Antimicrobial agents
Most microbiologists distinguish two groups of antimicrobial agents used in the treatment of infectious disease: antibiotics, which are natural substances produced by certain groups of microorganisms, and chemotherapeutic agents, which are chemically synthesized. The modern era of antimicrobial chemotherapy began in 1929, with Fleming’s discovery of the powerful bactericidal substance, penicillin, and Domagk’s discovery in 1935 of synthetic chemicals (sulfonamides) with broad antimicrobial activity.
Most of the natural antibiotics that are being used in agriculture and medicine are produced by Penicillium and Cephalosporium molds produce beta-lactam antibiotics such as penicillin and cephalosporin. Actinomycetes, mainly Streptomyces species, produce tetracyclines, aminoglycosides (streptomycin and its relatives), macrolides (erythromycin and its relatives), chloramphenicol, ivermectin, rifamycins. Bacillus species, such as B. polymyxa and B. subtilis, produce polypeptide antibiotics (e.g. polymyxin and bacitracin), and B. cereus produces zwittermicin.
The most important property of an antimicrobial agent is its selective toxicity, i.e., that the agent acts in some way that inhibits or kills bacterial pathogens but has little or no toxic effect on the patient.
The most important targets in bacteria that have been exploited so far are:-
- Cell wall synthesis inhibitors
Cell wall synthesis inhibitors generally inhibit some step in the synthesis of bacterial peptidoglycan. They exert their selective toxicity against bacteria because humans cells lack cell walls.
Example: penicillins andcephalosporins.
- Cell membrane inhibitors
These antibiotics disorganize the structure or inhibit the function of bacterial membranes. The integrity of the cytoplasmic and outer membranes is vital to bacteria, and compounds that disorganize the membranes rapidly kill the cells.
Example: Polymyxin is effective mainly against Gram-negative bacteria and is usually limited to topical usage. Polymyxins bind to membrane phospholipids and thereby interfere with membrane function.
- Protein synthesis inhibitors
Many therapeutically useful antibiotics owe their action to inhibition of some step in the complex process of protein synthesis. Their attack is always at one of the events occurring on the ribosome and never at the stage of amino acid activation or attachment to a particular tRNA.
Example: tetracyclines, chloramphenicol, erythromycin) and streptomycin.
- Effects on Nucleic Acids
Some antibiotics and chemotherapeutic agents affect the synthesis of DNA or RNA, or can bind to DNA or RNA so that their messages cannot be read.
Example: Nalidixic acid inhibits DNA gyrase activity.
Rifampicin acts quite specifically on the bacterial RNA polymerase and is inactive towards DNA polymerase or RNA polymerase from animal cells.
