to the potential development of VAP; suggesting that bacteria found in plaque can "migrate to the respiratory system." Microbiology The microbiologic flora responsible for VAP is different from that of the more common community-acquired pneumonia (CAP). In particular, viruses and fungi are uncommon causes in people who do not have underlying immune deficiencies. Though any microorganism that causes CAP can cause VAP, there are several bacteria which are particularly important causes of VAP because of their resistance to commonly used antibiotics. These bacteria are referred to as multidrug resistant (MDR). Pseudomonas aeruginosa is the most common MDR Gram-negative bacterium causing VAP. Pseudomonas has natural resistance to many antibiotics and has been known to acquire resistance to every antibiotic except for polymyxin B. Resistance is typically acquired through upregulation or mutation of a variety of efflux pumps which pump antibiotics out of the cell. Resistance may also occur through loss of an outer membrane porin channel (OprD) Klebsiella pneumoniae has natural resistance to some beta-lactam antibiotics such as ampicillin. Resistance to cephalosporins and aztreonam may arise through induction of a plasmid-based extended spectrum beta-lactamase (ESBL) or plasmid-based ampC-type enzyme Serratia marcescens has an ampC gene which can be induced by exposure to antibiotics such as cephalosporins. Thus, culture sensitivities may initially indicate appropriate treatment which fails due to bacterial response. Enterobacter as a group also have an inducible ampC gene. Enterobacter may also develop resistance by acquiring plasmids. Citrobacter also has an inducible ampC gene. Stenotrophomonas maltophilia often colonizes people who have tracheal tubes but can also cause pneumonia. It is often resistant to a wide array of antibiotics but is usually sensitive to co-trimoxazole Acinetobacter are becoming more common and may be resistant to carbapenems such as imipenem and meropenem Burkholderia cepacia is an important organism in people with cystic fibrosis and is often resistant to multiple antibiotics Methicillin-resistant Staphylococcus aureus is an increasing cause of VAP. As many as fifty percent of Staphylococcus aureus isolates in the intensive care setting are resistant to methicillin. Resistance is conferred by the mecA gene. The development of molecular diagnostic techniques is changing the understanding of the microbiology of VAP, with an increasing appreciation of the role of hard to culture bacteria and the change in the lung microbiome. A recent finding has highlighted the presence of Mycoplasma in the lavage of patients with VAP, a finding which was largely absent from ventilated