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Acinetobacter baumannii Infections in the ICU : Customization is the Key FREE TO VIEW

Jordi Rello, MD, PhD (Barcelona, Spain)
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Staff physician, Intensive Care Department, Hospital de Sabadell.

Correspondence to: Jordi Rello, MD, PhD, Intensive Care Department, Hospital de Sabadell, Parc Tauli s/n, E08208 Sabadell, Barcelona, Spain; e-mail: jrello@CSPT.ES



Chest. 1999;115(5):1226-1229. doi:10.1378/chest.115.5.1226
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A review of several series1indicates that Staphylococcus aureus and nonfermentative Gram-negative bacilli are the leading etiologic agents for ventilator-associated pneumonia (VAP). The incidence of S aureus and Pseudomonas aeruginosa is similar in the various series; the small differences depend on study population, exposure to antibiotics, and length of intubation. In contrast, the reported incidence of Acinetobacter baumannii varies widely: in three classical series of VAP using highly specific bronchoscopic techniques, Fagon et al,2 Rello et al,3and Torres et al4found A baumannii to be a cause of VAP in 9.5%, 3.5%, and 39.1% of episodes, respectively. The importance of nonfermentative Gram-negative bacilli as a cause of pneumonia is emphasized by the finding that these pathogens were responsible for 78.5% of deaths due to pneumonia with documented etiology in intubated patients.5 Using multivariate analysis, two studies67 found an independent association between this group of pathogens and death in a cohort of intubated patients. Whereas two matched case-control studies78 have demonstrated that VAP caused by P aeruginosa was associated with a significant excess of mortality, no such information is available for A baumannii.

In the late 1980s, methicillin-resistant strains of S aureus (MRSA) were responsible for large-scale outbreaks in North America and Europe, representing a major infection control problem. Critically ill patients were particularly affected, and the outbreaks were a major cause of mortality in those who were intubated.9In Spain, considerable clinical experience has been accumulated. The incidence of these strains declined unexpectedly in the early 1990s, and the A baumannii infection began to spread rapidly. In 1990, it was the eighth most frequent pathogen isolated from patients admitted to the ICU with a prevalence of 3.7%; 2 years later, it had risen to third place, with a prevalence of 8.2%.10 A national surveillance system (ENVIN-UCI) includes 49 Spanish hospitals that report ICU surveillance data for 1 month a year using standardized methods.11 Of the 7,151 patients reporting data in 1996, most VAPs caused by Gram-negative bacilli were due to P aeruginosa (22%) followed by A baumannii (13%); in contrast with P aeruginosa, the pattern of A baumannii distribution was patchy, and only 10 of the 49 hospitals had at least three A baumannii episodes. Moreover, broad differences in incidence were also reported between different ICUs from the same hospital.

In this issue of CHEST (see page 1378), Husni and colleagues report the characteristics of an outbreak of pneumonia caused by A baumannii in intubated patients. The work has its drawbacks—a deficiency in the number of potential risk factors evaluated, the absence of surveillance cultures, limiting the information only to patients with clinical manifestation of infection, and the lack of molecular typing in 11 of 15 strains. Despite these limitations (inevitable given the retrospective nature of the investigation), the study by Husni and colleagues is extremely important because it reopens the debate of whether new strategies are required to minimize the threat of newly resistant organisms.

Interventions for control or prevention of multiresistant organisms have been based on the following practices: (1) targeted surveillance and isolation; (2) decreasing the reservoir of organisms with potential for mutation; (3) decreasing antibiotic selective pressures; and (4) controlling dissemination within the institution by introducing barrier isolation precautions and maintaining proper use of equipment and procedures. The question facing clinicians worldwide is whether these measures are able to control an outbreak caused by this multiresistant organism in the ICU.

The study of Husni and coworkers and other reports12 establish clear-cut evidence of an association between some antimicrobials and A baumannii infection. Husni and colleagues found an association between certain cephalosporins and A baumannii infection. In contrast, other investigators14 report no association with prior antibiotic use. Moreover, A baumannii may rapidly colonize in patients who are admitted to ICUs when infection is endemic,13 or may even have been acquired by a patient in other hospital areas before ICU admission.12 My interpretation is that acquisition and dissemination of resistant organisms vary from one institution to another, whereas antimicrobial agents are not on their own a risk factor for acquisition of A baumannii; the role of exposure to certain antibiotics provides a selective advantage to a small resistant subpopulation of organisms in patients already colonized. Thus, the limitation of use to one antimicrobial agent may fuel a proliferation of resistant organisms,15 instead of helping to control an outbreak. Consequently, in meeting the challenge of eradicating an outbreak, major efforts should be devoted to measures of control, focusing on acquisition and dissemination.

A baumannii has been nicknamed the “Gram-negative MRSA,” and measures directed at controlling one organism have been implemented to control the other, ignoring the fact that the reservoir and the mode of spread for important resistant organisms vary dramatically. For some, like MRSA, the reservoir is the nares and the cutaneous surface both in hospitalized patients and those resident in the community.16 In addition, half of colonized patients will manifest signs of infection. For other organisms, like nonfermentative bacilli, the digestive tract of many patients in the ICU environment is the reservoir.13 In this scenario, the ratio of colonized to infected patients may be as high as 10:1, and patients with manifestations of infection are likely to be only the tip of the iceberg.13 Variability is also found in the mode of spread—MRSA appears to be passed from person to person, whereas nonfermenting Gram-negative bacillary infections appear to be spread through liquids and respiratory therapy devices.

Early detection of patients colonized, followed by strict attention to standard precautions and contact isolation, are essential components of any program designed to prevent nosocomial transmission of resistant organisms acquired from an exogenous route. Interestingly, the work by Husni and colleagues suggests an association between the development of VAP due to A baumannii and lower compliance with infection control practices. To compare their findings, one should look to other studies, such as a recent report of an outbreak of A baumannii pneumonia17in which the organism was found in various parts of the ICU, including those not normally touched by staff. Skin swabs from staff indicated colonization even though they had not cared for patients who had become infected. The organism was also detected in air near the output duct of ventilatory apparatus despite the presence of bacterial filters separating the patient from the ventilation tubing. Widespread environmental contamination is especially likely, while standard prevention measures and contact isolation are likely to be ineffective in preventing dissemination in intubated patients with A baumannii airway colonization. In my opinion, these observations demonstrate that these patients should be located in private, closed rooms with effective airborne isolation. Indeed, in hospitals with high prevalence, conversion from open rooms to isolation rooms has been reported to be highly effective in achieving successful control of transmission of airway colonization in intubated patients.18

In the clinical arena, clinicians should be aware that risk factors for pneumonia caused by A baumannii may vary from institution to institution. A recent study from Trouillet et al19has reported that pneumonia caused by A baumannii was confined to the subgroup of patients who received antibiotics and were ventilated for a period of > 1 week. Other French reports20 support these findings, suggesting that antimicrobial therapy active against these pathogens might be restricted to this subpopulation. In contrast, other authors reported that half of the episodes of VAP due to A baumannii developed within the first 8 days of ventilation.,14 The implications of these observations are that antimicrobial prescribing practices should be based on updated information customized to each institution, rather than on general guidelines.

In the research arena, studies are still needed to increase our knowledge of the specific risk factors of airway colonization by specific microorganisms in intubated patients. Variables associated with institutions with endemic colonization may be different from others in which infection by A baumannii is part of an expanding outbreak. In addition, different epidemiologic patterns may coexist with different clones. Thus, investigations should be customized to each individual clone (eg, those with carbapenem resistance) to avoid confusion between variables. Molecular typing methods are currently the cornerstone in such investigations. Finally, more epidemiologic investigations should be designed to investigate the relative importance of various modes of transmission and the specific risk factors associated with differentiated clones.

What we have learned, and what this author’s own experience confirms,14 is that the epidemiologic pattern of A baumannii differs from that of other nonfermentative Gram-negative bacilli.,20A baumannii has been nicknamed the“ Gram-negative MRSA,” but the same measures cannot be used to control the two pathogens. In addition, the epidemiologic pattern of the resistant organism may vary from hospital to hospital, and control measures or therapeutic approaches should be customized to each institution. Moreover, measures have to be individualized even to specific organism-antimicrobial pairs. Although antimicrobial control programs and increasing handwashing compliance may help in reducing the prevalence of A baumannii, as suggested by Husni and colleagues, I believe that these measures of control alone are not likely to be successful in preventing airway colonization in many institutions. Thus, in deciding how to prevent transmission or how to treat patients at risk, the approach needs to be customized to each individual institution.

References

Rello, J, Torres, A (1996) Microbial causes of ventilator-associated pneumonia.Semin Respir Infect11,24-31. [PubMed]
 
Fagon, JY, Chastre, J, Domart, Y, et al Nosocomial pneumonia in patients receiving continuous mechanical ventilation: prospective analysis of 52 episodes with use of a protected specimen brush and quantitative culture techniques.Am Rev Respir Dis1989;139,877-884. [PubMed]
 
Rello, J, Quintana, E, Ausina, V, et al Incidence, etiology and outcome of nosocomial pneumonia in mechanically ventilated patients.Chest1991;100,439-444. [PubMed] [CrossRef]
 
Torres, A, Aznar, R, Gatell, JM, et al Incidence, risk and prognosis factors of nosocomial pneumonia in mechanically ventilated patients.Am Rev Respir Dis1990;142,523-528. [PubMed]
 
Rello, J, Ausina, V, Ricart, M, et al Impact of previous antimicrobial therapy on the etiology and outcome of ventilator-associated pneumonia.Chest1993;104,1230-1235. [PubMed]
 
Kollef, MH, Silver, P, Murphy, DM, et al The effect of late-onset ventilator-associated pneumonia determining patient mortality.Chest1995;108,1655-1662. [PubMed]
 
Fagon, JY, Chastre, J, Domart, Y Nosocomial pneumonia in patients receiving continuous mechanical ventilation: prospective analysis of 52 episodes with use of a protected specimen brush and quantitative culture techniques.Am Rev Respir Dis1989;139,877-884. [PubMed]
 
Rello, J, Jubert, P, Vallés, J, et al Evaluation of outcome in intubated patients with pneumonia caused byPseudomonas aeruginosa.Clin Infect Dis1996;23,973-978. [PubMed]
 
Rello, J, Torres, A, Ricart, M, et al Ventilator-associated pneumonia byStaphylococcus aureus: comparison of methicillin-resistant and methicillin-sensitive episodes.Am J Respir Crit Care Med1994;150,1545-1549. [PubMed]
 
Vaqué, J, Roselló, J, Trilla, A, et al Nosocomial infections in Spain: results of five nationwide serial prevalence surveys (EPINE project, 1990 to 1994): nosocomial infections prevalence study in Spain. Infect Control Hosp Epidemiol. 1996;;17 ,.:293. [PubMed]
 
Palomar, M, Alvarez-Lerma, F, De la Cal, MA, et al ICU-acquired infections in Spain: predominant pathogens [abstract]. Intensive Care Med. 1996;;22(suppl 3) ,.:S325
 
Villers, D, Espaze, E, Coste-Burel, M, et al NosocomialAcinetobacter baumanniiinfections: microbiological and clinical epidemiology.Ann Intern Med1998;129,182-189. [PubMed]
 
Corbella, X, Pujol, M, Ayats, J, et al Relevance of digestive tract colonization in the epidemiology of nosocomial infections due to multiresistantAcinetobacter baumannii.Clin Infect Dis1996;23,329-334. [PubMed]
 
Baraibar, J, Correa, H, Mariscal, D, et al Risk factors for infection byAcinetobacter baumanniiin intubated patients with nosocomial pneumonia.Chest1997;112,1050-1054. [PubMed]
 
Schentag, JJ, Hyatt, JM, Carr, JR, et al Genesis of methicillin-resistantStaphylococcus aureus(MRSA), how treatment of MRSA infections has selected for vancomycin-resistantEnterococcus faecium, and the importance of antibiotic management and infection control.Clin Infect Dis1998;26,1204-1214. [PubMed]
 
Working Party.. Revised guidelines for the control of methicillin-resistantStaphylococcus aureusinfection in hospitals.J Hosp Infect1998;39,253-290. [PubMed]
 
Dealler, S Nosocomial outbreak of multiresistantAcinetobacter spon an intensive care unit: possible association with ventilation equipment [letter].J Hosp Infect1998;38,147-155. [PubMed]
 
Mulin, B, Rouget, C, Clèment, Ch, et al Association of private isolation rooms with ventilator-associatedAcinetobacter baumanniipneumonia in a surgical intensive care unit.Infect Control Hosp Epidemiol1997;18,499-503. [PubMed]
 
Trouillet, JL, Chastre, J, Vuagnat, A, et al Ventilator-associated pneumonia caused by potentially drug-resistant bacteria.Am J Respir Crit Care Med1998;157,531-539. [PubMed]
 
Rello, J, Ausina, V, Ricart, M, et al Risk factors for infection byPseudomonas aeruginosain patients with ventilator-associated pneumonia.Intensive Care Med1994;20,193-198. [PubMed]
 

Figures

Tables

References

Rello, J, Torres, A (1996) Microbial causes of ventilator-associated pneumonia.Semin Respir Infect11,24-31. [PubMed]
 
Fagon, JY, Chastre, J, Domart, Y, et al Nosocomial pneumonia in patients receiving continuous mechanical ventilation: prospective analysis of 52 episodes with use of a protected specimen brush and quantitative culture techniques.Am Rev Respir Dis1989;139,877-884. [PubMed]
 
Rello, J, Quintana, E, Ausina, V, et al Incidence, etiology and outcome of nosocomial pneumonia in mechanically ventilated patients.Chest1991;100,439-444. [PubMed] [CrossRef]
 
Torres, A, Aznar, R, Gatell, JM, et al Incidence, risk and prognosis factors of nosocomial pneumonia in mechanically ventilated patients.Am Rev Respir Dis1990;142,523-528. [PubMed]
 
Rello, J, Ausina, V, Ricart, M, et al Impact of previous antimicrobial therapy on the etiology and outcome of ventilator-associated pneumonia.Chest1993;104,1230-1235. [PubMed]
 
Kollef, MH, Silver, P, Murphy, DM, et al The effect of late-onset ventilator-associated pneumonia determining patient mortality.Chest1995;108,1655-1662. [PubMed]
 
Fagon, JY, Chastre, J, Domart, Y Nosocomial pneumonia in patients receiving continuous mechanical ventilation: prospective analysis of 52 episodes with use of a protected specimen brush and quantitative culture techniques.Am Rev Respir Dis1989;139,877-884. [PubMed]
 
Rello, J, Jubert, P, Vallés, J, et al Evaluation of outcome in intubated patients with pneumonia caused byPseudomonas aeruginosa.Clin Infect Dis1996;23,973-978. [PubMed]
 
Rello, J, Torres, A, Ricart, M, et al Ventilator-associated pneumonia byStaphylococcus aureus: comparison of methicillin-resistant and methicillin-sensitive episodes.Am J Respir Crit Care Med1994;150,1545-1549. [PubMed]
 
Vaqué, J, Roselló, J, Trilla, A, et al Nosocomial infections in Spain: results of five nationwide serial prevalence surveys (EPINE project, 1990 to 1994): nosocomial infections prevalence study in Spain. Infect Control Hosp Epidemiol. 1996;;17 ,.:293. [PubMed]
 
Palomar, M, Alvarez-Lerma, F, De la Cal, MA, et al ICU-acquired infections in Spain: predominant pathogens [abstract]. Intensive Care Med. 1996;;22(suppl 3) ,.:S325
 
Villers, D, Espaze, E, Coste-Burel, M, et al NosocomialAcinetobacter baumanniiinfections: microbiological and clinical epidemiology.Ann Intern Med1998;129,182-189. [PubMed]
 
Corbella, X, Pujol, M, Ayats, J, et al Relevance of digestive tract colonization in the epidemiology of nosocomial infections due to multiresistantAcinetobacter baumannii.Clin Infect Dis1996;23,329-334. [PubMed]
 
Baraibar, J, Correa, H, Mariscal, D, et al Risk factors for infection byAcinetobacter baumanniiin intubated patients with nosocomial pneumonia.Chest1997;112,1050-1054. [PubMed]
 
Schentag, JJ, Hyatt, JM, Carr, JR, et al Genesis of methicillin-resistantStaphylococcus aureus(MRSA), how treatment of MRSA infections has selected for vancomycin-resistantEnterococcus faecium, and the importance of antibiotic management and infection control.Clin Infect Dis1998;26,1204-1214. [PubMed]
 
Working Party.. Revised guidelines for the control of methicillin-resistantStaphylococcus aureusinfection in hospitals.J Hosp Infect1998;39,253-290. [PubMed]
 
Dealler, S Nosocomial outbreak of multiresistantAcinetobacter spon an intensive care unit: possible association with ventilation equipment [letter].J Hosp Infect1998;38,147-155. [PubMed]
 
Mulin, B, Rouget, C, Clèment, Ch, et al Association of private isolation rooms with ventilator-associatedAcinetobacter baumanniipneumonia in a surgical intensive care unit.Infect Control Hosp Epidemiol1997;18,499-503. [PubMed]
 
Trouillet, JL, Chastre, J, Vuagnat, A, et al Ventilator-associated pneumonia caused by potentially drug-resistant bacteria.Am J Respir Crit Care Med1998;157,531-539. [PubMed]
 
Rello, J, Ausina, V, Ricart, M, et al Risk factors for infection byPseudomonas aeruginosain patients with ventilator-associated pneumonia.Intensive Care Med1994;20,193-198. [PubMed]
 
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