Bases of Antimicrobial Chemotherapy and Selection of Antibiotics

Chemotherapy is the term originally used to describe the use of drugs that are selectively toxic to invading microorganisms while having minimal effects on the host.

In conventional manner the definition of chemotherapy which is done by Ehrlich himself who was the inventor of drug against syphilis according to him ** the use of synthetic chemical which destroy the invading infective agents. But now a day antibiotics also included in chemotherapy. ** Antibiotics** are those substance produced by microorganisms  and that stop the growth or kill other microorganisms. But chemotherapy definition is broaden further and cancer cells killing or inhibiting substances are also included in this category.

Note *now a days antibiotics also included those substance which are semi-synthetic just like flouro-quinolones i.e (ciprofloxacin)

Principles of antimicrobial therapy

The difference between the host cell and the invading entity is of immense importance. These differences give the specificity to the drug to be only toxic for the invading pathogens and not to be toxic toward host cells.

Selection of antimicrobial agents

Selection of proper antibiotic depend upon certain factor before the therapy is initiated. The factors are 1) identity of pathogen 2) organism susceptibility to a particular agent 3 ) site of infection 4) patient factor 5 ) safety of the agent and 6) cost.

1)    Identity of the pathogen )__ through several ways the pathogen is identified. Gram staining give a through knowledge about the organism. If the identification process take some time and the infection is sever so treatment should be started before the laboratory reports with an ideal agent this strategy is called empirical therapy. Empirical treatment should be started on the bases of patient’s history. Broad spectrum therapy is initiated initially. In some special situation specific agent are preferred to be used for example gram positive cocci in the spinal fluid of a newborn infant is unlikely to be streptococcus pneumoniae and most likely to be streptococcus agalactiae which is sensitive to penicillin G. Contrast to this gram positive cocci in the spinal fluid of a 40 year old patient is most likely to be S.pneumoniae. This is frequently resistant to penicillin G and often requires treatment with high dose third generation cephalosporin (ceftriaxone) or vancomycin.

2)    Determining antimicrobial susceptibility of infective organism)__  certain drugs are preferred over others in case of certain specific infections. Certain laboratory procedures are followed to identify the susceptibility. These procedures give knowledge about a specific agent which is most active. These procedure give knowledge of selection of bactericidal and bacteriostatic agents also tell us about Minimum inhibitory concentration (MIC) and minimum bactericidal concentration. 

1)    Site of infection)--- site of infection plays an immense role in the finding a specific agent.  Site of just like CNS which is protected by blood brain barrier which impeded the flow of certain drugs. Site of infection depend upon the following factors like size of the drug molecule just like vancomycin cant cross the blood brain barrier and in the same way lipid solubility of a drug may affect it just like lipophilic metronidazole and chloramphenicol can easily cross the blood brain barrier at the other side penicillin G.

The other three Factors like patient factors, safety of the agent when some co-morbidities are there and Cost factors are decided as accordingly to rational drug therapies.