HPD and cystic fibrosis: An in vitro transcriptomic study investigating the physiology of P. aeruginosa grown in CF sputum revealed that the genes associated with PQS metabolism, such as those coding for the aromatic amino acid aminotransferase, 4-hydroxyphenylpyruvate dioxygenase (hpd) and pqsABCDE, were expressed 10-fold greater than the expression when P. aeruginosa was cultured in media containing glucose alone as the carbon source (Palmer et al., 2005).