Rules for antibiotic dilution

Calculating dosage using ampicillin as an example

If the drug vial indicates dosage in milligrams or grams rather than standard units of action, the medical professional uses the so-called "third" dilution rule.

Key parameter for mathematical conversion: 1 gram of dry substance corresponds to 1,000,000 units of action (IU).

For example, if a physician prescribes a patient 750,000 IU, and a vial containing 1 gram of the drug is available (equivalent to 1000 mg or 1,000,000 IU), the calculation is performed similarly to synthetic antibacterial agents. The required volume of solvent is calculated so that the syringe contains the strictly prescribed dose.

Determining microorganism susceptibility

For therapy to be successful, it is necessary to determine the pathogen's susceptibility spectrum in vitro. The outcome of treatment depends not only on the proper selection of the drug, but also on the patient's immunity and concomitant diseases.

The main task of laboratory tests is to find the minimum inhibitory concentration (MIC). This is the dose of the drug that completely prevents visible reproduction of bacteria on the nutrient medium.

Two basic groups of studies are distinguished:

• Diffusion of the active substance into a solid medium.
• Serial dilutions in liquid or solid medium.

Note: for the cultivation and isolation of anaerobic bacteria in the laboratory, the use of an anaerostat is mandatory.

Disk diffusion method and E-test

Disk diffusion method is the most popular qualitative analysis due to its simplicity. A pure culture is seeded as a dense lawn onto dried standard Mueller-Hinton agar. Standard industrial paper disks impregnated with the antibiotic are placed on top using sterile tweezers or an automatic dispenser (maximum 6 disks per dish). It is important to strictly observe the distance: at least 1.5–2 cm from the edge of the dish and between the disks themselves. The dishes are inverted, placed in a thermostat, and incubated for 18–20 hours at 37°C. Afterward, the diameters of the growth inhibition zones are measured and conclusions are drawn according to the manufacturer's instructions.

E-test is a hybrid option, occupying an intermediate position between disks and serial dilutions. Instead of disks, a narrow polymer strip is placed on the inoculated agar. Concentration values are printed on it, and the strip is impregnated with a drug gradient (from minimum to maximum doses). When the flora is sensitive, the zone of no growth acquires a teardrop (ellipsoid) shape. The MIC is taken as the mark on the strip closely approached by the microorganism colonies.

Serial dilution method

This is a quantitative, highly accurate, and informative approach (also known as Koch's method, allowing quantitative accounting of CFU). A series of test tubes with sequentially decreasing antibiotic concentrations is prepared in a liquid nutrient medium. Then, a strictly equal volume of the studied bacterial culture is added to each container. Based on the presence or absence of visible growth in the test tubes, the MIC is calculated accurately.

Features of preparing topical antimicrobial agents

In addition to systemic antibiotics, topical combined iodine preparations are used in practice, which also require compliance with preparation rules. For example, Iodonat solution (containing iodine, potassium iodide, alkyl sulfonate, and phosphoric acid) must be diluted exactly 4.5 times before surgical hand scrubbing, preparing the surgical field, or treating wound edges before suturing. Only sterile or boiled water is used for this.

Other agents are used without dilution:

• Jox spray (allantoin and povidone-iodine) is prescribed in ENT practice for streptococcal pharyngitis, tonsillitis, glossitis, laryngitis, and early symptoms of influenza. Rarely causes transient burning.
• Iodopyron (povidone-iodine and potassium iodide) is available as a solution, ointment, and aerosol. It has a broad bactericidal effect against gram-positive and gram-negative flora (including resistant strains of E. coli, S. aureus, Proteus), as well as fungicidal and anti-inflammatory effects. The solution is used for disinfecting gloves and donor elbow bends, while ointments are used to treat infected wounds, burns, trophic ulcers, dermatitis, and pressure ulcers.

Class and exam questions

What else to study on this topic

• Bacteriological research method: inoculation, isolation of a pure culture, and study of physiological properties.
• Macroorganism factors influencing the outcome of antibiotic therapy (immune status, concomitant diseases).
• Mechanisms of action of combined iodine preparations and their effect on pharmacokinetics.
• Features of cultivating anaerobic microorganisms using an anaerostat.

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.