants, marine organisms, and total- or semisynthetic procedures. Since the discovery of penicillin by A. Fleming in 1928, thousands of antibiotics have been isolated and identified; some have been found to be of value in the treatment of infectious disease. They differ markedly in physicochemical and pharmacological properties, antimicrobial spectra, and mechanisms of action.
Penicillin is produced by strains of the fungus Penicillium notatum and P. chrysogenum. Most of the other antibiotics in clinical use are produced by actinomycetes, particularly streptomycetes (natural antibiotics). Other antibiotics are produced by chemical synthesis (synthetic antibiotics). Based on structure, the major antibiotic classes are the β-lactams (penicillins and cephalosporins), aminoglycosides, macrolides, tetracyclines, quinolones, rifamycins, polyenes, azoles, glycopeptides, and polypeptides.
The key step in the production of natural antibiotics is a fermentation process. Strains of microorganisms, selected by elaborate screening procedures from randomly isolated pure cultures, are inoculated into sterile nutrient medium in large vats and incubated for varying periods of time. Different strains of a single microbial species may differ greatly in the amounts of antibiotics they produce. Strain selection is thus the most powerful tool in effecting major improvements in antibiotic yield. In addition, variations in culturing conditions often markedly affect the amount of antibiotic that is produced by a given strain. Chemical modifications of antibiotics produced by fermentation processes have led to semisynthetic ones with improved antimicrobial activity or pharmacological properties. See Bacterial physiology and metabolism, Fermentation
All microorganisms can cause infectious diseases in animals and humans, though the majority of infections are caused by bacteria. Most antibiotics are active against bacteria. Although for the proper treatment of serious infections cultures and antibiotic sensitivities are required, antibiotic therapy is often empiric, with etiology being inferred from the clinical features of a disease.
Bacteria are divided into the gram positive and the gram negative; each group comprises a wide variety of different species. Staphylococci, pneumococci, and streptococci are the more common gram-positive organisms, while enterobacteria, Pseudomonas, and Hemophilus are the most common gram negative. Certain antibiotics are effective only against gram-positive bacteria. Others are effective against both gram-positive and gram-negative bacteria and are referred to as broad-spectrum antibiotics. See Bacteria, Medical bacteriology
Pathogenic fungi may be divided on the basis of their pathogenicity into true pathogens and opportunistic pathogens. The opportunistic occur mainly in debilitated and immunocompromised patients. Clinically useful antibiotics include amphotericin B, nystatin, griseofulvin and the azole antifungals. See Fungi, Medical mycology, Opportunistic infections
With some viruses that cause mild infections, such as the common-cold viruses (rhinoviruses), treatment is symptomatic. With others, such as the polio, smallpox (now eradicated), and hepatitis B viruses, the only way to prevent disease is by vaccination. With still other viruses, antibiotics, mostly synthetic, are the appropriate treatment. Clinically useful antibiotics are ribavirin, acyclovir, and zidovudine, which are active against, respectively, respiratory, herpes, and human immunodeficiency viruses. See Animal virus, Vaccination
Protozoa may be divided, on the basis of the site of infection, into intestinal, urogenital, blood, and tissue. Protozoan diseases such as malaria, trypanosomiasis, and amebiasis are particularly common in the tropics, in populations living under poor housing and sanitary conditions. In the developed countries, P. carinii is the most important opportunistic pathogen, being associated almost exclusively with acquired immune deficiency syndrome (AIDS). Antibiotics active against protozoa include metronidazole, trimethoprim-sulfamethoxazole, and quinine. See Acquired immune deficiency syndrome (AIDS), Medical parasitology, Protozoa
The observation of the antitumor activity of actinomycin sparked an intensive search for antitumor antibiotics in plants and microorganisms. Among the antibiotics used clinically against certain forms of cancer are daunorubicin, doxorubicin, mitomycin C, and bleomycin.
Antibiotics active against bacteria are bacteriostatic or bacteriocidal; that is, they either inhibit growth of susceptible organisms or destroy them. On the basis of their mechanism of action, antibiotics are classified as (1) those that affect bacterial cell-wall biosynthesis, causing loss of viability and often cell lysis (penicillins and cephalosporins, bacitracin, cycloserine, vancomycin); (2) those that act directly on the cell membrane, affecting its barrier function and leading to leakage of intracellular components (polymyxin); (3) those that interfere with protein biosynthesis (chloramphenicol, tetracyclines, erythromycin, spectinomycin, streptomycin, gentamycin); (4) those that affect nucleic acid biosynthesis (rifampicin, novobiocin, quinolones); and (5) those that block specific steps in intermediary metabolism (sulfonamides, trimethoprim). See Enzyme, Sulfonamide
Antibiotics active against fungi are fungistatic or fungicidal. Their mechanisms of action include (1) interaction with the cell membrane, leading to leakage of cytoplasmic components (amphotericin, nystatin); (2) interference with the synthesis of membrane components (ketoconazole, fluconazole); (3) interference with nucleic acid synthesis (5-fluorocytosine); and (4) interference with microtubule assembly (griseofulvin). See Fungistat and fungicide
For an antibiotic to be effective, it must first reach the target site of action on or in the microbial cell. It must also reach the body site at which the infective microorganism resides in sufficient concentration, and remain there long enough to exert its effect. The concentration in the body must remain below that which is toxic to the human cells. The effectiveness of an antibiotic also depends on the severity of the infection and the immune system of the body, being significantly reduced when the immune system is impaired. Complete killing or lysis of the microorganism may be required to achieve a successful outcome. See Immunity
Antibiotics may be given by injection, orally, or topically. When given orally, they must be absorbed into the body and transported by the blood and extracellular fluids to the site of the infecting organisms. When they are administered topically, such absorption is rarely possible, and the antibiotics then exert their effect only against those organisms present at the site of application.
The therapeutic value of every antibiotic class is gradually eroded by the microbial resistance that invariably follows broad clinical use.
Some bacteria are naturally resistant to certain antibiotics (inherent resistance). Clinical resistance is commonly due to the emergence of resistant organisms following antibiotic treatment (acquired resistance). This emergence, in turn, is due to selection of resistant mutants of the infective species (endogenous resistance) or, usually, to transfer of resistance genes from other, naturally resistant species (exogenous resistance). A major challenge in antimicrobial chemotherapy is the horizontal spread of resistance genes and resistant strains, mostly in the hospital but also in the community. The consequences are increased patient morbidity and mortality, reduced drug options, and more expensive and toxic antibiotics.
Rapid detection of resistance and pathogen identification are critical for the rational use of antibiotics and implementation of infection control measures. In the absence of such information, treament is empiric, usually involving broad-spectrum agents, which exacerbates resistance development. Inadequate infection control measures encourage dissemination of resistant strains.
It is estimated that the average duration of many infectious diseases and the severity of certain others have decreased significantly since the introduction of antibiotic therapy. The dramatic drop in mortality rates for such dreaded diseases as meningitis, tuberculosis, and septicemia offers striking evidence of the effectiveness of these agents. Bacterial pneumonia, bacterial endocarditis, typhoid fever, and certain sexually transmitted diseases are also amenable to treatment with antibiotics. So are infections that often follow viral or neoplastic diseases, even though the original illness may not respond to antibiotic therapy. See Epidemiology
Antibiotics in small amounts are widely used as feed supplements to stimulate growth of livestock and poultry. They probably act by inhibiting organisms responsible for low-grade infections and by reducing intestinal epithelial inflammation. Many experts believe that this use of antibiotics contributes to the emergence of antibiotic-resistant bacteria that could eventually pose a public health problem.
In cattle, sheep, and swine, antibiotics are effective against economically important diseases. The use of antibiotics in dogs and cats closely resembles their use in human medical practice. In fish farms, antibiotics are usually added to the food or applied to the fish by bathing. The incidence of infections in fish, and animals in general, may be reduced by the use of disease-resistant stock, better hygiene, and better diet.
Although effective against many microorganisms causing disease in plants, antibiotics are not widely used to control crop and plant diseases. Some of the limiting factors are instability of the antibiotic under field conditions, the possibility of harmful residues, and expense. Nevertheless, antibiotic control of some crop pathogens is being practiced, as is true of the rice blast in Japan, for example. See Plant pathology