Under what circumstances might a doctor prescribe a narrow-spectrum antibiotic? & broad-spectrum antibiotic?

If a blood test has revealed what infection you have, there may be a antibiotic that is directly and specifically used for this infection as it's known that this antibiotic is the most effective. Or was developed for this specific infection.

A broad spectrum antibiotic such as penicillin is used more for if they are not certain what the infection is and what's causing it.

Using a broad spectrum antibiotic will hopefully help with the infection and usually does. Except if the unknown infection/cause is something that will not be helped by this sort of antibiotic.

Spectrum Antibiotic

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Good antibiotic stewardship practices include using a narrow spectrum antibiotic whenever possible. This is especially true when one has either 1. positive culture results to target treatment (assuming not a multi-drug resistant bug) or 2. high index of suspicion of what bug and that bug not multi-drug resistant. Penicillin is generally considered narrow spectrum in the US.

Broad spectrum ABX are used when clinician has no idea yet about what bug and believes either a gram positive or gram negative organism could be responsible or a multi-drug resistant one.

Use of broad spectrum ABX are associated with inducement of resistance not only of target bug, but others. I will not go into mechanisms of resistance here. Suffice it to say that MRSA, MRSE, multidrug resistant Klebsiella and VRE are only some of the bad strains of bacteria thought d/t overuse of broad spectrum ABX such as levofloxacin. Also note that the broader the spectrum of the ABX used, the more likelihood of overgrowth of yeast &/or Clostridia, thus more side effects like vaginitis, bloody diarrhea, etc. such as with Augmentin and later generation cephalosporins.

All penicillins are Beta-lactam antibiotics and are used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms. β-Lactam antibiotics are a broad class of antibiotics, consisting of all antibiotic agents that contains a β-lactam nucleus in its molecular structure. the spectrum depends upon type of semisynthetic penicillin drug see the following list Narrow-spectrum β-lactamase sensitive benzathine penicillin benzylpenicillin (penicillin G) phenoxymethylpenicillin (penicillin V) procaine penicillin Penicillinase-resistant penicillins methicillin oxacillin nafcillin cloxacillin dicloxacillin flucloxacillin β-lactamase-resistant penicillins temocillin Moderate-spectrum amoxicillin ampicillin Broad-spectrum co-amoxiclav (amoxicillin+clavulanic acid) Extended-spectrum azlocillin carbenicillin ticarcillin mezlocillin piperacillin