Table 1.

Pathogens commonly encountered in the different types of bacterial pneumonia.

Class	CAP (a) [11,12,13]	HAP/VAP [11,14]	HCAP [12,15]
Gram-positive	Streptococcus pneumoniae Staphylococcus aureus (b)	Streptococcus pneumoniae Streptococcus spp. Staphylococcus aureus (b)	Streptococcus pneumoniae Staphylococcus aureus (b)
Gram-negative	Mycoplasma pneumoniae Chlamydophila pneumoniae Haemophilus influenzae Legionella spp. Pseudomonas aeruginosa Enterobacteriaceae (Escherichia coli, Klebsiella spp., Enterobacter spp., Proteus mirabilis)	Enterobacteriaceae (Escherichia coli, Klebsiella spp., Enterobacter spp., Proteus spp.) Serratia marcescens Pseudomonas aeruginosa Acinetobacter spp. Haemophilus influenza Stenotrophomonas maltophilia	Haemophilus influenzae Pseudomonas aeruginosa Acinetobacter baumannii Stenotrophomonas maltophilia

spp.: species; ^(a) pathogen identification fails in about 50% of cases [16]; ^(b) both methicillin-resistant (MRSA) and methicillin-sensitive S. aureus (MSSA).