Cotrimoxazole is effective against which organism?

Cotrimoxazole, which is a combination of sulfamethoxazole and trimethoprim, is primarily effective against a variety of Gram-positive and Gram-negative bacteria, including some strains of Staphylococcus aureus. While it is not first-line treatment for Methicillin-resistant Staphylococcus aureus (MRSA), certain strains of MRSA are susceptible to cotrimoxazole. This is due to its ability to inhibit folate synthesis, which is essential for bacterial growth.

Cotrimoxazole is less effective against certain organisms such as Pseudomonas aeruginosa, which has intrinsic resistance to many antibiotics due to its unique membrane structure and efflux pumps. Although cotrimoxazole is sometimes used in treating uncomplicated urinary tract infections caused by Mycoplasma, the activity against mycoplasmas is not its primary strength compared to others. Additionally, anaerobes are typically treated with different antibiotics that are more effective in anaerobic environments. Thus, despite the complexities of bacterial resistance and susceptibility, the association of cotrimoxazole with certain strains of MRSA makes it a relevant choice in specific clinical scenarios.