Stills from video. Staphylococci bacteria, which have been labelled with a green fluorescent protein, express a resistance gene for the antibiotic chloramphenicol. Black Streptococcus pneumoniae bacteria do not have the resistance gene. In a medium containing the antibiotic, the green cells begin to grow and divide whereas the non-resistant black cells don't. After a time, individual black cells begin to divide and they even outgrow their green companions.