For the effect on the cultures of staphylococci that Fleming observed, the mould had to be growing before the bacteria began to grow, because penicillin is only effective on bacteria when they are reproducing. Fortuitously, the temperature in the laboratory during that August was optimum first for the growth of the mould, below 20 °C, and later in the month for the bacteria, when it reached 25 °C. Had Fleming not left the cultures on his laboratory bench and put them in an incubator, the phenomenon would not have occurred.[39]
From wikipedia:
For the effect on the cultures of staphylococci that Fleming observed, the mould had to be growing before the bacteria began to grow, because penicillin is only effective on bacteria when they are reproducing. Fortuitously, the temperature in the laboratory during that August was optimum first for the growth of the mould, below 20 °C, and later in the month for the bacteria, when it reached 25 °C. Had Fleming not left the cultures on his laboratory bench and put them in an incubator, the phenomenon would not have occurred.[39]
https://en.wikipedia.org/wiki/History_of_penicillin