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Is MRSA More Pathogenic in Critically Ill Patients?

Is MRSA More Pathogenic in Critically Ill Patients? I read with great interest the recent Original Investigation "Outcome and Attributable Mortality in Critically Ill Patients With Bacteremia Involving Methicillin-Susceptible [MSSA] and Methicillin-Resistant Staphylococcus aureus [MRSA]."1 The attributable mortality rates in intensive care unit (ICU) patients with staphylococcal bacteremia obtained in this study by using 2 independent case-control studies is clearly unique to this study. Even though the study did not specifically use hemodynamic instability, acute renal failure, and MRSA bacteremia as explanatory variables, it has established these statistically as independent predictors of mortality. I would like to make the following observations in this regard: 1. It is a clear possibility that the issue of relative efficacy of penicillinase-resistant β-lactam antibiotics against MSSA bacteremias compared with that of glycopeptides against MRSA bacteremias may be magnified and become crucial in critically ill patients. 2. Perhaps the use of APACHE (Acute Physiology and Chronic Health Evaluation) II scores for critically ill patients allows better assessment of the severity of staphylococcal bacteremia than the sepsis severity score of McCabe and Jackson, used in some powerful studies done before. 3. We are already well aware of the probability of overestimation of MRSA virulence using cohort studies.2 What may need to be better understood is the impact of using 2 independent case-control studies in this investigation vs using patients with MSSA bacteremia as control patients.3 In patients with Staphylococcus bacteremia the mortality correlates fairly well with the degree of change in APACHE II scores on the day of onset of the bacteremia compared with the prebacteremic APACHE II score.4,5 So it is difficult to exactly specify how critical a case patient is compared with a control patient at the time of admission to the ICU. The only truly known fact about a specific case-control group at the time of admission to the ICU is the diagnostic category leading to admission to the ICU; the time course of bacteremia will remain unknown and so will be the expected mortality. 4. As in every study comparing MRSA with MSSA bacteremia, it is important to adjust the length of hospitalization prior to the onset of infection. This study, like others, also confirmed that length of hospitalization (ICU stay, in this situation) was independently associated with mortality in the MRSA case-control group. I differ with the questioning of its confounding effect. If the increased attributable mortality from MRSA bacteremia is accepted as above, the central question that remains to be addressed is what leads to increased mortality from MRSA: virulence, inadequate treatment, or both. To answer that question we may need to address the following: 1. It is poorly understood as to why MRSA colonization has a higher autoinfection rate than MSSA colonization. Is it perhaps in some way also related to the underlying disease states, increased age, ICU stay, postsurgical state, or relative immune compromise and not to any higher relative virulence of MRSA? 2. Do we need more basic studies than have been done already that have failed to show convincing differences among MRSA and MSSA strains with regard to possible virulence markers, such as adherence capacity, production of toxins, hemolysins, and intraleukocytic survival? 3. Even if we were to find some reasons for higher relative virulence of MRSA, can we demonstrate its precise role in explaining the possible higher attributable mortality, given the numerous and intricately interlinked variables that contribute to mortality in these patients? References 1. Blot SIVandewoude KHHoste EAColardyn FA Outcome and attributable mortality in critically ill patients with bacteremia involving methicillin-susceptible and methicillin-resistant Staphylococcus aureus. Arch Intern Med. 2002;1622229- 2235Google ScholarCrossref 2. Soriano AMartinez JMensa J et al. Pathogenic significance of methicillin resistance for patients with Staphylococcus aureus bacteremia. Clin Infect Dis. 2000;30368- 373Google ScholarCrossref 3. Harbarth SRutschmann OSudre PPittet D Impact of methicillin resistance on the outcome of patients with bacteremia caused by Staphylococcus aureus. Arch Intern Med. 1998;158182- 189Google ScholarCrossref 4. Yzerman EBoelens HTjhie JKluytmans JMouton JVerbrugh H Delta APACHE II for predicting the course and outcome of nosocomial Staphylococcus bacteremia and its relation to host defense. J Infect Dis. 1996;173914- 919Google ScholarCrossref 5. Mylotte JMAeschlimann JRRotella DL Staphylococcus aureus bacteremia: factors predicting hospital mortality. Infect Control Hosp Epidemiol. 1996;17165- 168Google ScholarCrossref http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Internal Medicine American Medical Association

Is MRSA More Pathogenic in Critically Ill Patients?

Archives of Internal Medicine , Volume 163 (6) – Mar 24, 2003

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Publisher
American Medical Association
Copyright
Copyright © 2003 American Medical Association. All Rights Reserved.
ISSN
0003-9926
eISSN
1538-3679
DOI
10.1001/archinte.163.6.739
Publisher site
See Article on Publisher Site

Abstract

I read with great interest the recent Original Investigation "Outcome and Attributable Mortality in Critically Ill Patients With Bacteremia Involving Methicillin-Susceptible [MSSA] and Methicillin-Resistant Staphylococcus aureus [MRSA]."1 The attributable mortality rates in intensive care unit (ICU) patients with staphylococcal bacteremia obtained in this study by using 2 independent case-control studies is clearly unique to this study. Even though the study did not specifically use hemodynamic instability, acute renal failure, and MRSA bacteremia as explanatory variables, it has established these statistically as independent predictors of mortality. I would like to make the following observations in this regard: 1. It is a clear possibility that the issue of relative efficacy of penicillinase-resistant β-lactam antibiotics against MSSA bacteremias compared with that of glycopeptides against MRSA bacteremias may be magnified and become crucial in critically ill patients. 2. Perhaps the use of APACHE (Acute Physiology and Chronic Health Evaluation) II scores for critically ill patients allows better assessment of the severity of staphylococcal bacteremia than the sepsis severity score of McCabe and Jackson, used in some powerful studies done before. 3. We are already well aware of the probability of overestimation of MRSA virulence using cohort studies.2 What may need to be better understood is the impact of using 2 independent case-control studies in this investigation vs using patients with MSSA bacteremia as control patients.3 In patients with Staphylococcus bacteremia the mortality correlates fairly well with the degree of change in APACHE II scores on the day of onset of the bacteremia compared with the prebacteremic APACHE II score.4,5 So it is difficult to exactly specify how critical a case patient is compared with a control patient at the time of admission to the ICU. The only truly known fact about a specific case-control group at the time of admission to the ICU is the diagnostic category leading to admission to the ICU; the time course of bacteremia will remain unknown and so will be the expected mortality. 4. As in every study comparing MRSA with MSSA bacteremia, it is important to adjust the length of hospitalization prior to the onset of infection. This study, like others, also confirmed that length of hospitalization (ICU stay, in this situation) was independently associated with mortality in the MRSA case-control group. I differ with the questioning of its confounding effect. If the increased attributable mortality from MRSA bacteremia is accepted as above, the central question that remains to be addressed is what leads to increased mortality from MRSA: virulence, inadequate treatment, or both. To answer that question we may need to address the following: 1. It is poorly understood as to why MRSA colonization has a higher autoinfection rate than MSSA colonization. Is it perhaps in some way also related to the underlying disease states, increased age, ICU stay, postsurgical state, or relative immune compromise and not to any higher relative virulence of MRSA? 2. Do we need more basic studies than have been done already that have failed to show convincing differences among MRSA and MSSA strains with regard to possible virulence markers, such as adherence capacity, production of toxins, hemolysins, and intraleukocytic survival? 3. Even if we were to find some reasons for higher relative virulence of MRSA, can we demonstrate its precise role in explaining the possible higher attributable mortality, given the numerous and intricately interlinked variables that contribute to mortality in these patients? References 1. Blot SIVandewoude KHHoste EAColardyn FA Outcome and attributable mortality in critically ill patients with bacteremia involving methicillin-susceptible and methicillin-resistant Staphylococcus aureus. Arch Intern Med. 2002;1622229- 2235Google ScholarCrossref 2. Soriano AMartinez JMensa J et al. Pathogenic significance of methicillin resistance for patients with Staphylococcus aureus bacteremia. Clin Infect Dis. 2000;30368- 373Google ScholarCrossref 3. Harbarth SRutschmann OSudre PPittet D Impact of methicillin resistance on the outcome of patients with bacteremia caused by Staphylococcus aureus. Arch Intern Med. 1998;158182- 189Google ScholarCrossref 4. Yzerman EBoelens HTjhie JKluytmans JMouton JVerbrugh H Delta APACHE II for predicting the course and outcome of nosocomial Staphylococcus bacteremia and its relation to host defense. J Infect Dis. 1996;173914- 919Google ScholarCrossref 5. Mylotte JMAeschlimann JRRotella DL Staphylococcus aureus bacteremia: factors predicting hospital mortality. Infect Control Hosp Epidemiol. 1996;17165- 168Google ScholarCrossref

Journal

Archives of Internal MedicineAmerican Medical Association

Published: Mar 24, 2003

Keywords: critical illness,methicillin-resistant staphylococcus aureus

References

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