Genotyping of S. marcescens and K. pneumoniae nosocomial isolates from newborns

Objectives: Serratia marcescens and Klebsiella pneumoniae are wellknowncausative agents of nosocomial infections in newborns.The aim of the present work is to investigate, by the use ofmolecular typing methods, two putative epidemic clusters of newborncross-infections due to S. marcescens and K. pneumoniae,respectively, in order to establish clonal relationships among theisolates and to identify the possible source and mode of transmission.Methods : Over a 6-month period, 11 newborns, from an intensivecare unit, where either infected or colonised by S. marcescens, 10by K. pneumoniae, three by both of them. Four premature neonatesamong these patients yielded invasive infections. Repetitivepositive blood cultures were associated in two cases withK. pneumoniae, in one case with S. marcescens, in another fatal casewith both microorganisms. The nurses and the environment werechecked by microbiological studies. The antibiograms and theresults from two molecular typing methods (automated Riboprintingand ERIC-PCR) of all bacteraemic or colonising isolates fromnewborns and from patients elsewhere in the hospital during thesame time period, were compared in order to investigate theepidemiology of the clusters.Results: Totally 20 S. marcescens isolates, including 12 from newbornsand eight from other patients of the same hospital, and 10K. pneumoniae strains from newborns were examined. An initialanalysis performed after the identification of the first nine newbornscontaminated by S. marcescens established clonal relationshipsamong all the isolates, including the two bacteraemic strains. Asecond analysis revealed one different pattern in three S. marcescensstrains successively isolated. Six genotypes unrelated tonewborns isolates were found in the eight strains from the otherexamined patients. The three K. pneumoniae bacteraemic strainsresulted genetically related among them but unrelated to the colonisingisolates. S. marcescens isolates among different molecularpatterns showed identical antibiograms. A focal source for bothmicroorganisms was not identified. Conclusion: Typing results revealed that two different microorganisms(S. marcescens and K. pneumoniae) were involved at the sametime in invasive nosocomial infections in premature newborns anddocumented two contemporaneous clonal clusters of cases. Heterogeneousgenotypes among both species were also demonstrated tobe simultaneously present in the neonatal intensive care unit.