Brief Answer:
Possible causes explained below.
Detailed Answer:
Hello,
First of all, please note that normal respiratory flora is called so because they're a group of micro-organisms harboring in our respiratory tract harmlessly, in fact, protects us from other infections to some extent.
"Normal respiratory Flora very reduced" in sputum culture can be seen after antibiotic treatment (generally immediately after the treatment or in case of prolonged antibiotic course) which can also kill those normal florae along with the harmful ones.
Another possibility would be a diluted sample in which the normal flora would get washed off/diluted to that extent.
So, it does not mean an infection as such, but a state or phase that the body is going through rather. In that case, well-balanced diet, avoidance of dusty atmosphere, regular exercise, immune boosting foods (soups, porridge, broths etc.) along with multivitamin and antioxidants would be sufficient to regain normalcy within a couple of days only.
Let me know if I can assist you further. Take Care.
Regards