In Petri you attempt to grow a cell culture in a Petri dish. You deposit individual 'good' cells which grow and divide, spreading across the dish. However, 'bad' cells appear from the ambient environment which you must battle for control of the dish. You can use agar to speed up your cells or fight off the bad cells with numerous tools like lasers, temperature-control, penicillin, radiation, bleach or intentional phage infections. Additionally, spontaneous phage infections can occur which may threaten to wipe our your colony. You must fight those off as well.
Level progression is divided into 'stages', where each stage consists of a series of 'rounds'. Increasing stages indicate increasing difficulty while the rounds within a stage provide variations in gameplay (by varying the round's end-condition and the possible bonuses such that different strategies and approaches are required for each round).
Lastly, an online global scoreboard lets you show off your success to the whole world! This scoreboard is available either through the app or at http://petri.keithwiley.com.
Petri should provide high 'replayability' since there are no secrets. Just keep trying to improve on your earlier high scores (or try to beat other people's high scores for dominance of the global scoreboard).