It was just an ordinary Monday morning, but what happened that day was going to change the rest of my life. I was walking down the street, looking cool in my new Reeboks, when this lumbering oaf of a human nearly steps on me.

"Excuse me, " I said politely, "but we bacteria are only four inches tall and very squashable. Do you mind looking where you place your size twelve boots?"

A simple apology was all I wanted, but I couldn't believe what he said next.

"You're not a bacteria, mate," he said, "you're just a figment of some advertising executive's overfertile imagination."

I was about to say "I'm a bacteri-um, pal, 'cos there's only one of me", but he was gone before I could open my mouth.

And then I thought, "Figment? What does he mean, a figment? I'm a real bacterium, aren't I?..."