Preparing inoculum for susceptibility testing of anaerobes

Area of Science:

• Microbiology
• Clinical Laboratory Science
• Antimicrobial Susceptibility Testing

Background:

• Accurate inoculum preparation is critical for reliable anaerobic bacteria antimicrobial susceptibility testing.
• Current methods may not consistently yield optimal viable bacterial counts.

Purpose of the Study:

• To evaluate six different methods for preparing anaerobic bacteria inoculum for susceptibility testing.
• To determine the most effective method for obtaining consistent and high viable bacterial counts across multiple anaerobic species.

Main Methods:

• Six distinct inoculum preparation methods were assessed.
• Viable bacterial counts were determined for 15 anaerobic species using each method.
• Methods included broth microdilution, broth-disk elution, and the National Committee for Clinical Laboratory Standards agar method.

Main Results:

• Inocula prepared from 24- and 48-hour anaerobe blood agar plates, adjusted to a 0.5 McFarland standard, provided the most consistent and highest viable counts.
• These agar plate-derived suspensions yielded higher counts compared to overnight growth in thioglycolate or Schaedler broth.

Conclusions:

• Preparation of inoculum directly from 24- and 48-hour anaerobe blood agar plates is recommended for reliable susceptibility testing.
• This optimized method can potentially expedite anaerobic bacteria susceptibility testing, improving clinical relevance.