Hello Welcome to HCM, thanks for posting your query.
I read your question and understood it completely.. The place where I practice, I see a lot of throat swabs with such organisms namely,
Klebsiella
Proteus
E coli Stap Aureus etc
See, 1st thing I want to tell you that, you stop worrying.
Your doctor was right in saying so.
See, it's simple. There is an infection and it has to be treated.
Along with cultures, they will usually do antibiotic sensitivity and prescribe medications accordingly.
And I am pretty sure, your doctor would have done it.
Coming to your worry, as why and how you got that infection.
1. Anything that you eat can have those organisms.
2. Check your blood sugars, diabetics are more prone for such infections.
3. In immuno compromised states you may end up with such an infection.
So chill, take medications this time. And if you repeatedly get such problems then you need thorough evaluation.
Your one more concern that, is it infectious!?
I say, yes if your saliva comes in contact with some other person . At least initially. So treat yourself, do not worry and come back to me in case you need any help. Take care.
Hope this helps you.