Do Anaerobic Bacteria Cause Ventilator-Associated Pneumonia?

Abstract & Commentary

Synopsis: Of 26 mechanically ventilated patients, 22 developed bacterial lower respiratory tract colonization, and in 15 patients anaerobes were recovered; 2 of 5 patients diagnosed with ventilator-associated pneumonia had anaerobes present in sufficient quantity to suggest that they were considered pathogenic.

Source: Robert R, et al. Colonization of lower respiratory tract with anaerobic bacteria in mechanically ventilated patients. Intensive Care Med. 2003;29: 1062-1068.

Robert and colleagues in Poitiers, France, carried out this study to determine the frequency with which the lower airways of intubated ICU patients become colonized with anaerobic bacteria, and to see whether such colonization was associated with actual clinical pneumonia. They studied 26 patients who were intubated and mechanically ventilated within 24 hours of admission to their university hospital’s ICU. They performed protected tracheal aspiration and protected tracheal brushings on the day following intubation and then twice weekly until the patients were extubated. Special techniques for recovering anaerobic bacteria were used. Ventilator-associated pneumonia (VAP) was defined using a quantitative culture threshold of 103 colony-forming units (cfu) per mL and the following clinical criteria: a new infiltrate on the chest x-ray that persisted at least 24 hours, plus at least 2 of the 3 signs of fever (or hypothermia), purulent tracheal aspirate, and/or leukocytosis.

Twenty-two patients (85%) became colonized by at least 1 bacterial species, 21 by aerobic organisms, and 15 by anaerobes. The latter comprised 28 isolates of 9 different species of anaerobic bacteria. Fourteen of the 15 patients with anaerobic colonization were also colonized by aerobic bacteria; only 1 had only anaerobes. Robert et al classified the colonization as early (within 5 days of intubation) in 16 of the 22 patients and late (after 5 days) in 6 patients. The criteria for VAP were satisfied by 5 patients, in 2 of whom the bacteria recovered at > 103 cfu/mL included anaerobes. The latter were both Fusobacterium nucleatum and Prevotella melaninogenica in both patients and also P oralis in 1. In 1 of these patients, Streptococcus viridans was also recovered in pathogenic numbers. Robert et al conclude that lower airway colonization with anaerobic bacteria is common in mechanically ventilated patients, and that these organisms may have a role in the pathogenesis of VAP.

Comment by David J. Pierson, MD

Using specialized techniques for sampling lower respiratory tract secretions and quantitatively culturing the specimens, Robert et al recovered anaerobic bacteria from 15 of the 26 intubated patients they studied. Twenty-eight different anaerobic isolates were recovered from the 15 patients, and in all but 1 instance aerobic bacteria grew as well. While there is nothing included in the paper to make me suspect that the techniques used were faulty or that there were other study design problems, it seems odd to me that such a large proportion of the patients were colonized and/or infected concurrently by such a large number of organisms. Bronchoscopy was not used in obtaining the bacteriologic specimens, and the sampling technique used was different from that generally used by intensivists in this country. It is tempting to speculate, as Marik does in the accompanying editorial,1 that at least some of them may have been contaminated, or at least that the organisms recovered did not constitute actual pulmonary pathogens.

Do anaerobes cause VAP? I imagine they can, but it must be exceedingly rare, and certainly much less frequent than the 40% of VAP cases observed in this study. Anaerobes rarely cause community-acquired pulmonary infections in the absence of airway occlusion, trapping the bacteria after aspiration of particulate matter from the mouth and establishing the local oxidation-reduction environment necessary for proliferation of these generally fastidious organisms. Anaerobic chest infections are often subacute, at least in their clinical features. I suppose they have to have an initial, less fulminant phase, and so it is possible that the patients in this study were in the very early stages of what is more typically observed in patients with community-acquired anaerobic infections.

Should antibiotic treatment of suspected VAP include anaerobic coverage? Marik1 points out that it very often does—perhaps resulting in more morbidity and mortality than would occur from untreated anaerobic infections. I am not sure what to do with the findings of this paper. For now, and until these data are replicated by investigators at other institutions, I am not inclined to change either the way I culture lower airway specimens or the way I order initial antimicrobial coverage in patients with suspected VAP.

Dr. Pierson is Professor of Medicine University of Washington Medical Director Respiratory Care Harborview Medical Center Seattle.

Reference

1. Marik P. Aliens, anaerobes, and the lung! Intensive Care Med. 2003;29:1035-1037.