Shaking incubator

Baseline imaging of printed arrays was conducted at an exposure time of 1s using a Nikon Ti-2 inverted microscope. Images were captured at 4× and 10× magnifications. Stability testing involved imaging under the same parameters after 30 mins of immersed water wash at 220 RPM using a shaking incubator (VWR, Ontario, Canada). Food overlay imaging using romaine lettuce, ground beef, and whole chicken was done under the same parameters, but with 10 mm thick samples placed on the arrays.

A bioluminescent strain of MRSA USA300 was cultured overnight on BHI agar plates with 5% cow blood (VMBS, UC Davis). The plate was scanned on an IVIS Spectrum (PerklinElmer, Waltham, MA, USA) to confirm bioluminescence. Single colonies were picked and placed in 6 mL of Tryptic Soy Broth medium with chloramphenicol (100 ug/mL) and kanamycin (50 ng/mL). The bacteria were cultured overnight in a shaking incubator (VWR) at 37 °C and 300 RPM. The next day, the bacteria were diluted in 6 mL of Tryptic Soy Broth medium with chloramphenicol (100 ug/mL) and cultured in a shaking incubator (VWR) at 37 °C and 300 RPM. The bacteria were cultured to an optical density of 0.0418 ± 0.0026, which corresponded to 1.14–1.29 × 10⁷ CFU in 50 uL (in the SEM study presented in Figure 4a, the final dose was 9.04 × 10⁶ CFU in 50 uL). Bacteria optical density (OD600) was measured using a NanoDrop spectrophotometer (ThermoFisher, Waltham, MA, USA). After that, the bacteria were centrifuged at 3850 RPM for 10 min at 4 °C (Beckman Coulter, Brea, CA, USA) and resuspended in 1 mL of ice-cold PBS (Gibco, Waltham, MA, USA).