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