Comparative Assessment of Biofilm formation of Pseudomonas aeruginosa Isolates by Crystal Violet Assay and Viable Count Assay

This study was performed to detect biofilm formation by Pseudomonas aeruginosa by qualitative and quantitative methods. Pseudomonas aeruginosa isolates were tested for their ability to form biofilm by the tube method, the spectrophotometric and the viable count methods. Sixteen isolates (69.6%) were strong biofilm forming; two isolates (8.7%) were moderate biofilm forming, while five isolates (21.7%) were weak biofilm forming by both the tube and spectrophotometric methods. On the other hand, the viable count method was poorly correlated with either of the tube and spectrophotometric methods. High viable counts were recorded for biofilms formed by thirteen isolates (56.5%), one of which was moderate biofilm forming by the spectrophotometric method. Intermediate viable counts were found for biofilms formed by seven isolates (30.4%) including three strong biofilm forming isolates by the tube method, four strong biofilm forming isolates by the spectrophotometric method and two moderate biofilm forming isolates by both the spectrophotometric and the tube methods. This discrepancy of results may be attributed to the fact that, the matrix material and dead cells, in addition to the viable cells, are measured by the tube and spectrophotometric method, while the viable count method detects only viable cells within the biofilm.