What are Antibiotics?
The word "antibiotics" comes from the Greek anti ("against") and bios ("life"). Antibiotics are drugs that either destroy bacteria or prevent their reproduction. Antibiotics that kill bacteria are called "bactericidal" and the ones that stop the growth of bacteria are called "bacteriostatic".
Since penicillin's introduction during the 1940s, scientists developed numerous other antibiotics. Today, over 100 different antibiotics are available. About 90% of antibiotics are made from living organisms such as bacteria, others are produced synthetically, either in whole or in part.
Antibiotics classification
Although there are several classification schemes for antibiotics, based on bacterial spectrum (broad, narrow) or route of administration (injectable, oral, topical), or type of activity (bactericidal, bacteriostatic), the most useful is based on chemical structure. Antibiotics within a structural class will generally have similar patterns of effectiveness, toxicity, and allergic potential.
Most commonly used types of antibiotics are: Penicillins, Fluoroquinolones, Cephalosporins, Macrolides, and Tetracyclines. While each class is composed of multiple drugs, each drug is unique in some way.
Penicillins
The penicillins are the oldest class of antibiotics. Penicillins have a common chemical structure which they share with the cephalopsorins. Penicillins are generally bactericidal, inhibiting formation of the cell wall.
There are four types of penicillins:- The natural pencillins are based on the original penicillin-G structure. Penicillin-G types are effective against gram-positive strains of streptococci, staphylococci, and some gram-negative bacteria such as meningococcus.
- Penicillinase-resistant penicillins are active even in the presence of the bacterial enzyme that inactivates most natural penicillins.
- Extended spectrum penicillins which are effective against a wider range of bacteria.
- Aminopenicillins such as Ampicillin and Amoxicillin have an extended spectrum of action compared with the natural penicillins.
Penicillins side effects
Penicillins are among the least toxic drugs known. The most common side effect of penicillin is diarrhea. Nausea, vomiting, and upset stomach are also common. In rare cases penicillins can cause immediate and delayed allergic reactions - specifically, skin rashes, fever, and anaphylactic shock. Penicillins are classed as category B during pregnancy.
Cephalosporins
Cephalosporins have a mechanism of action identical to that of the penicillins. However, the basic chemical structure of the penicillins and cephalosporins differs in other respects, resulting in some difference in the spectrum of antibacterial activity. Like the penicillins, cephalosporins interfere with synthesis of the bacterial cell wall and so are bactericidal.
Cephalosporins are among the most diverse classes of antibiotics, they are grouped into "generations" by their antimicrobial properties. Each generation has a broader spectrum of activity than the one before.
The first generation cephalosporins include: cephalothin, cefazolin, cephapirin, cephradine, cephalexin, cefadroxil. Their spectrums of activity are quite similar. They possess generally excellent coverage against most gram-positive pathogens and variable to poor coverage against most gram negative pathogens.
The second generation cephalosporins include: cefaclor, cefamandole, cefonicid, ceforanide, cefuroxime. In addition to the gram-positive spectrum of the first generation cephalosporins, these agents have expanded gram-negative spectrum. Cefoxitin and cefotetan also have good activity against Bacteroides fragilis.
The third generation cephalosporins have much expanded gram-negative activity. However, some members of this group have decreased activity against gram-positive organisms. The third generation cephalosporins include: cefcapene, cefdaloxime, cefditoren, cefetamet, cefixime, cefmenoxime, cefodizime, cefoperazone, cefotaxime, cefpimizole, cefpodoxime, ceftibuten, ceftriaxone. They have the advantage of convenient dosing schedules, but they are expensive.
The fourth generation cephalosporins are extended-spectrum agents with similar activity against gram-positive organisms as first-generation cephalosporins. They also have a greater resistance to beta-lactamases (bacterial enzymes that may destroy antibiotic before it can do its work) than the third generation cephalosporins. Many fourth generation cephalosporins can cross blood brain barrier and are effective in meningitis. The fourth generation cephalosporins include: cefclidine, cefepime, cefluprenam, cefozopran, cefpirome, cefquinome.
Cephalosporin side effects
Cephalosporins generally cause few side effects. Common side effects associated these drugs include: diarrhoea, nausea, mild stomach cramps or upset. Approximately 5�10% of patients with allergic hypersensitivity to penicillins will also have cross-reactivity with cephalosporins. Thus, cephalosporin antibiotics are contraindicated in people with a history of allergic reactions (urticaria, anaphylaxis, interstitial nephritis, etc) to penicillins or cephalosporins. Cephalosporin antibiotics are classed as pregnancy category B.
Fluoroquinolones
Fluoroquinolones are the newest class of antibiotics. Their generic name often contains the root "floxacin". They are synthetic antibiotics that belong to the family of antibiotics called quinolones. The older quinolones are not well absorbed and are used to treat mostly urinary tract infections. The newer fluoroquinolones are broad-spectrum bacteriocidal drugs that are chemically unrelated to the penicillins or the cephaloprosins. Because of their excellent absorption fluoroquinolones can be administered not only by intravenous but orally as well.
Commonly used fluoroquinolones include Ciprofloxacin, levofloxacin, lomefloxacin, norfloxacin, sparfloxacin, clinafloxacin, gatifloxacin, ofloxacin, trovafloxacin.
Fluoroquinolones side effects
Fluoroquinolones are well tolerated and relatively safe. The most common side effects include nausea, vomiting, diarrhea, abdominal pain. Other more serious but less common side effects are central nervous system effects (headache, confusion and dizziness), phototoxicity (more common with lomefloxacin and sparfloxacin). All drugs in this class have been associated with convulsions. Fluoroquinolones are classed as pregnancy category C.
Tetracyclines
Tetracyclines got their name because they share a chemical structure that has four rings. They are derived from a species of Streptomyces bacteria. Tetracycline antibiotics are broad-spectrum bacteriostatic agents, that inhibit bacterial protein synthesis. Tetracyclines may be effective against a wide variety of microorganisms, including rickettsia and amebic parasites.
Tetracyclines are used in the treatment of infections of the respiratory tract, sinuses, middle ear, urinary tract, skin, intestines. Tetracyclines also are used to treat Gonorrhoea. Their most common current use is in the treatment of moderately severe acne and rosacea.
The most commonly prescribed tetracycline antibiotics are: tetracycline, doxycycline, minocycline, oxytetracycline.
Tetracycline side effects
Drugs in the tetracycline class become toxic over time. Expired drugs can cause a dangerous syndrome resulting in damage to the kidneys.
Common side effects associated with tetracyclines include cramps or burning of the stomach, diarrhea, sore mouth or tongue. Tetracyclines can cause skin photosensitivity, which increases the risk of sunburn under exposure to UV light. This may be of particular importance for those intending to take on holidays long-term doxycyline as a malaria prophylaxis. Rarely, tetracyclines may cause allergic reactions. Very rarely severe headache and vision problems may be signs of dangerous secondary intracranial hypertension.
Tetracycline antibiotics should not be used in children under the age of 8, and specifically during periods of tooth development. Tetracyclines are classed as pregnancy category D. Use during pregnancy may cause alterations in bone development.
Macrolides
The macrolide antibiotics are derived from Streptomyces bacteria, and got their name because they all have a macrocyclic lactone chemical structure. The macrolides are bacteriostatic, binding with bacterial ribosomes to inhibit protein synthesis. Erythromycin, the prototype of this class, has a spectrum and use similar to penicillin. Macrolide antibiotics are used to treat respiratory tract infections (such as pharyngitis, sinusitis, and bronchitis), genital, gastrointestinal tract, and skin infections.
The most commonly prescribed macrolide antibiotics are: erythromycin, clarithromycin, azithromycin, roxithromycin, troleandomycin.
Macrolides side effects
Side effects associated with macrolides include nausea, vomiting, and diarrhea; infrequently, there may be temporary auditory impairment. Azithromycin has been rarely associated with allergic reactions, including angioedema, anaphylaxis, and dermatologic reactions. Oral erythromycin may be highly irritating to the stomach and when given by injection may cause severe phlebitis. Macrolide antibiotics should be used with caution in patients with liver dysfunction. Pregnancy category B: Azithromycin, erythromycin. Pregnancy category C: Clarithromycin, dirithromycin, troleandomycin.
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