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MRSA pathogenicity and bacteraemia
Letters
to the Editor
171
References 1. Patterson
2. 3.
4.
5.
6.
7.
WJ, Seal DV, Curran E, Sinclair TM, McLuckie JC. Fatal nosocomial Legionnaires’ Disease: relevance of contamination of hospital water supply by temperature-dependent buoyancy-driven flow from spur pipes. Epidemiol Infect 1994; 112: 513-525. de Beer D, Srinivasan R, Stewart PS. Direct measurement of chlorine penetration into biofilms during disinfection. Appl Environ Microbial 1994; 60: 433944. Kuchta JM, Navratil SJ, Wadowski RM, Dowling JN, States SJ, Yee RB. Effect of chlorine on the survival and growth of Legionella pneumophila and Hartmannella vermiformis. In: Barbaree JM, Breiman KF, Dufour AP, Eds. Legionella Current Status and Emerging Perspectives. Washington DC: American Society for Microbiology, 1993; 242-245. McLuckie JC, Campbell I, Hay J, Patterson W, Seal DV. The Colonisation of Water Supplies in United Kingdom Transplant Units with Legionella Bacteria and Protozoa, and the Risk to Patients. HMSO Scotland 1995. ISBN 0-7480-3036-O. Struelens MJ, Rost F, Maes N, Maas A, Serruys E. Control of nosocomial legionellosis based on water system disinfection by heat and UV light: a four-year evaluation. In: Barbaree JM, Breiman KF, Dufour AI’, Eds. Legionella Current Status and Emerging Perspectives. Washington DC: American Society for Microbiology, 1993; 253-254. Liu Z, Stout JE, Tedesco L, Boldin N, Hwang C, Diven WF, Yu VL. Controlled evaluation of copper-silver ionization in eradicating Legionellapneumophila from a hospital water distribution system. 3 Infect Dis 1994; 169: 919-922. Patterson WJ, Hay J, Seal DV, McLuckie JC. Colonization of transplant unit water supplies with Legionella and protozoa: precautions required to reduce the risk of legionellosis. 3 Hosp Znfect 1997; (in press).
Sir, MRSA
pathogenicity
and
bacteraemia
Catchpole et al.’ provide evidence supporting findings from Spain2 that patients with methicillin-resistant Staphylococcus aureu~ (MRSA) bacteraemia suffer an excess mortality compared to those with bacteraemia with methicilin-sensitive strains (MSSA). In studying hospitalized patients with complex histories, the difficulties in pinpointing a single final cause of death and of knowing that all relevant differences between groups have been controlled, leave uncertainties that a patient would have survived had their bloodstream S. aureus not been methicillin-resistant. Although excess mortality in an MRSA group may indicate enhanced virulence, it is not possible to reach a firm conclusion that MRSA per se is more lethal. Its persistence in terminal patients may reflect an ability to withstand antibiotic therapy separate from a contribution to eventual demise. Moreover there are other studies suggesting that MRSA and MSSA bacteraemias have similar outcomes, as noted by the authors of the Spanish report. An alternative and less extreme comparative measure of pathogenicity is the likelihood of patients with MRSA and MSSA developing, rather than succumbing to, bacteraemia with these organisms. At this hospital, this was assessed by examination of laboratory records of patients from whom specimens had yielded MRSA or MSSA during a one-year period. The
Letters
172 Table
Type
I.
to the Editor
Patients with MRSAor MSSA-bacteraemia proportion of those positive for the organism
of Ward
General medical (2) Renal medicine (1) Care of elderly (1) General surgical (2) Vascular surgery (1) Intensive care (1)
MRSA
MSSA
14/51 s/30 2124 8178 2146 21179
8127 28161 l/10 2141 3124 2120
as a
analysis was limited to hospital areas where MRSA was common and which regularly took large numbers of blood cultures. MRSA patients detected only as part of the MRSA screening programme were excluded from the analysis. The results of this analysis are shown in Table I. MRSA and MSSA caused bacteraemia in 16.9 and 24.0% of their respective patient groups (chi-square=3.742, P=O.O53). Although MRSA was more likely than MSSA to be invasive in intensive care, the reverse was true in renal medicine and overall there was no statistically significant difference. The apparent increased virulence of MRSA in intensive care patients is compatible with the findings of Pujol et ~1.~ and argues for the policy now adopted in some centres of limiting MRSA control to critical areas only. Perhaps the explanation for differences in virulence found in different studies lies in the existence of numerous strains of both MRSA and MSSA, varying in their virulence, as well as patients having different susceptibilities.
Department of Bacteriology, St. Mary’s Hospital, Praed St., London, W2 1NY
S. P. Barrett
References 1. Catchpole C, Wise R, Fraise A. MRSA bacteraemia. J Hasp Infect 1997; 35: 159-161. 2. Romero-Vivas J, Rubio M, Fernandez C, Picazo JJ. Mortality associated with nosocomial bacteremia due to methicillin resistant Staphylococcus aureus. Clin Infect Dis 1995; 21: 1417-1423. 3. Pujol M, Pefia C, Pallares R, Ariza J, Ayats J, Dominguez MA, Gudiol F. Nosocomial Staphylococcus aureus bacteremia among nasal carriers of methicillin-resistant and methicillin-susceptible strains. Am J Med 1996; 100: 509416.
to the Editor
171
References 1. Patterson
2. 3.
4.
5.
6.
7.
WJ, Seal DV, Curran E, Sinclair TM, McLuckie JC. Fatal nosocomial Legionnaires’ Disease: relevance of contamination of hospital water supply by temperature-dependent buoyancy-driven flow from spur pipes. Epidemiol Infect 1994; 112: 513-525. de Beer D, Srinivasan R, Stewart PS. Direct measurement of chlorine penetration into biofilms during disinfection. Appl Environ Microbial 1994; 60: 433944. Kuchta JM, Navratil SJ, Wadowski RM, Dowling JN, States SJ, Yee RB. Effect of chlorine on the survival and growth of Legionella pneumophila and Hartmannella vermiformis. In: Barbaree JM, Breiman KF, Dufour AP, Eds. Legionella Current Status and Emerging Perspectives. Washington DC: American Society for Microbiology, 1993; 242-245. McLuckie JC, Campbell I, Hay J, Patterson W, Seal DV. The Colonisation of Water Supplies in United Kingdom Transplant Units with Legionella Bacteria and Protozoa, and the Risk to Patients. HMSO Scotland 1995. ISBN 0-7480-3036-O. Struelens MJ, Rost F, Maes N, Maas A, Serruys E. Control of nosocomial legionellosis based on water system disinfection by heat and UV light: a four-year evaluation. In: Barbaree JM, Breiman KF, Dufour AI’, Eds. Legionella Current Status and Emerging Perspectives. Washington DC: American Society for Microbiology, 1993; 253-254. Liu Z, Stout JE, Tedesco L, Boldin N, Hwang C, Diven WF, Yu VL. Controlled evaluation of copper-silver ionization in eradicating Legionellapneumophila from a hospital water distribution system. 3 Infect Dis 1994; 169: 919-922. Patterson WJ, Hay J, Seal DV, McLuckie JC. Colonization of transplant unit water supplies with Legionella and protozoa: precautions required to reduce the risk of legionellosis. 3 Hosp Znfect 1997; (in press).
Sir, MRSA
pathogenicity
and
bacteraemia
Catchpole et al.’ provide evidence supporting findings from Spain2 that patients with methicillin-resistant Staphylococcus aureu~ (MRSA) bacteraemia suffer an excess mortality compared to those with bacteraemia with methicilin-sensitive strains (MSSA). In studying hospitalized patients with complex histories, the difficulties in pinpointing a single final cause of death and of knowing that all relevant differences between groups have been controlled, leave uncertainties that a patient would have survived had their bloodstream S. aureus not been methicillin-resistant. Although excess mortality in an MRSA group may indicate enhanced virulence, it is not possible to reach a firm conclusion that MRSA per se is more lethal. Its persistence in terminal patients may reflect an ability to withstand antibiotic therapy separate from a contribution to eventual demise. Moreover there are other studies suggesting that MRSA and MSSA bacteraemias have similar outcomes, as noted by the authors of the Spanish report. An alternative and less extreme comparative measure of pathogenicity is the likelihood of patients with MRSA and MSSA developing, rather than succumbing to, bacteraemia with these organisms. At this hospital, this was assessed by examination of laboratory records of patients from whom specimens had yielded MRSA or MSSA during a one-year period. The
Letters
172 Table
Type
I.
to the Editor
Patients with MRSAor MSSA-bacteraemia proportion of those positive for the organism
of Ward
General medical (2) Renal medicine (1) Care of elderly (1) General surgical (2) Vascular surgery (1) Intensive care (1)
MRSA
MSSA
14/51 s/30 2124 8178 2146 21179
8127 28161 l/10 2141 3124 2120
as a
analysis was limited to hospital areas where MRSA was common and which regularly took large numbers of blood cultures. MRSA patients detected only as part of the MRSA screening programme were excluded from the analysis. The results of this analysis are shown in Table I. MRSA and MSSA caused bacteraemia in 16.9 and 24.0% of their respective patient groups (chi-square=3.742, P=O.O53). Although MRSA was more likely than MSSA to be invasive in intensive care, the reverse was true in renal medicine and overall there was no statistically significant difference. The apparent increased virulence of MRSA in intensive care patients is compatible with the findings of Pujol et ~1.~ and argues for the policy now adopted in some centres of limiting MRSA control to critical areas only. Perhaps the explanation for differences in virulence found in different studies lies in the existence of numerous strains of both MRSA and MSSA, varying in their virulence, as well as patients having different susceptibilities.
Department of Bacteriology, St. Mary’s Hospital, Praed St., London, W2 1NY
S. P. Barrett
References 1. Catchpole C, Wise R, Fraise A. MRSA bacteraemia. J Hasp Infect 1997; 35: 159-161. 2. Romero-Vivas J, Rubio M, Fernandez C, Picazo JJ. Mortality associated with nosocomial bacteremia due to methicillin resistant Staphylococcus aureus. Clin Infect Dis 1995; 21: 1417-1423. 3. Pujol M, Pefia C, Pallares R, Ariza J, Ayats J, Dominguez MA, Gudiol F. Nosocomial Staphylococcus aureus bacteremia among nasal carriers of methicillin-resistant and methicillin-susceptible strains. Am J Med 1996; 100: 509416.