Real-time imaging of bacterial colony growth dynamics for cells with Type IV-A1 CRISPR-Cas activity

The Type IV-A1 CRISPR-Cas system of Pseudomonas oleovorans provides defense against mobile genetic elements in the absence of target DNA degradation. In recent studies, Escherichia coli BL21-AI cells with Type IV-A1 CRISPR-Cas activity displayed a heterogeneous colony growth phenotype. Here, we developed a convenient smartphone-mediated automatic remote-controlled time-lapse imaging system (SMARTIS), that enables monitoring of growing bacteria over time. The system's design includes a custom-built imaging box equipped with LED lights, an adjustable heating system and a smartphone that can be remotely controlled using freely available, user-friendly applications. SMARTIS allowed long-term observation of growing colonies and was utilized to analyze different growth behaviors of E. coli cells expressing Type IV-A1 CRISPR ribonucleoproteins. Our findings reveal that heterogeneity in colonies can emerge within hours of initial growth. We further examined the influence of different expression systems on bacterial growth and CRISPR interference activity and demonstrated that the observed heterogeneity of colony-forming units is strongly influenced by plasmid design and backbone identity. This study highlights the importance of careful assessment of heterogenous colony growth dynamics and describes a real-time imaging system with wide applications beyond the study of CRISPR-Cas activity in bacterial hosts.