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Repeated epidemics caused by extended-spectrum beta-lactamase-producing Serratia marcescens strains
F Luzzaro1, M Perilli, R Migliavacca
1Laboratory of Microbiology, Hospital of Varese, Italy.
Abstract:
An outbreak of Serratia marcescens involving 42 patients admitted to the general intensive care unit of the Hospital of Varese, Italy, occurred from March 1994 to August 1995. The causative strains were resistant to oxyimino-cephalosporins and monobactams due to their production of an extended-spectrum beta-lactamase. Another outbreak caused by Serratia marcescens strains had occurred in the same unit a few months earlier, from February to October 1993, with the strains involved producing a novel TEM-derived extended-spectrum beta-lactamase. In order to verify whether there were any relationships between isolates from the two epidemics, the strains and their enzymes were characterized. Biochemical data and gene amplification experiments showed that the isolates of the second outbreak harbored a non-conjugative plasmid of approximately 48 kb, codifying for the production of an SHV-derived extended-spectrum beta-lactamase with pI 8.2. Restriction fragment length polymorphism analysis of total genomic DNA by pulsed-field gel electrophoresis of Serratia marcescens isolates unambiguously identified two different bacterial clones responsible for the two epidemics. Epidemiological and microbiological investigations demonstrated the long persistence of Serratia marcescens strains and their circulation in other hospital wards, thus suggesting their possible role as a long-term reservoir for further epidemic spread.
Insights
Two distinct Serratia marcescens clones caused separate intensive care unit outbreaks, linked to extended-spectrum beta-lactamase production. These persistent bacteria may act as reservoirs for future hospital-wide infections.
Area of Science:
- Medical Microbiology
- Infectious Diseases
- Hospital Epidemiology
Background:
- An outbreak of Serratia marcescens occurred in an intensive care unit (ICU) from March 1994 to August 1995.
- The causative strains exhibited resistance to oxyimino-cephalosporins and monobactams due to extended-spectrum beta-lactamase (ESBL) production.
- A prior outbreak in the same ICU (February-October 1993) involved strains producing a novel TEM-derived ESBL.
Purpose of the Study:
- To investigate potential relationships between Serratia marcescens isolates from two distinct ICU outbreaks.
- To characterize the bacterial strains and their beta-lactamase enzymes involved in both epidemics.
Main Methods:
- Biochemical analysis and gene amplification to characterize enzyme production.
- Plasmid profiling to identify genetic elements responsible for resistance.
- Restriction fragment length polymorphism (RFLP) analysis using pulsed-field gel electrophoresis (PFGE) for clonal identification.
Main Results:
- Isolates from the second outbreak possessed a 48 kb non-conjugative plasmid encoding an SHV-derived ESBL (pI 8.2).
- PFGE analysis unequivocally identified two different Serratia marcescens clones responsible for the two separate outbreaks.
- Epidemiological data indicated long-term persistence and circulation of these strains within the hospital.
Conclusions:
- The two ICU outbreaks were caused by distinct Serratia marcescens clones.
- The study highlights the potential for prolonged persistence and inter-ward circulation of ESBL-producing Serratia marcescens.
- These persistent strains may serve as reservoirs, increasing the risk of further epidemic spread within healthcare settings.