On the right, a lactose (-) strain that has the particularity of producing a red pigment (bacterium: Serratia marcescens), which is most often lactose (+)

On the left, a lactose (+) strain (bacterium: Escherichia coli), with an area that has turned green again, probably due to the re-alkalization of the medium following the exhaustion of lactose at this location (utilization of peptones)