HELP my friend, a heart valve replacement, Streptococcus infection

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Would anybody have any opinion or theory?

My friend recently had a stroke and blood culture showed the infection
of streptococcus.  A few days later, blood culture no longer shows
streptococcus due to antibiotic medicare. He has an artificial heart
valve, and the doctor says the valve must have the infection. 

Is there any possibility that the bacteria comes back? 

If the bacteria comes back, does my friend need the valve replacement?
Can the infectin place be anywhere else?

I would appreciate it if anybody can email me or post any opinion and
information. Of course, I just want to verify the doctor's theory. (BTW
I agree with this doctor, but I need third person's opinion to calm
down his family, or to look for more resources for the alternative
thory.)

Thank you.

Brian Moriguchi