Which bacterium has variable sensitivity to TMP/SMZ?

Staphylococcus aureus demonstrates variable sensitivity to TMP/SMZ (trimethoprim/sulfamethoxazole), reflecting the complexity of treating infections caused by this organism. This variability arises because some strains of Staphylococcus aureus, particularly methicillin-resistant Staphylococcus aureus (MRSA), may not respond well to this antibiotic combination due to resistance mechanisms.

In contrast, other organisms like Escherichia coli and Streptococcus pneumoniae typically show more predictable patterns of susceptibility or resistance to TMP/SMZ, with E. coli often being effectively treated by this combination, although resistance can occur in certain strains. Pneumocystis carinii (also known as Pneumocystis jirovecii), while treated effectively with TMP/SMZ, is not variable in sensitivity since it is almost entirely reliant on this medication for treatment.

Thus, the reason Staphylococcus aureus is the correct choice is rooted in its unpredictable sensitivity patterns to TMP/SMZ, highlighting the need for susceptibility testing in clinical practice for effective treatment planning.