Evaluation of E test as a rapid method for determining MICs for nutritionally variant streptococci

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Susceptibility Testing

Background:

• Nutritionally variant streptococci (NVS) pose challenges in clinical laboratories.
• Accurate Minimum Inhibitory Concentration (MIC) estimation is crucial for effective antimicrobial therapy.
• Rapid and reliable methods for NVS susceptibility testing are needed.

Purpose of the Study:

• To evaluate the E test as a rapid and simple alternative for MIC estimation of NVS.
• To compare E test performance with conventional broth and plate dilution methods.
• To determine optimal media and conditions for E test application with NVS.

Main Methods:

• The E test was assessed using various supplemented media.
• MICs obtained with the E test were compared to agar and broth dilution reference methods.
• Media included isosensitest agar, Mueller-Hinton, and Columbia-based agar, supplemented with pyridoxal hydrochloride (vitamin B6) and defibrinated horse blood.
• The effect of cysteine supplementation and carbon dioxide (CO2) incubation was also investigated.

Main Results:

• Isosensitest agar supplemented with 5% defibrinated horse blood and 0.001% pyridoxal HCl demonstrated high agreement (within one twofold-dilution titer) with agar (complete agreement) and broth (86-93% agreement) reference methods.
• E test interpretation was challenging with other media due to hazy growth or double zoning.
• Neither cysteine addition nor CO2 incubation significantly impacted MIC results.

Conclusions:

• The E test, particularly on isosensitest agar with specific supplements, is a viable and accurate method for MIC estimation of NVS.
• This method offers a simpler and faster alternative to conventional techniques for NVS antimicrobial susceptibility testing.
• Careful selection of media is essential for reliable E test results with NVS.