when would you choose a narrow-spectrum rather than a broad-spectrum antibiotic?why?

Answer 1

If you can't determine the type of bacteria involved in the infection, then an antibiotic targets a wide spectrum of bacteria has a better chance of hitting it.

If you know with some accuracy what type of bacteria you're dealing with, then it's better to attack it with a narrow-spectrum antibiotic that targets just that type of bacteria.

This is for two reasons, resistance, and side-effects.

The first point is that if you use too many wide-spectrum antibiotics, then over time other bacteria will become resistant to them and they will become less effective in fighting future infections.

The second point is that a wide-spectrum antibiotic may also kill off many of the body's "good" bacteria ... such as those involved in digestion, or resisting yeast cultures, which is why some antibiotics can cause diarrhea, yeast infections, or other side-effects.

Answer 2

If you have culture proof of a specific sensitive bacterial infection, it is usually best to target that with a narrow spectrum antibiotic simply to avoid encouraging resistance of other bacteria in the environment. That has been a concern for decades. The more we use broad spectrum drugs, the more we select multiply resistant bacterial species that forever require new antibiotic development. We are seeing problems now with many microorganisms including some staphylococci and (recently in the news) mycobacterium tuberculosis.
A multiply resistant TB is a major concern for the world. Remember that TB was the number one killer in the U.S. just 100 years ago. TB has been a worldwide killer for many centuries and is still a worldwide threat. TB has assaulted humans throughout recorded history.