Microbiología. Antibiograma por difusión en disco.

Antibiogram by Kirby-Bauer Method: Overview and Key Considerations

General Introduction to Antibiogram Methods

• Various methods can be used to measure the in vitro susceptibility of pathogenic microorganisms to antimicrobials. The standardized methods include macro and micro dilution in broth, as well as the disk diffusion method, commonly known as the Kirby-Bauer test.

Characteristics of the Kirby-Bauer Method

• The Kirby-Bauer method is widely utilized in bacteriology laboratories and is standardized for rapidly growing microorganisms and some fastidious organisms. It provides qualitative results that correlate with quantitative minimum inhibitory concentration (MIC) determinations.

• This method involves using a constant concentration of an antimicrobial agent impregnated on filter paper disks placed on solid culture media inoculated with the bacteria being tested for sensitivity. Once in contact, the antibiotic diffuses radially into the medium.

Interpretation of Results

• Bacterial sensitivity is interpreted based on the clear zone of inhibition around the disk; its diameter (measured in millimeters) is proportional to bacterial susceptibility. Proper execution requires a standardized inoculum and conditions that allow adequate diffusion of antibiotics, which can affect interpretation.

Technical Conditions Affecting Antibiogram Results

• Several factors must be considered:

• Bacterial Conditions: Growth kinetics, number of bacteria, and growth phase are crucial.

• Antimicrobial Disks: Must be stored at specific temperatures; beta-lactams should remain frozen until use.

• Culture Media: Agar pH should be between 7.2 and 7.4; deviations can lead to false results regarding susceptibility or resistance due to chemical interactions within the medium.

Specific Recommendations for Culture Media

• Recommended agar depth is between 3 to 5 mm; less than this may yield false susceptibility readings while more than this may produce false resistance readings.

• Excess thymidine can cause false resistance for sulfonamides and trimethoprim; high levels of calcium/magnesium ions can also lead to misleading results for aminoglycosides and tetracyclines in Pseudomonas aeruginosa cases. Low cation content may result in false susceptibility readings for certain strains like Lyon zinc excess leading to false resistance against carbapenems.

Quality Control Measures

• For quality control, strains such as Escherichia coli and Staphylococcus aureus (non-beta-lactamase producers) should be used routinely alongside other specific strains depending on testing requirements.

• Regular checks on McFarland standards are necessary monthly, ensuring uniform turbidity through vortex agitation before storage under appropriate conditions (darkness at room temperature). Incubation conditions including temperature, time, and atmosphere significantly influence bacterial growth outcomes during testing procedures.

Microbial Culture and Antibiotic Sensitivity Testing

Preparation of Bacterial Cultures

• The process begins with selecting specific discs based on the microorganism being evaluated, assessing the turbidity to McFarland standard 0.5, and using a pure bacterial culture.

• Colonies should be taken from an 18 to 24-hour growth period, preferably from selective media, and suspended in sterile saline solution for proper dilution.

• The suspension is adjusted visually to match the McFarland standard, which corresponds to approximately 1 to 2 x 10^8 colony-forming units per milliliter.

Inoculation Techniques

• After adjusting turbidity, inoculate the agar plate by streaking in one direction with a sterile swab; rotate the plate at a 60-degree angle for thorough coverage.

• Repeat the streaking process in different directions (total of three extractions) to ensure complete and homogeneous inoculation across the agar surface.

Application of Antibiotic Discs

• For antibiotic susceptibility testing (e.g., Staphylococcus aureus), place discs at least 15 to 26 mm apart on the agar surface after allowing it to dry for a few minutes.

• A maximum of five discs can be applied per plate; however, up to seven may fit on a larger plate (100 mm), or twelve on even larger plates (150 mm).

Incubation Conditions

• Once all discs are placed, incubate plates inverted within 15 minutes at 35°C for standard bacteria or under CO2 conditions for fastidious organisms like Haemophilus spp.

Reading Results

• Post-incubation, assess growth by measuring inhibition zones against a black background; clear areas indicate effective inhibition where no growth occurs.

• If no inhibition is observed, classify as resistant; consider medium type when interpreting results—translucent media require specific lighting techniques for accurate measurement.

Interpretation Challenges

• Special attention is needed when reading results involving opaque media or certain antibiotics that may show atypical growth patterns due to delayed inhibition effects.

• In cases of double zones of inhibition or mixed cultures within zones, further subculturing may be necessary for accurate identification and susceptibility assessment.

This structured approach ensures clarity in understanding microbial culture preparation and antibiotic sensitivity testing processes while providing direct references through timestamps.

Antimicrobial Susceptibility Testing and Interpretation

Understanding Inhibition Zones in Antimicrobial Testing

• The opacity around an oxacillin disk in a Staphylococcus aureus test indicates significant inhibition, showing resistance only up to the edge of the opacity. This suggests homogeneous growth around the disk, which may extend to its border or exhibit a reduced halo after measuring inhibition zones.

Interpreting Results Based on CLSI Guidelines

• Interpretation of inhibition results is based on CLSI tables, considering the type of microorganism evaluated, the antimicrobial used, and its concentration. The diameter of inhibition is compared against criteria to classify microorganisms as sensitive, intermediate, resistant, or non-sensitive.

Categories of Antimicrobial Resistance

• Sensitive isolates are inhibited by recommended antimicrobial concentrations for treatment. Intermediate includes isolates with minimum inhibitory concentrations (MIC) close to achievable blood or tissue levels; response rates may be lower than sensitive isolates. Resistant implies no inhibition at recommended doses due to specific resistance mechanisms like beta-lactamase production.

Non-Sensitive Isolates and Reporting Practices

• Non-sensitive categories apply when isolates show low inhibition zones below sensitivity cut-off values but do not necessarily indicate a resistance mechanism. Identification and susceptibility testing should be repeated for these isolates; selective reporting helps clinicians choose narrow-spectrum antibiotics effectively to reduce multi-resistant strain selection from broad-spectrum use.

Case Study: Staphylococcus aureus Antibiogram Results

• A culture identified Staphylococcus aureus with varying diameters of inhibition for selected antimicrobials:

• Oxacillin: 30 mm

• Erythromycin: 31 mm

• Clindamycin: 32 mm

• Sulfamethoxazole: 30 mm

• Linezolid: 30 mm

• Tetracycline: 32 mm

• Rifampicin: 16 mm

Results must be compared with CLSI criteria for susceptibility categorization.

Final Reporting Considerations

• The report indicated that Staphylococcus aureus was sensitive to oxacillin, sulfamethoxazole, tetracycline, erythromycin, clindamycin but resistant to rifampicin. It’s crucial that reports specify oxacillin sensitivity/resistance even if tested with an oxacillin disk; if resistant, note that it indicates resistance to all beta-lactams except certain cephalosporins effective against methicillin-resistant strains (MRSA).