A novel simple method for quantifying bacteria from endotracheal aspirates

Researchers evaluated a dipstrip method (Bacteruritest) for quantifying bacteria in endotracheal aspirates from 42 ventilated patients, comparing results with conventional surface plate counting. The dipstrip technique uses filter paper strips of standard porosity that absorb a known fluid volume and deposit it onto agar plates. Of 33 specimens yielding quantifiable counts, 31 showed results within a 10-fold range of surface plate counts, with two additional specimens within a 100-fold range. Six specimens produced confluent growth in both methods, and three showed no detectable growth by dipstrip but low counts by surface plate method. The dipstrip method accurately quantified bacterial loads from 10³ to >10¹¹ cfu/ml. The authors conclude that dipstrips provide a practical, cost-effective alternative to labor-intensive serial dilution methods for routine bacterial quantification in endotracheal aspirates. Each dipstrip costs approximately 0.5 pence compared to 30 pence per blood agar plate, and multiple imprints can be applied to single plates, potentially enabling routine quantification of bacterial loads in respiratory specimens.

Key findings

• Dipstrip method yielded bacterial counts within a 10-fold range of surface plate counts in 31 of 33 evaluable specimens
• Dipstrips accurately quantified bacterial loads in the range of 10³ to >10¹¹ cfu/ml, encompassing clinically relevant diagnostic thresholds
• Dipstrip method is significantly more cost-effective (0.5 pence per strip versus 30 pence per agar plate) and less labor-intensive than conventional serial dilution techniques
• Three proposed dilutions (2-fold, 10², and 10⁴) would allow quantitative estimation of endotracheal aspirate loads between 10⁴-10¹⁰ cfu/ml