Antibiotic is a chemical substance produced by a microorganism that inhibits the growth of or kills other microorganisms especially bacteria and fungi.

Illnesses such as syphilis, tuberculosis, salmonella, and some forms of meningitis are caused by bacteria and are treated by antibiotics. The earliest account of the usage of microbes against microbes was laid by Fleming. Fleming (1928) described the effect on staphylococci of a substance produced by Penicillium notatum and researched its possible use as a treatment for bacterial infections.

The noun “antibiotic” was first used in 1942 by Dr. Selman A. Waksman, soil microbiologist. Dr. Waksman and his colleagues discovered several Actinomycetes derived antibiotics. In forties to sixties, the term “antibiotic” was clearly different from the term “chemotherapeutic drug”. Chemotherapeutic drugs were man-made substances used to cure the diseases.

“All antibiotics are chemotherapeutic agents, but not all chemotherapeutic agents are antibiotics”

    The word antibiotic denotes:

“A chemical substance derived from a microorganism or produced by chemical synthesis that is able to kill or inhibit microorganisms and cure infections.”

Originally, earliest records say that antibiotics were derived from fungal sources. These can be referred to as “natural” antibiotics. But these Natural antibiotics are more toxic than synthetic antibiotics.

Examples of the natural antibiotics are:

• Benzyl penicillin
• Gentamicin

SOURCES OF ANTIBIOTICS:

• Bacteria (most prolific source is the Streptomycete group)

For example Streptomycin, Tetracycline, Chloramphenicol and Kanamycin etc.

• Moulds (filamentous fungi) 

For example Penicillins and Cephalosporins

• Synthetic  antibiotics

For example Chloramphenicol

• Semi-synthetic

Part of molecule is produced by a microorganism and part of it is modified/ altered chemically to chemically improve it.

E.g. many types of Penicillin - Ampicillin, Methicillin, Carbenicillin, Cloxacillin

CLASSIFICATION:

There are several classification schemes for antibiotics:

• Based on bacterial spectrum (broad versus narrow)
• Route of administration (Injectable versus oral versus topical)
• Type of activity (bactericidal vs. bacteriostatic)
• Chemical structure.

The most important method of classification is based upon chemical structure. Antibiotics of a particular structural class generally show similar patterns of effectiveness, toxicity, and allergic potential.

The classification according to their structure:

1.   Amino glycosides (Amikacin, Gentamicin, Streptomycin etc.)

     They have very strong and rapid bactericidal effect on bacteria. Amino glycosides are not effective against anaerobes. They act upon several sites of bacterial cell (outer membrane, Ribosomes). Amino glycosides are preferably used in combination with other antibiotics.

• Streptomycin is an old drug used in the treatment of tuberculosis.
• Gentamicin is a standard and most widely used amino glycoside.
• Amikacin and Isepamicin- These antibiotics resist various bacterial destructive enzymes.

2.   Glycopeptides (Vancomycin and Fusfomycin)

They are bactericidal drugs inhibiting bacterial cell wall synthesis in a step prior to beta lactam action. They interfere with RNA synthesis or injure the protoplast. They have large molecular size and hence are not able to penetrate the cell wall and are not absorbed from the gastrointestinal tract. The drugs are excreted almost exclusively by glomerular filtration.

• Vancomycin

Used for Staph or Strep infections RESISTANT to Methicillin.

Vancomycin potentiates the nephrotoxicity of amino glycosides.

• Fusfomycin

Inhibits cell wall synthesis

Treatment of uncomplicated UTI only in females

3.   β-lactum:

They are bactericidal drugs. The bacterial enzymes that are affected by beta-lactam are called penicillin binding proteins. They inhibit cell wall synthesis.

• Penicillins-the usage is against the following:Carbapenems(Imipenem)
  • Gram +ve infections
  • Treponema (Syphilis)
  • Pseudomonas (Penicillin IV + Amino glycosides)
• Cephalosporins (1^st - 4^th generation)

4.   Fluoroquinolones (Nalidixic acid):

They are synthetic antibacterial agents, and not derived from bacteria. They are chemically unrelated to the penicillin or the cephalosporin. They get so well absorbed that in general they are as effective by the oral route as by intravenous infusion.

5.   Oxazolidinones (Linezolid)

They bind to 50S and prevent formation of initiation complex and prevent the cell replication. They are used in people who are allergic to Vancomycin. 

6.   Macrolides(Erythromycin)

They are static antibiotics reversibly inhibit protein synthesis on ribosomal level. They are derived from Streptomyces bacteria. Erythromycin, the prototype of this class is used similar to penicillin. After the introduction of more modern members into this class of antibiotics they became very popular because of low frequency of side effects and comfortable usage.

• Lincosamides (Clindamycin): it is  effective against
• gram +ve organisms
• anaerobes (B.fragilis)

7.   Miscellaneous (Metronidazole, Tetracyclin)

• Tetracyclin: They are derived from a species of Streptomyces bacteria. They are bacteriostatic and used against a large variety of microorganisms, including Rickettsia and amebic parasites.

WHAT IS A PERFECT ANTIBIOTIC?

 A perfect antibiotic should have following features:

• Non-toxic
• Non-allergenic
• Selective target
• Broad spectrum of activity
• No development of resistance to it by bacteria
• Stable
• Cheap
• Long shelf life
• High therapeutic index
• Variable excretion rate from body (high rate for acute infections and UTIs, low rate for chronic infections)

HOW DO THE ANTIBIOTICS WORK?

Antibiotics act by inhibiting crucial life sustaining processes in the cell/ organism. They work by inhibiting synthesis of cell wall materials like the synthesis of DNA, RNA, Ribosomes and proteins etc. They can be both: bactericidal or bacteriostatic i.e. they can either kill the bacteria or inhibit their multiplication.

DOSAGE RECOMMENDED:

Dosage varies with drug, route of administration, pathogen, site of infection, and severity. Renal function, age of patient, and other factors also impact the dosage. Consultation from the doctor is recommended. Most antibiotics start having an effect on an infection within a few hours.  It is important to remember to complete the whole course of the medication. Some antibiotics should not be consumed with certain foods and drinks while some others should not be taken with the food in your stomach hence they should be taken at least an hour before or after the food intake.

SIDE EFFECTS:

All antibiotics cause risk of overgrowth by non-susceptible bacteria. The common side effects are:

• Feeling and being sick
• Diarrhea
• Fungal infections of the mouth, digestive tract and vagina
• Allergic reactions, especially from penicillin

Some rare side effects are:

• Abnormal blood clotting 
• Sensitivity to sun
• Formation of kidney stones 

PRECAUTIONS:

• While taking antibiotic do not take any other OTC/herbal medicine without telling your doctor
• If you are taking metronidazole do not consume alcohol
• Dairy products should not be consumed if you are taking tetracycline
• Do not over use the antibiotics