Properties of Antimicrobials

Antimicrobial chemicals differ widely in their strength, the pathogens they are effective against, and their modes of action, so each must be evaluated based on selective toxicity, spectrum of activity, mode of action, side effects, and potential for resistance.
Antimicrobials are chemicals that kill or inhibit the growth of microorganisms. All disinfectants that affect microbes, such as bleach, are antimicrobial. Disinfectants reduce microorganisms from inanimate objects or surfaces while sterilization aims to completely eliminate microorganisms. An antibiotic is an antimicrobial chemical agent used as a drug for the treatment of a bacterial infection. Antibiotics can be classified based on a number of factors. Different antibiotics target microbes in different ways and can be categorized based on the target or mode of action. Depending on the mode of action, each antibiotic is effective against a particular range of microbial species. Antibiotics are often categorized as narrow-spectrum or broad-spectrum. Narrow-spectrum antibiotics are effective against smaller, more specific groups of microbes, such as gram-negative or gram-positive bacteria but not both. Broad-spectrum antibiotics are effective against many different types of microbes. There are a number of tests available for finding the effectiveness of antimicrobials against specific species or strains of microbes. Similarly, there are tests available to determine the dose necessary to effectively kill or inhibit the growth of specific microbes.

Common Antimicrobial Drugs and Target Organisms

Antimicrobial Drug Organism Gram-Negative/Positive
Beta-lactam (penicillin and cephalosporins) Staphylococci and streptococci Positive
Macrolides Pneumonia, H. pylori, chlamydia, and acute nonspecific urethritis. Both
Aminoglycosides Aerobic bacilli, staphylococci, and certain mycobacteria Both
Tetracyclines Escherichia coli and Haemophilus influenzae Both
Polypeptides Escherichia coli and several filamentous fungi Positive
Sulfonamides Staphylococci and streptococci Both
Fluoroquinolones Staphylococci and streptococci Negative

The effectiveness of antimicrobial drugs is dependent upon the type of organism and how it stains.

No antibiotic is always safe in all patients. Side effects should be monitored and managed in patients. Side effects are often mild but can be quite serious. Fortunately, the side effects common to different antimicrobials are well documented, allowing clinicians to screen for them. The means of antimicrobial dosing has a major impact on side effects. Some antimicrobials have serious side effects when administered orally or intravenously but are safe when applied topically to the skin.

A major dilemma in the evolution and spread of antimicrobial resistance is a global public health problem. Bacteria rapidly evolve resistance to antimicrobials, especially when the drugs are not managed appropriately. There is a constant struggle between the development of new antimicrobials and bacterial evolution to circumvent them. Microbial genes conferring resistance to antimicrobials are often coded on DNA strands separate from the microbe's chromosome called plasmids. These extrachromosomal plasmid resistance genes are readily spread among different species and strains of bacteria and have led to the rapid acquisition of resistance to multiple drugs by some strains.