“Support bacteria; it is the only culture we have left. Happy Birthday!”
“The only culture some people have is between their toes! Happy Birthday!”
Maybe something about the bacteria she will be spreading all over the cake when she blows out the candles?
What is her name? Can that be used in a punny way?
Here’s a site that has lots of biology jokes, puns, poetry…maybe that will give you some ideas:
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