In general, the most important pathogens are Pseudomonas aeruginosa Methicillin-sensitive Staphylococcus aureus Methicillin-resistant S aureus (MRSA) Other important pathogens include enteric gram-negative bacteria (mainly Enterobacter species, Klebsiella pneumoniae, Escherichia coli, Serratia marcescens, Proteus species, and Acinetobacter species).