Incubation time and culture medium affect gram staining accuracy: A Cellpose image analysis study

Gram staining remains the cornerstone rapid technique for bacterial identification in clinical microbiology. The quantitative influence of culture medium and incubation duration on staining fidelity has not been systematically evaluated using automated image analysis.

270 Gram-stained images were acquired from Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922 grown on blood agar (BA), brain-heart infusion broth (BHI), and species-selective media at 24, 48, and 72 h. Cellpose 2.0 yielded 90 quantitative variables across staining intensity, cell wall characteristics, and cell morphology. Data were analysed by ANOVA, Pearson correlation, and principal component analysis (PCA).

Both medium and incubation time significantly affected all variable groups (p < 0.001). The Blue/Red staining ratio showed greater temporal sensitivity in S. aureus (|Δ|: 0.319 vs 0.109). Peptidoglycan retention score declined from 54.47 ± 19.46 at 24 h to 25.29 ± 11.87 at 72 h (p < 0.001) in S. aureus. Gram-positive classification fell to ≤26% at 72 h across all media. Principal component analysis (PCA) explained 63.6% of total variance, separating species (PC1) and time points (PC2). Blue/Red ratio correlated strongly with Gram-positive percentage (r = 0.910) and peptidoglycan retention score (r = 0.740).

Extended incubation (>48 h) and selective media introduce systematic bias, particularly in S. aureus ATCC 25923. The 90-variable Cellpose framework supports 24-48 h cultures from nutrient-rich media as optimal for reliable Gram stain-based identification and laboratory quality assurance.