Treatment completion for latent tuberculosis infection: a retrospective cohort study comparing 9 months of isoniazid, 4 months of rifampin and 3 months of isoniazid and rifapentine.

BACKGROUND: The U.S. Centers for Disease Control and Prevention (CDC) recommended a new regimen for treatment of latent tuberculosis (three months of weekly isoniazid and rifapentine) in late 2011. While completion rates of this regimen were reported to be higher than nine months of isoniazid, little is known about the completion rates of three months of isoniazid and rifapentine compared to nine months of isoniazid or four months of rifampin in actual use scenarios. METHODS: We conducted a retrospective cohort study comparing treatment completion for latent tuberculosis (TB) infection in patients treated with nine months of isoniazid, three months of isoniazid and rifapentine or four months of rifampin in outpatient clinics and a public health TB clinic in Seattle, Washington. The primary outcome of treatment completion was defined as 270 doses of isoniazid within 12 months, 120 doses of rifampin within six months and 12 doses of isoniazid and rifapentine within four months. RESULTS: Three hundred ninety-three patients were included in the study. Patients were equally likely to complete three months of weekly isoniazid and rifapentine or four months of rifampin (85% completion rate of both regimens), as compared to 52% in the nine months of isoniazid group (p < 0.001). These associations remained statistically significant even after adjusting for clinic location and type of monitoring. Monitoring type (weekly versus monthly versus less often than monthly) had less impact on treatment completion than the type of treatment offered. CONCLUSIONS: Patients were equally as likely to complete the three months of isoniazid and rifapentine as four months of rifampin. Four months of rifampin is similar in efficacy compared to placebo as isoniazid and rifapentine but does not require directly observed therapy (DOT), and is less expensive compared to combination therapy with isoniazid and rifapentine, and thus can be the optimal treatment regimen to achieve the maximal efficacy in a community setting.

Clinical Impact and Cost-effectiveness of Xpert MTB/RIF Testing in Hospitalized Patients With Presumptive Pulmonary Tuberculosis in the United States.

Background: Microscopic examination of acid-fast-stained sputum smears is the current standard of care in the United States to determine airborne infection isolation (AII) of inpatients with presumptive pulmonary tuberculosis (PTB). However, nucleic acid amplification testing (NAAT) with the Xpert MTB/RIF assay (Xpert) may be more efficient and less costly. Methods: This prospective observational cohort study enrolled a consecutive sample of 318 AII-eligible inpatients from a public hospital in Seattle, Washington, from March 2012 to October 2013. Sputum samples were collected from each inpatient and analyzed using smear microscopy, culture, drug susceptibility testing, and NAAT. The performance, clinical utility (AII duration and survival), and cost-effectiveness from an institutional perspective were compared for 5 testing strategies. Results: Among the 318 admissions with presumptive PTB, 20 (6.3%) were culture-positive for Mycobacterium tuberculosis. The sensitivity of 1 Xpert, 2 Xperts, 2 smears, or 3 smears compared to culture was 0.85 (95% confidence interval [CI], .61­.96), 0.95 (95% CI, .73­1.0), 0.70 (95% CI, .46­.88), and 0.80 (95% CI, .56­.93), respectively. A cost-effectiveness analysis of the study results demonstrated that an Xpert test on 1 unconcentrated sputum sample (assuming equivalent results for unconcentrated and concentrated sputum samples) is the most cost-effective strategy (99.9% preferred at willingness-to-pay of US$50000) and on average would save 51.5 patient-hours in AII and up to $11466 relative to microscopy without a compromise in sensitivity. Conclusions: In hospitalized patients with presumptive PTB in a low-burden setting, NAAT can reduce AII and is comparably sensitive, more specific, and more cost-effective than smear microscopy.

Proteomic landscape of bronchoalveolar lavage fluid in human immunodeficiency virus infection.

The lung is an important reservoir of human immunodeficiency virus (HIV). Individuals infected with HIV are more prone to pulmonary infections and chronic lung disorders. We hypothesized that comprehensively profiling the proteomic landscape of bronchoalveolar lavage fluid (BALF) in patients with HIV would provide insights into how this virus alters the lung milieu and contributes to pathogenesis of HIV-related lung diseases. BALF was obtained from five HIV-negative (HIV(-)) and six asymptomatic HIV-positive (HIV(+)) subjects not on antiretroviral therapy. Each sample underwent shotgun proteomic analysis based on HPLC-tandem mass spectrometry. Differentially expressed proteins between the groups were identified using statistical methods based on spectral counting. Mechanisms of disease were explored using functional annotation to identify overlapping and distinct pathways enriched between the BALF proteome of HIV(+) and HIV(-) subjects. We identified a total of 318 unique proteins in BALF of HIV(-) and HIV(+) subjects. Of these, 87 were differentially up- or downregulated between the two groups. Many of these differentially expressed proteins are known to interact with key HIV proteins. Functional analysis of differentially regulated proteins implicated downregulation of immune responses in lungs of HIV(+) patients. Combining shotgun proteomic analysis with computational methods demonstrated that the BALF proteome is significantly altered during HIV infection. We found that immunity-related pathways are underrepresented in HIV(+) patients. These findings implicate mechanisms whereby HIV invokes local immunosuppression in the lung and increases the susceptibility of HIV(+) patients to develop a wide range of infectious and noninfectious pulmonary diseases.

Fentanyl-induced chest wall rigidity.

Fentanyl and other opiates used in procedural sedation and analgesia are associated with several well-known complications. We report the case of a man who developed the uncommon complication of chest wall rigidity and ineffective spontaneous ventilation following the administration of fentanyl during an elective bronchoscopy. His ventilation was assisted and the condition was reversed with naloxone. Although this complication is better described in pediatric patients and with anesthetic doses, chest wall rigidity can occur with analgesic doses of fentanyl and related compounds. Management includes ventilatory support and reversal with either naloxone or a short-acting neuromuscular blocking agent. This reaction does not appear to be a contraindication to future use of fentanyl or related compounds. Chest wall rigidity causing respiratory compromise should be readily recognized and treated by bronchoscopists.

Environment or host?: A case-control study of risk factors for Mycobacterium avium complex lung disease.

RATIONALE: Mycobacterium avium complex lung disease is an increasingly common and chronically debilitating problem. Several host traits have been suggested or confirmed as risk factors. Potential environmental and behavioral risk factors have also been proposed. Few have been evaluated in comparative studies. OBJECTIVES: To determine if aerosol-generating activities in the home and garden, features of the home water supply, or several pulmonary and immune-compromising conditions are associated with Mycobacterium avium complex lung disease. METHODS: Cases were recruited from academic medical centers and by informal referrals from nonuniversity practices in Washington and Oregon. Control subjects were recruited by random-digit dialing and matched to cases by age, sex, and partial telephone number. Associations were measured as odds ratios (OR) estimated using conditional logistic regression. MEASUREMENTS AND MAIN RESULTS: Known and potential risk factors were measured by in-home interview. Fifty-two matched pairs were studied. Six of 12 examined host traits were associated with disease, including history of chronic obstructive pulmonary disease (OR, 10; 95% confidence interval [CI], 1.2-80), pneumonia hospitalization (OR, 3.4; 95% CI, 1.1-11), and steroid use (OR, 8; 95% CI, 1.6-41). In contrast, 11 of the 14 aerosol-generating activities and all five features of home water supply studied bore little or no association with disease. CONCLUSIONS: Aerosol-generating activities seem not to be risk factors for Mycobacterium avium complex lung disease in HIV-negative adults, but prior lung disease and immune-suppressing drugs seem to be associated with susceptibility.

Predictive value of bronchoscopy in assessing the severity of inhalation injury.

Inhalation injury is associated with severe pulmonary complications as inhaled products of combustion cause lung inflammation and loss of natural defenses. A bronchoscopic grading for inhalation injury has been proposed but has not yet been validated in burn patients. In this study, the authors evaluated whether bronchoscopic grading of injury clinically correlated with indices of gas exchange over the first 72 hours or predicted differences in hospitalization outcomes. They conducted a single-center retrospective review of all mechanically ventilated adults with suspected inhalation injury and thermal injury over an 18-month period. All recorded bronchoscopy examinations were reviewed and categorized injury according to the published abbreviated injury score (AIS 0: no injury, 1: mild, 2: moderate, 3: severe, and 4: massive injury). They also compared changes in oxygenation, airway pressures, chest radiograph findings, fluid administration, and early development of pneumonia and organ failure, by severity of inhalation injury according to the AIS. Thirty-two adult patients met inclusion criteria over the study period. This cohort was 69% male with a mean age of 44.5 ± 14 years and a mean % TBSA burn of 33.9 ± 17%. Of these 32 patients, 11 patients (34%) were classified as grade 0, 9 patients (28%) were classified as grade 1, 7 patients (22%) were classified as grade 2, and 5 patients (16%) were classified as grade 3. Measured carboxyhemoglobin levels increased significantly with higher AIS grade. Oxygenation indices were worse as grade worsened by 24, 48, and 72 hours. The incidence of acute respiratory distress syndrome increased by grade of injury: 0, 22, 57, and 80%, respectively, at 24 hours (P < .01), and remained statistically different at 48 and 72 hours. After adjustment for age, % TBSA burn, and full-thickness component, severe inhalation injury (grades 2 and 3) was associated with an increased risk of acute respiratory distress syndrome at 24 and 72 hours, as well as ventilator days >21 days, and a trend toward multiple organ dysfunction syndrome and mortality. Better understanding of the relationship between inhalation injury and lung physiologic sequelae is a burn research priority. The bronchoscopic grading of inhalation injury moderately correlates with early indices of impaired gas exchange in this cohort and may be a promising tool for staging lower airway injury. Prospective studies should definitively answer whether AIS bronchoscopy staging predicts hospitalization outcomes in inhalation injury.

The biological activity of FasL in human and mouse lungs is determined by the structure of its stalk region.

Acute lung injury (ALI) is a life-threatening condition in critically ill patients. Injury to the alveolar epithelium is a critical event in ALI, and accumulating evidence suggests that it is linked to proapoptotic Fas/FasL signals. Active soluble FasL (sFasL) is detectable in the bronchoalveolar lavage (BAL) fluid of patients with ALI, but the mechanisms controlling its bioactivity are unclear. We therefore investigated how the structure of sFasL influences cellular activation in human and mouse lungs and the role of oxidants and proteases in modifying sFasL activity. The sFasL in BAL fluid from patients with ALI was bioactive and present in high molecular weight multimers and aggregates. Oxidants generated from neutrophil myeloperoxidase in BAL fluid promoted aggregation of sFasL in vitro and in vivo. Oxidation increased the biological activity of sFasL at low concentrations but degraded sFasL at high concentrations. The amino-terminal extracellular stalk region of human sFasL was required to induce lung injury in mice, and proteolytic cleavage of the stalk region by MMP-7 reduced the bioactivity of sFasL in human cells in vitro. The sFasL recovered from the lungs of patients with ALI contained both oxidized methionine residues and the stalk region. These data provide what we believe to be new insights into the structural determinants of sFasL bioactivity in the lungs of patients with ALI.

Factors associated with mortality in patients with tuberculosis.

BACKGROUND: Known risk factors for death following a diagnosis of tuberculosis may not be applicable to current U.S. cases. We evaluated the factors associated with all-cause mortality in patients with tuberculosis in Washington State. METHODS: Using data from the Tuberculosis Information Management System of Washington State, we conducted a cohort study of all residents diagnosed with tuberculosis from 1993 through 2005. Death from any cause was ascertained through the Washington State Death Certificate Data Files. Proportional hazards models were used to estimate the independent effect on all-cause mortality of demographic, clinical, and behavioral characteristics. RESULTS: During a median follow-up of 6 years in 3451 patients treated for tuberculosis, there were 417 deaths. Mortality was independently associated with increasing age, male gender, HIV-coinfection, and U.S. birth. Within 1 year of tuberculosis diagnosis, treatment by a private provider and the use of directly observed therapy were also independently associated with increased mortality. In addition, an interaction term of private provider times directly observed therapy was also significantly associated with mortality. CONCLUSIONS: We identified factors independently associated with increased all-cause mortality following a diagnosis of tuberculosis. The associations between mortality and provider type should be evaluated with more thorough adjustment for severity of illness, but suggest important directions for future research.

Long-term survival after hospitalization for community-acquired and healthcare-associated pneumonia.

BACKGROUND: Elderly patients surviving community-acquired pneumonia (CAP) have subsequent increased mortality. However, little is known regarding long-term survival in younger adults or those with healthcare-associated pneumonia (HCAP). OBJECTIVES: To identify factors associated with mortality and compare long-term survival in patients hospitalized with HCAP to that of patients with CAP. METHODS: We determined survival after discharge as of December 2002 in a patient cohort admitted with pneumonia between June 1994 and May 1996. We used the Cox proportional hazard model to estimate differences in survival after controlling for confounders. RESULTS: Of the 522 patients hospitalized with pneumonia, 457 survived to discharge. One hundred sixty-four patients (36%, 95% confidence interval, CI, 31-40%) were admitted with HCAP, while 293 (64%, 95% CI 60-69%) were admitted with CAP. Of the 181 deaths in the follow-up period, 70 occurred in patients under age 65 years admitted with HCAP (53% death rate, 95% CI 44-62%). Nineteen of these deaths (27%, 95% CI 17-39%) occurred in the absence of HIV infection. In patients under the age of 65 whose only risk factor for HCAP was treatment for pneumonia or hospitalization in the previous 90 days, 4 of 13 patients (31%, 95% CI 9-61%) died. Twenty percent (95% CI 15-26%) of patients under age 65 years admitted with CAP died during the follow-up. CONCLUSIONS: Admission for HCAP, and to a lesser degree CAP, is associated with increased long-term mortality even in young patients. Future studies are warranted to identify interventions to improve survival in this population.

Evidence for limited genetic compartmentalization of HIV-1 between lung and blood.

BACKGROUND: HIV-1 is frequently detected in the lungs of infected individuals and is likely important in the development of pulmonary opportunistic infections. The unique environment of the lung, rich in alveolar macrophages and with specialized local immune responses, may contribute to differential evolution or selection of HIV-1. METHODOLOGY AND FINDINGS: We characterized HIV-1 in the lung in relation to contemporaneous viral populations in the blood. The C2-V5 region of HIV-1 env was sequenced from paired lung (induced sputum or bronchoalveolar lavage) and blood (plasma RNA and proviral DNA from sorted or unsorted PBMC) from 18 subjects. Compartmentalization between tissue pairs was assessed using 5 established tree or distance-based methods, including permutation tests to determine statistical significance. We found statistical evidence of compartmentalization between lung and blood in 10/18 subjects, although lung and blood sequences were intermingled on phylogenetic trees in all subjects. The subject showing the greatest compartmentalization contained many nearly identical sequences in BAL sample, suggesting clonal expansion may contribute to reduced viral diversity in the lung in some cases. However, HIV-1 sequences in lung were not more homogeneous overall, nor were we able to find a lung-specific genotype associated with macrophage tropism in V3. In all four subjects in whom predicted X4 genotypes were found in blood, predicted X4 genotypes were also found in lung. CONCLUSIONS: Our results support a picture of continuous migration of HIV-1 between circulating blood and lung tissue, with perhaps a very limited degree of localized evolution or clonal replication.

Mapping the lung proteome in cystic fibrosis.

The pathophysiology of cystic fibrosis (CF) lung disease remains incompletely understood. Novel mechanisms in the pathogenesis of CF lung disease may be discovered by studying the patterns of protein expression in bronchoalveolar lavage fluid (BALF). We used shotgun proteomics to analyze BALF samples from 8 CF and 4 control subjects. Differential protein expression between CF and control subjects was determined using spectral counting and statistical analysis. Using Gene Ontology analysis, we identified enriched biological modules and then applied network analysis to construct a protein interaction map in CF lung disease. Shotgun proteomics analysis of BALF identified hundreds of proteins whose differential enrichment or depletion robustly distinguished the CF phenotype from normal controls. Functional categorization and network analysis identified key processes, including the immune response and proteolytic activity that are known contributors to CF lung disease. Importantly, this approach also implicated abnormalities in previously unsuspected pathways, such as dysregulation of the complement system that may have critical roles in the pathogenesis of CF lung disease. By integrating shotgun proteomics with statistical and computational analyses, we have developed a promising approach to understand the pathophysiology of CF lung disease. Our approach should be applicable to a wide range of proteomics-based clinical research.

Implementation and impact of a translational research training program in pulmonary and critical care medicine.

BACKGROUND: The translation of basic research advances to the clinical arena has been slow and inefficient. With the goal of improving interactions and collaboration between basic science and clinical investigators, we instituted a Translational Research Training Program (TRTP) in acute lung injury to complement our basic science and clinical research training programs in pulmonary and critical care medicine. METHODS: We developed a TRTP in which trainees select a primary research discipline for rigorous development of skills in either basic science research or clinical research. This primary foundation is complemented by cross-training in the other discipline through a specifically designed program of study. To measure the impact of the program, we analyzed publication rates, coauthorship to reflect collaboration between research disciplines, and publication of papers with a translational focus by members of our division before and after the institution of the TRTP. RESULTS: We describe our new training program, including modifications to our preexisting program and development of new components. We found significant increases in multidisciplinary authorship and translational articles following institution of TRTP. CONCLUSIONS: An explicit TRTP appears to increase collaboration between basic and clinical investigators. Our goal is to share our experiences and provide a template for other pulmonary and critical care programs interested in developing similar curricula. We speculate that this training will improve the translation of basic research findings into clinical advances, thus increasing the probability that successful treatments will be developed for patients with lung diseases.

Proteomic and computational analysis of bronchoalveolar proteins during the course of the acute respiratory distress syndrome.

RATIONALE: Acute lung injury causes complex changes in protein expression in the lungs. Whereas most prior studies focused on single proteins, newer methods allowing the simultaneous study of many proteins could lead to a better understanding of pathogenesis and new targets for treatment. OBJECTIVES: The purpose of this study was to examine the changes in protein expression in the bronchoalveolar lavage fluid (BALF) of patients during the course of the acute respiratory distress syndrome (ARDS). METHODS: Using two-dimensional difference gel electrophoresis (DIGE), the expression of proteins in the BALF from patients on Days 1 (n = 7), 3 (n = 8), and 7 (n = 5) of ARDS were compared with findings in normal volunteers (n = 9). The patterns of protein expression were analyzed using principal component analysis (PCA). Biological processes that were enriched in the BALF proteins of patients with ARDS were identified using Gene Ontology (GO) analysis. Protein networks that model the protein interactions in the BALF were generated using Ingenuity Pathway Analysis. MEASUREMENTS AND MAIN RESULTS: An average of 991 protein spots were detected using DIGE. Of these, 80 protein spots, representing 37 unique proteins in all of the fluids, were identified using mass spectrometry. PCA confirmed important differences between the proteins in the ARDS and normal samples. GO analysis showed that these differences are due to the enrichment of proteins involved in inflammation, infection, and injury. The protein network analysis showed that the protein interactions in ARDS are complex and redundant, and revealed unexpected central components in the protein networks. CONCLUSIONS: Proteomics and protein network analysis reveals the complex nature of lung protein interactions in ARDS. The results provide new insights about protein networks in injured lungs, and identify novel mediators that are likely to be involved in the pathogenesis and progression of acute lung injury.

Spectral index for assessment of differential protein expression in shotgun proteomics.

Detecting differentially expressed proteins is a key goal of proteomics. We describe a label-free method, the spectral index, for analyzing relative protein abundance in large-scale data sets derived from biological samples by shotgun proteomics. The spectral index is comprised of two biochemically plausible features: relative protein abundance (assessed by spectral counts) and the number of samples within a group with detectable peptides. We combined the spectral index with permutation analysis to establish confidence intervals for assessing differential protein expression in bronchoalveolar lavage fluid from cystic fibrosis and control subjects. Significant differences in protein abundance determined by the spectral index agreed well with independent biochemical measurements. When used to analyze simulated data sets, the spectral index outperformed four other statistical tests (Student's t-test, G-test, Bayesian t-test, and Significance Analysis of Microarrays) by correctly identifying the largest number of differentially expressed proteins. Correspondence analysis and functional annotation analysis indicated that the spectral index improves the identification of enriched proteins corresponding to clinical phenotypes. The spectral index is easily implemented and statistically robust, and its results are readily interpreted graphically. Therefore, it should be useful for biomarker discovery and comparisons of protein expression between normal and disease states.

Essential role of MMP-12 in Fas-induced lung fibrosis.

Acute lung injury (ALI) is characterized by an early inflammatory response followed by a late fibroproliferative phase, and by an increase in the bronchoalveolar lavage fluid (BALF) concentrations of bioactive soluble FasL (sFasL). Activation of Fas (CD95) has been associated with the development of lung fibrosis in mice. The goal of this study was to determine the mechanisms that link Fas activation with the development of fibrosis in the lungs. We treated mice with three daily intratracheal instillations of a Fas-activating monoclonal antibody (Jo2) or a control IgG, and studied the animals at sequential times. Mice treated with Jo2 had increased caspase-3 activation in alveolar wall cells on Days 2, 4, and 7; an inflammatory response peaking on Day 7, and increased total lung collagen on Day 21. Gene expression profiling performed on Days 2, 4, and 7 showed sequential activation of co-regulated profibrotic genes, including marked up-regulation of matrix metalloproteinase 12 (MMP-12). Targeted deletion of MMP-12 protected mice from Fas-induced pulmonary fibrosis, even though the inflammatory responses in the lungs were similar to those of wild-type mice. Compared with wild-type mice, the mmp12(-/-) mice showed decreased expression of the profibrotic genes egr1 and cyr61. We conclude that Fas activation in the lungs induces a complex response that includes apoptosis, inflammation, and eventually fibrosis, and that MMP-12 is essential for the fibrotic phenotype. We speculate that MMP-12 activity is required for activation of the profibrotic genes egr1 and cyr61.

cDNA microarray analysis reveals fundamental differences in the expression profiles of primary human monocytes, monocyte-derived macrophages, and alveolar macrophages.

We report the systematic use of large-scale cDNA microarrays to study the gene expression profiles of primary human peripheral blood monocytes (MONO) in comparison with in vitro-differentiated, M-CSF-induced MONO-derived macrophages (MAC) and primary human alveolar MAC (AM), obtained by bronchoalveolar lavage from the lungs of normal volunteers. These studies revealed large-scale differences in the gene expression profile between both MAC types (MAC and AM) and MONO. In addition, large differences were observed in the gene expression profiles of the two MAC types. Specifically, 21% of genes on the array (2904 out of 13,582) were differentially expressed between AM and MONO, and 2229 out of 13,583 probes were differentially expressed between MAC and AM. Our expression data show remarkable differences in gene expression between different MAC subpopulations and emphasize the heterogeneity of different MAC populations. This study underscores the need to scrutinize models of MAC biology for relevance to specific disease processes.

Antimicrobial treatment of ventilator-associated pneumonia.

Ventilator-associated pneumonia is a common complication of ventilatory support for patients with acute respiratory failure and is associated with increased morbidity, mortality, and costs. Optimal antimicrobial therapy is an essential part of successful management of ventilator-associated pneumonia. Numerous safe and effective antimicrobial drugs are available, and their efficacy can be optimized by attention to basic pharmacokinetic and pharmacodynamic principles. An adequate initial empiric antimicrobial regimen is essential, because inadequate initial therapy is consistently associated with increased mortality. This regimen must be selected before final microbiology results become known, but likely pathogens and antimicrobial resistance patterns can be predicted based on published guidelines, patient-specific factors, and local epidemiologic data. Nevertheless, the initial regimen must often be broad-spectrum and typically requires combination therapy, with 2 or 3 different drugs, if there are risk factors for multidrug-resistant pathogens. The antimicrobial regimen can be narrowed or discontinued as culture and susceptibility results permit. This de-escalation strategy ensures adequate initial antimicrobial therapy for most patients but lessens unnecessary antimicrobial exposure. The best diagnostic approach used to guide therapy, the optimum duration of therapy, and the roles of combination therapy, rotating therapy, and unconventional approaches to antimicrobial therapy all remain uncertain.

The microbiology of ventilator-associated pneumonia.

Ventilator-associated pneumonia (VAP) is a common complication of ventilatory support for patients with acute respiratory failure and is associated with increased morbidity, mortality, and costs. Awareness of the microbiology of VAP is essential for selecting optimal antibiotic therapy and improving these outcomes. The specific microbial causes of VAP are many and varied. Most cases of VAP are caused by bacterial pathogens that normally colonize the oropharynx and gut, or that are acquired via transmission by health-care workers from environmental surfaces or from other patients. Common pathogens include Pseudomonas species and other highly resistant Gram-negative bacilli, staphylococci, the Enterobacteriaceae, streptococci, and Haemophilus species. Antibiotic-resistant pathogens such as Pseudomonas and Acinetobacter species and methicillin-resistant strains of Staphylococcus aureus are much more common after prior antibiotic treatment or prolonged hospitalization or mechanical ventilation, and when other risk factors are present. The bacterial pathogens responsible for VAP also vary depending on patient characteristics and in certain clinical circumstances, such as in acute respiratory distress syndrome or following tracheostomy, traumatic injuries, or burns. But these differences appear to be due primarily to the duration of mechanical ventilation and/or degree of prior antibiotic exposure of these patients. The causes of VAP can vary considerably by geographic location (even between units in the same hospital), emphasizing the importance of local epidemiological and microbiological data. Atypical bacteria, viruses, and fungi also have been implicated as causes of VAP, but these pathogens have not been studied systematically and their role is presently unclear. In conclusion, information about the microbiology of VAP serves to guide optimal antibiotic therapy. The risk of antibiotic-resistant pathogens can be estimated using simple clinical features and awareness of local microbiology patterns. The roles of atypical bacterial and nonbacterial pathogens in VAP are incompletely understood and should be investigated further.