• New antibiotics from Calbiochem
• Now supplied in a convenient liquid format
• Easy-to-use, sterile-filtered, and cell culture-tested

Humans serve as hosts to a wide variety of disease-causing organisms. They include bacteria; viruses; fungi; protozoans; and helminths. Research over the past century has provided us with chemical agents to overcome these infections. Antibiotics are chemicals that can either kill the bacteria (bactericidal) or prevent their growth (bacteriostatic). Almost all bacteria can be classified as Gram-positive or Gram-negative. This classification is based on the positive or negative results from Gram’s staining method, which uses complex purple dye and iodine. Because Gram-positive bacteria have more layers of peptidoglycan in their cell walls than Gram-negative, they can retain the dye.

Antibiotics are natural substances secreted by one microorganism to ward off attack from other microorganisms. Although humans have inadvertently used natural antibiotics for centuries, these powerful agents were not discovered until the late 1920s and not exploited until the late 1940s. There are many chemicals that are lethal to bacteria, but cannot be used to cure infections because they are lethal to the host as well. The critical problem is to find substances that attack a metabolic pathway found in the bacterium but not in the host. Many metabolic activities of the bacterial cell differ from those in the human cell. These differences can be exploited in the development of antibiotic agents. Antibiotics act on bacteria in one of the following manners:

A. Inhibition of cell wall synthesis

B. Inhibition of protein synthesis

C. Inhibition of nucleic acid synthesis

D. Anti-metabolic activity or competitive antagonism

A. Inhibition of cell wall synthesis:
Bacterial cell wall contains repeating units of peptidoglycan, which are not found in human cells. Hence, the bacterial cell wall presents an ideal target for antibiotic development and therapy. The following antibiotics have been developed as inhibitors of cell wall synthesis:

1.	b -lactam antibiotics
	a. Penicillins
	b. Cephalosporins (e.g, Cefotaxime)
2.	Cycloserine, Ethionamide, Isoniazid
3.	Vancomycin
4.	Bacitracin

B. Protein Synthesis Inhibition
An antibiotic that inhibits either transcription or translation will inhibit protein synthesis. Antibiotics target transcription step because ribosomes of bacteria are different from those of higher organisms. Antibiotics that either alter the structure of the template DNA or inhibit the RNA polymerase will interfere with the synthesis of RNA, and consequently with protein synthesis. For example, Actinomycin D binds to guanine in DNA, distorting the DNA, and thus blocking transcription and Rifampin (Rifampicin or Rifamycin) inhibits protein synthesis by inhibiting the DNA-dependent RNA polymerase.

C. Nucleic Acid Synthesis Inhibitors
In order to multiply and spread infection, the pathogen must be able to replicate its DNA. Most inhibitors of DNA replication bind to DNA and are too toxic for any clinical use. The major group of antibacterial agents that act by blocking DNA synthesis/activity is the quinolone group. The quinolones block the A subunit of DNA gyrase (a topoisomerase that uncoils and coils the DNA) and induce the formation of a relaxation complex analogue. Antibiotics, such as nalidixic acid and ciprofloxacin are good inhibitors of DNA gyrase.

D. Competitive Antagonistic Antibiotics
Inhibitors of metabolic pathways via competitive antagonism include:

• Sulfonamides
• Trimethoprim
• Isoniazid