'Pseudomonas aeruginosa' is a Gram-negative bacterium of the proteobacteria class, and one of the most common causes of nosocomial infections. For example, it causes chronic pneumonia in cystic fibrosis patients. Patient sputum contains 2-heptyl-4-hydroxyquinoline N-oxide [HQNO] and 'Pseudomonas' quorum sensing molecules such as the 'Pseudomonas' quinolone signal [PQS]. It is known that HQNO inhibits the enzyme activity of mitochondrial and bacterial complex III at the Qi (quinone reduction) site, but the target of PQS is not known. In this work we have shown that PQS has a negative effect on mitochondrial respiration in HeLa and A549 cells. It specifically inhibits the complex I of the respiratory chain. In vitro analyses showed a partially competitive inhibition with respect to ubiquinone at the IQ site. In competing studies with Rotenone, PQS suppressed the ROS-promoting effect of Rotenone, which is typical for a B-type inhibitor. Prolonged incubation with PQS also had an effect on the activity of complex III.