Which of these antibiotics has the most narrow-spectrum activity?

Isoniazid is recognized for its narrow-spectrum activity, particularly against Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). This targeted action makes isoniazid particularly effective for treating tuberculosis while having minimal effects on other types of bacteria. Its specific mechanism of action involves inhibiting cell wall synthesis in mycobacteria, a feature that sets it apart from broader-spectrum antibiotics that target a wider variety of both Gram-positive and Gram-negative bacteria.

In contrast, streptomycin, amoxicillin, and tetracycline have broader-spectrum capabilities, meaning they are effective against a wider range of bacterial species. Streptomycin targets both Gram-positive and Gram-negative bacteria but is primarily effective against aerobic bacteria; amoxicillin, which is a penicillin derivative, has utility against numerous bacterial infections; and tetracycline is broad-spectrum, effective against a multitude of organisms, including some unusual pathogens like chlamydia and mycoplasma. This wider range of activity differentiates them from isoniazid, making it the most focused in terms of the spectrum of bacteria it can effectively target.