Final answer:
The client with a sputum culture confirming Legionella pneumophila should be placed on airborne precautions due to the risk of transmitting the bacteria through airborne water droplets.
Step-by-step explanation:
The nurse should place the client with a sputum culture that confirms Legionella pneumophila on airborne precautions. Among the options given, Legionella, the bacterium that causes Legionnaires' disease, is known to be transmitted through airborne water droplets. Unlike MRSA, E. coli UTIs, and Pseudomonas in a leg ulcer, which are typically spread by direct contact, Legionella can become aerosolized and inhaled, hence the need for airborne precautions.