HLAR is another significant acquired resistance factor. Since the standard approach for treating systemic infections is a combination of a cell wall-targeted antibiotic with an aminoglycoside, assessment of resistance to both classes of antibiotics is important. High-level resistance to aminoglycosides will negate the synergistic effect of combined therapy with either penicillin or vancomycin.
Standard susceptibility methods (either disk diffusion or broth dilution) will not detect HLAR patterns unless the protocol incorporates testing at increased concentrations of gentamicin, streptomycin, or both. CLSI document M100 outlines recommended protocols for screening for HLAR.
Table 8. HLAR Screening.| Gentamicin HLAR |
| Method | Conditions | Interpretation |
| Disk Diffusion | MHA agar
120 µg gentamicin disk
Standard inoculum
Standard incubation temperature (35 ± 2°C; ambient air) for 16–18 hours incubation duration | Resistant = 6 mm
Inconclusive = 7–9 mm
Susceptible ≥10 mm |
| Broth Microdilution | BHI broth
500 µg/mL gentamicin
Standard inoculum
Standard incubation conditions for 24 hours incubation duration | Any growth equates to resistant
|
| Streptomycin HLAR |
| Disk Diffusion | MHA agar
300 µg streptomycin disk
Standard inoculum
Standard incubation conditions for 16–18 hours incubation duration | Resistant = 6 mm
Inconclusive = 7–9 mm
Susceptible ≥10 mm
|
| Broth Microdilution | BHI broth
1000 µg/mL streptomycin
Standard inoculum
Standard incubation conditions for 24–48 hours incubation duration. If susceptible at 24 hours, reincubate and re-read at 48 hours. | Any growth equates to resistant
|
Clinical Correlation
- A resistant result indicates that synergistic effects will not be achieved between the indicated aminoglycoside and the cell wall active agent (e.g., ampicillin, penicillin, or vancomycin).
- A susceptible result indicates that synergistic effects are possible.
