Antimicrobial Therapy Overview

Selection of Antimicrobials

• Bacteriostatic medications slow the growth of micro-organisms, but the immune system response of phagocytic cells (macrophages, neutrophils) actually destroys the bacteria.
• Three principal factors to consider when selecting an antibiotic: identity of the causative agent, sensitivity of the infecting organism to an antimicrobial, and other factors (location of infection, age, allergies, and immune status of host).

Identification of Causative Agent

• Laboratory testing of body fluids (blood, urine, sputum, and wound drainage) identifies the micro-organism causing the infection.
• Gram stain: technicians examine an aspirate of the body fluid under a microscope to identify the micro-organisms directly.
• Culture: technicians apply the aspirate to a culture medium, where colonies of the micro-organism grow over several days.

Sensitivity of a Micro-organism to an Antimicrobial

• Disk diffusion test (Kirby-Bauer test): determines the degree of medication sensitivity.
• Serial dilution: a quantitative method using several test tubes with varying concentrations of the antimicrobial to determine the amount necessary to treat a specific infection.
• Minimum inhibitory concentration (MIC): the lowest concentration of antibiotic that inhibits bacterial growth completely but does not kill the bacteria.
• Minimum bactericidal concentration: the lowest concentration of the antibiotic that kills 99.9% of the bacteria.

Host Factors

• Immune system: people with an intact immune system require either bactericidal or bacteriostatic antibiotics, while those with immune system compromise need strong bactericidal antibiotics.
• Site of infection: some sites are difficult for antimicrobials to reach, such as cerebrospinal fluid, heart, and purulent abscesses.
• Age: infants and older adults are at increased risk for antimicrobial toxicity due to undeveloped kidney and liver function or reduced medication metabolism and excretion.

Antimicrobial Therapy

• Antimicrobial therapy: the use of medications to treat infections due to bacteria, viruses, or fungi.
• Antimicrobials use selective toxicity to kill or control microbes without destroying host cells.
• Changes in the DNA of micro-organisms (conjugation) produce resistance to multiple existing medications, requiring the creation of new antimicrobials.

Methods of Antimicrobial Actions

• Destroying the cell wall present in bacteria but not in mammals.
• Inhibiting the conversion of an enzyme unique for a particular bacterium's survival.
• Impairing protein synthesis in the bacteria's ribosomes, which are never identical to mammalian cells.
• Disrupting bacterial synthesis or function of DNA and RNA.
• Inhibiting viral replication.

Classification of Antimicrobial Medications

• Narrow-spectrum antibiotics: effective against a few types of bacteria.
• Broad-spectrum antibiotics: effective against a wide variety of bacteria.
• Bactericidal medications: directly lethal to the micro-organism.

Prophylaxis

• Indications for prophylactic use include prevention of infections in clients undergoing surgery, sexually transmitted infections following sexual exposure, and individuals with prosthetic heart valves prior to dental or other procedures.
• Use antimicrobials for individuals who have recurring urinary tract infections.

Preventive Measures

• Perform hand hygiene before and after each client contact to prevent the spread of infection.
• Recognize invasive procedures that increase the risk of infection.
• Encourage clients to maintain an up-to-date immunization status.
• Instruct clients to take the full course of antimicrobials prescribed to prevent medication resistance and recurrence of infection.
• Use infection-control procedures to prevent transmission of resistant micro-organisms.