TBBT episode 9.22 "The Fermentation Bifurcation" is interesting as that it shows Sheldon and Bernadette together. There are comparatively few Sheldon-Bernardette stories in TBBT. It would have been interesting if Melissa had played Penny or Amy. The height difference between Melissa and Jim could make some great physical comedy :-)
Everything belongs to Lorre and Prady. Story starts in mid-season 3 before Amy appeared.
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Leonard came out of Penny's apartment and said "Penny isn't feeling well, she will not be at the Cheesecake Factory tonight."
Sheldon said "that's not acceptable, Penny may be a terrible waitress but at least she knows my burger order."
"Well someone else will serve us tonight, do you want a sick person serving food? Come on Raj and Howard are meeting us there".
At the Cheesecake Factory Sheldon said to Howard "We are going to talk Physics, I am sure you can find something to amuse yourself on your phone in the meantime."
"I have a Masters degree" but before Howard could say anything else a small blonde lady appeared.
"Welcome to the Cheesecake Factory, my name is Bernardette, what can I get for you gentlemen tonight?"
Sheldon said "Penny's replacement, another dumb blonde."
"Who are you calling dumb there Mr Skinny? Like your little friend there I have a Masters degree and I am studying for a PhD."
"Really, I am DOCTOR Sheldon Cooper, I have two PhDs. My friend here with the glasses is Doctor Hofstadter and the quiet Indian is Doctor Koothrapalli, we are all Physicists. However, Mr Wolowitz here is a mere engineer."
"I am studying for a PhD in microbiology of bacteria in yeast."
"Biology, well at least it isn't Geology."
"Would you like to see the menu?"
"That would be a good idea"