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[Practical problems raised by antibiotic minimal inhibitory concentration determination in Legionella species
Abstract:
Because of culture medium constitution and growth conditions of Legionella species, antibiotic Minimal inhibitory Concentration (MIC) determination is difficult and does not fit with the accepted standards. The following strains have been tested : references strains : Staphylococcus aureus Oxford CNCM 53154, S. aureus 209P CNCM 53156, E. coli CNCM A224, E. coli CNCM 7324. L. pneumophila, 9 strains representative of the 6 serogroups. L. micdadei, L. dumoffii. 5 clinical isolates of L. pneumophila serogroup 1 (3 isolated in our Laboratory and 2 in Belgium). MIC of 17 antibiotics have been determined, among them macrolids not tested before. The method used was the Agar Dilution Method, with several media compared. MIC values are different in function of the various media tested Charcoal yeast extract medium, which is recommended for Legionella growth, does not inhibit activity of penicillin G, cefalotin, cefoxitin, amikacin, erythromycin and lincosamins, in contrast of tetracyclin, gentamicin, dibekacin, chloramphenicol, oleandomycin, pristinamycin, spiramycin and rifampicin. Charcoal, ferric pyrophosphate and L-cystein present in the culture medium are inactivating factors. However our results confirm good activity of rifampicin, erythromycin, cefoxitin, chloramphenicol, gentamicin and amikacin. Among the others macrolids tested, pristinamycin is the most active.
Insights
Determining antibiotic Minimal Inhibitory Concentration (MIC) for Legionella is challenging due to culture media. Charcoal yeast extract medium affects certain antibiotics, but rifampicin and erythromycin show good activity.
Area of Science:
- Microbiology
- Antimicrobial Susceptibility Testing
Background:
- Legionella species pose diagnostic challenges, particularly in determining antibiotic susceptibility.
- Standardized antibiotic Minimal Inhibitory Concentration (MIC) determination is difficult due to specific culture medium requirements and growth conditions for Legionella.
Purpose of the Study:
- To evaluate the impact of different culture media on antibiotic MIC determination for Legionella species.
- To assess the activity of 17 antibiotics, including novel macrolides, against various Legionella strains.
Main Methods:
- Agar Dilution Method was employed to determine MIC values.
- Multiple culture media were compared, focusing on Charcoal yeast extract medium, recommended for Legionella.
- Reference strains (Staphylococcus aureus, E. coli) and clinical isolates of Legionella (L. pneumophila, L. micdadei, L. dumoffii) were tested.
Main Results:
- Culture medium composition significantly influences MIC values.
- Charcoal yeast extract medium, while supporting Legionella growth, inhibited the activity of several antibiotics including tetracycline, gentamicin, and chloramphenicol.
- Components like charcoal, ferric pyrophosphate, and L-cysteine were identified as potential inactivating factors.
- Despite medium effects, rifampicin, erythromycin, cefoxitin, chloramphenicol, gentamicin, and amikacin demonstrated good activity.
- Pristinamycin emerged as the most active among the tested macrolides.
Conclusions:
- Standardized MIC determination for Legionella requires careful consideration of culture medium composition.
- Certain components in recommended Legionella media can interfere with antibiotic activity.
- Rifampicin, erythromycin, and pristinamycin show promising activity against Legionella species, warranting further investigation.