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Antibiotics may be informally defined as the subgroup of anti-infectives that are derived from bacterial sources and are used to treat bacterial infections. Other classes of drugs, most notably the sulfonamides, may be effective antibacterials. Similarly, some antibiotics may have secondary uses, such as the use of demeclocycline (Declomycin, a tetracycline derivative) to treat the syndrome of inappropriate antidiuretic hormone (SIADH) secretion. Other antibiotics may be useful in treating protozoal infections.


Antibiotics are used for treatment or prevention of bacterial infection.



Although there are several classification schemes for antibiotics, based on bacterial spectrum (broad versus narrow) or route of administration (injectable versus oral versus topical), or type of activity (bactericidal vs. bacteriostatic), the most useful is based on chemical structure. Antibiotics within a structural class will generally show similar patterns of effectiveness, toxicity, and allergic potential.
PENICILLINS. The penicillins are the oldest class of antibiotics, and have a common chemical structure which they share with the cephalopsorins. The two groups are classed as the beta-lactam antibiotics, and are generally bacteriocidal—that is, they kill bacteria rather than inhibiting growth. The penicillins can be further subdivided. The natural pencillins are based on the original penicillin G structure; penicillinase-resistant penicillins, notably methicillin and oxacillin, are active even in the presence of the bacterial enzyme that inactivates most natural penicillins. Aminopenicillins such as ampicillin and amoxicillin have an extended spectrum of action compared with the natural penicillins; extended spectrum penicillins are effective against a wider range of bacteria. These generally include coverage for Pseudomonas aeruginaosa and may provide the penicillin in combination with a penicillinase inhibitor.
CEPHALOSPORINS. Cephalosporins and the closely related cephamycins and carbapenems, like the pencillins, contain a beta-lactam chemical structure. Consequently, there are patterns of cross-resistance and cross-allergenicity among the drugs in these classes. The "cepha" drugs are among the most diverse classes of antibiotics, and are themselves subgrouped into 1st, 2nd and 3rd generations. Each generation has a broader spectrum of activity than the one before. In addition, cefoxitin, a cephamycin, is highly active against anaerobic bacteria, which offers utility in treatment of abdominal infections. The 3rd generation drugs, cefotaxime, ceftizoxime, ceftriaxone and others, cross the blood-brain barrier and may be used to treat meningitis and encephalitis. Cephalopsorins are the usually preferred agents for surgical prophylaxis.
FLUROQUINOLONES. The fluroquinolones are synthetic antibacterial agents, and not derived from bacteria. They are included here because they can be readily interchanged with traditional antibiotics. An earlier, related class of antibacterial agents, the quinolones, were not well absorbed, and could be used only to treat urinary tract infections. The fluroquinolones, which are based on the older group, are broad-spectrum bacteriocidal drugs that are chemically unrelated to the penicillins or the cephaloprosins. They are well distributed into bone tissue, and so well absorbed that in general they are as effective by the oral route as by intravenous infusion.
TETRACYCLINES. Tetracyclines got their name because they share a chemical structure that has four rings. They are derived from a species of Streptomyces bacteria. Broad-spectrum bacteriostatic agents, the tetracyclines may be effective against a wide variety of microorganisms, including rickettsia and amoebic parasites.
MACROLIDES. The macrolide antibiotics are derived from Streptomyces bacteria, and got their name because they all have a macrocyclic lactone chemical structure. Erythromycin, the prototype of this class, has a spectrum and use similar to penicillin. Newer members of the group, azithromycin and clarithyromycin, are particularly useful for their high
Different antibiotics destroy bacteria in different ways. Some short-circuit the processes by which bacteria receive energy. Others disturb the structure of the bacterial cell wall, as shown in the illustration above. Still others interfere with the production of essential proteins.
Different antibiotics destroy bacteria in different ways. Some short-circuit the processes by which bacteria receive energy. Others disturb the structure of the bacterial cell wall, as shown in the illustration above. Still others interfere with the production of essential proteins.
(Illustration by Electronic Illustrators Group.)
level of lung penetration. Clarithromycin has been widely used to treat Helicobacter pylori infections, the cause of stomach ulcers.
OTHERS. Other classes of antibiotics include the aminoglycosides, which are particularly useful for their effectiveness in treating Pseudomonas aeruginosa infections; the lincosamindes, clindamycin and lincomycin, which are highly active against anaerobic pathogens. There are other, individual drugs which may have utility in specific infections.

Recommended dosage

Dosage varies with drug, route of administration, pathogen, site of infection, and severity. Additional considerations include renal function, age of patient, and other factors. Consult manufacturers' recommendations for dose and route.

Key terms

Bacteria — Tiny, one-celled forms of life that cause many diseases and infections.
Inflammation — Pain, redness, swelling, and heat that usually develop in response to injury or illness.
Meningitis — Inflammation of tissues that surround the brain and spinal cord.
Microorganism — An organism that is too small to be seen with the naked eye.
Organism — A single, independent unit of life, such as a bacterium, a plant or an animal.
Pregnancy category — A system of classifying drugs according to their established risks for use during pregnancy. Category A: Controlled human studies have demonstrated no fetal risk. Category B: Animal studies indicate no fetal risk, but no human studies; or adverse effects in animals, but not in well-controlled human studies. Category C: No adequate human or animal studies; or adverse fetal effects in animal studies, but no available human data. Category D: Evidence of fetal risk, but benefits outweigh risks. Category X: Evidence of fetal risk. Risks outweigh any benefits.

Side effects

All antibiotics cause risk of overgrowth by non-susceptible bacteria. Manufacturers list other major hazards by class; however, the health care provider should review each drug individually to assess the degree of risk. Generally, breastfeeding is not recommended while taking antibiotics because of risk of alteration to infant's intestinal flora, and risk of masking infection in the infant. Excessive or inappropriate use may promote growth of resistant pathogens.
Penicillins: Hypersensitivity may be common, and cross allergenicity with cephalosporins has been reported. Penicillins are classed as category B during pregnancy.
Cephalopsorins: Several cephalopsorins and related compounds have been associated with seizures. Cefmetazole, cefoperazone, cefotetan and ceftriaxone may be associated with a fall in prothrombin activity and coagulation abnormalities. Pseudomembranous colitis has been reported with cephalosporins and other broad spectrum antibiotics. Some drugs in this class may cause renal toxicity. Pregnancy category B.
Fluroquinolones: Lomefloxacin has been associated with increased photosensitivity. All drugs in this class have been associated with convulsions. Pregnancy category C.
Tetracyclines: Demeclocycline may cause increased photosensitivity. Minocycline may cause dizziness. Do not use tetracyclines in children under the age of eight, and specifically avoid during periods of tooth development. Oral tetracyclines bind to anions such as calcium and iron. Although doxycycline and minocycline may be taken with meals, patients must be advised to take other tetracycline antibiotics on an empty stomach, and not to take the drugs with milk or other calcium-rich foods. Expired tetracycline should never be administered. Pregnancy category D. Use during pregnancy may cause alterations in bone development.
Macrolides: Erythromycin may aggravate the weakness of patients with myasthenia gravis. Azithromycin has, rarely, been associated with allergic reactions, including angioedema, anaphylaxis, and dermatologic reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis. Oral erythromycin may be highly irritating to the stomach and when given by injection may cause severe phlebitis. These drugs should be used with caution in patients with liver dysfunction. Pregnancy category B: Azithromycin, erythromycin. Pregnancy category C: Clarithromycin, dirithromycin, troleandomycin.
Aminoglycosides: This class of drugs causes kidney and ototoxicity. These problems can occur even with normal doses. Dosing should be based on renal function, with periodic testing of both kidney function and hearing. Pregnancy category D.

Recommended usage

To minimize risk of adverse reactions and development of resistant strains of bacteria, antibiotics should be restricted to use in cases where there is either known or a reasonable presumption of bacterial infection. The use of antibiotics in viral infections is to be avoided. Avoid use of fluroquinolones for trivial infections.
In severe infections, presumptive therapy with a broad-spectrum antibiotic such as a 3rd generation cephalosporin may be appropriate. Treatment should be changed to a narrow spectrum agent as soon as the pathogen has been identified. After 48 hours of treatment, if there is clinical improvement, an oral antibiotic should be considered.



"Consumer Alert: Antibiotic Resistance Is Growing!" People's Medical Society Newsletter 16 (August 1997): 1.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.

Patient discussion about Antibiotics

Q. Can I stop taking my Antibiotics? The Doctor prescribed me Antibiotics for 10 days. I have been taking them for 5 days and feel better. Can I stop taking them?

A. you need to take all of your pills,if not it could come back.

Q. Why Is it Important to Not Use Antibiotics Often? Why is my doctor always so reluctant to prescribe me antibiotics?

A. Antibiotic resistance has become a serious problem in both developed and underdeveloped nations. By 1984 half of those with active tuberculosis in the United States had a strain that resisted at least one antibiotic. In certain settings, such as hospitals and some childcare locations, the rate of antibiotic resistance is so high that the usual, low-cost antibiotics are virtually useless for treatment of frequently seen infections. This leads to more frequent use of newer and more expensive compounds, which in turn leads to the rise of resistance to those drugs. A struggle to develop new antibiotics ensues to prevent losing future battles against infection. Therefore the doctors try to avoid using antibiotics when it is not necessary, and try to keep a certain limited use of these medications.

Q. Do Antibiotics cure a cold? I have a cold and a runny nose, should I take Antibiotics?

A. Taking antbiotics when you only have a cold can harm your chances of the effectiveness of using antibiotics when you have a severe problem. Your body can build up an immunity to antibiotics so it is only recommended to take them when your immune system can't fight off the infections. Most of the time, a cold just needs to run it's course , so drinking plenty of fluids and resting can allow your body to rejuvinate and fight the cold. To help prevent colds and viruses, look for products that help to maintain a good immune system like vitamin C. Aloe juice is another good product for your immune system. When we deal with stress and don't get enough rest, we cause havoc on our immune system, so prevention can be the best thing to do. Wishing you well!

More discussions about Antibiotics
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References in periodicals archive ?
Alcohol and antibiotics shouldn't be mixed for many reasons, but there is one commonly heard misconception when it comes it comes to alcohol and antibiotics.
At the same time, many low- and middle-income countries experience vast gaps in access to effective and appropriate antibiotics. Childhood deaths due to pneumonia (estimated globally at close to one million per year) because of lack of access to antibiotics remain frequent in many parts of the world.
Cross-tabulation was done to determine the association between level of knowledge around the appropriate use of antibiotics, when to discontinue antibiotics course and the conditions/illnesses antibiotics treat.
For every 1,000 women taking antibiotics for at least two months, just six would be likely to experience damage to their hearts or arteries, they said.
Simply using antibiotics creates resistance the drugs should only be used to treat infections.
Dr Kumar quoted a British study that now challenges the traditional belief that patients need to be prescribed five-10 days courses of antibiotics. "The study points out that antibiotic courses should not be beyond three-four days as the patient's immune system should be allowed to kick in to counter the condition.
The more often someone with a UTI is treated with antibiotics, the more likely she is to experience a recurrence due to the reservoir of antibiotic-resistant bacteria.
'Most viral infections don't even require antibiotics' he stressed.
The authorities concerned should take this matter seriously and take measures to minimise the use of antibiotics.
Smart use of antibiotics is key to controlling the spread of resistance.' He added that in Pakistan, alarming 88.9% rates of unnecessary antibiotic prescriptions are for self-limiting Upper Respiratory Tract infections.
Prof Dr Khalid Hussain Mahida said antibiotics are not effective against viral infections like the common cold, flu, most sore throats, bronchitis, and many sinus and ear infections.
Staff at Mersey Care NHS Foundation Trust are urging people to become antibiotic guardians to help spread the message that taking antibiotics when you don't need them puts you and your family at risk.

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