I did a plaque assay with an arbovirus in Vero cells. I infected my confluent cell monolayer with 200ul virus diluent followed by 1 hr infection with rocking to coat after each 15-minutes interval. I added agarose (1.6%) as the first overlay and neutral red in the second overlay.

When I went to count the plaques the next day after pouring the second overlay, I found this contamination (shown on the attached plate). I would like to figure out what that contamination was and its source. I would highly appreciate any suggestions or help.

Have a great day!