Aminoglycoside resistance patterns of Serratia marcescens strains of clinical origin.

Aminoglycoside resistance patterns of 147 Serratia marcescens strains of clinical origin were studied. All strains analysed belonged to three different bacterial populations. The periods of study and the institutions the strains were isolated from correlated significantly with the resistance patterns shown by the strains. The most frequent resistance patterns found were the following: ACC (6')-I at the Hospital Infantil de México (Children's Hospital of México), and ANT (2'') + AAC(6')-I at the Instituto Nacional de Pediatría (INPed or National Institute of Pediatrics) in Mexico City. Furthermore, the isolation frequency of aminoglycoside-sensitive strains decreased remarkably at the INPed over a 12-year period. These results suggest that there has been a selection of Serratia marcescens strains that are very resistant to aminoglycosides.

Biotyping of Serratia marcescens strains of clinical origin.

One hundred and ninety five Serratia marcescens strains of clinical origin isolated at the Children's Hospital of Mexico (Hospital Infantil de México) in 1978 and at the National Institute of Pediatrics (Instituto Nacional de Pediatría) in Mexico City in 1977 and from 1988 to 1989, were studied and compared. All strains were identified using the biotyping system described by Grimont and Grimont, without modification. The most numerous biogroup found was A5/8, and the frequencies of isolation of each biotype varied depending on the institution where it was isolated and the period of study.

Partial characterization of Serratia marcescens nosocomial strains.

A nosocomial infection outbreak occurred in the Intensive Care Unit (ICU) of the Instituto Nacional de Pediatría (INP) in México City, during the months of March, April and May in 1988 Serratia marcescens was isolated as the etiological agent for this epidemic. Up to date, the source of contamination, the spreading and the pathogenic mechanisms which were involved in this outbreak remain unknown. In order to study the dynamics of the bacterial population involved in this outbreak, all strains of nosocomial S. marcescens isolated during 1988 were collected and studied. Eighty nosocomial strains were analysed. For this purpose we used four different markers: antibiotic susceptibility, presence of plasmids, exoenzyme production and pigment synthesis from a precursor. Using these markers, we were able to establish that five subpopulations of bacteria were present during the ICU outbreak, and that one of these subpopulations, VIII-A, was the most frequently isolated. A short time after this outbreak, we obtained S. marcescens isolates with similar properties which proceeded from other hospital units, suggesting intrahospital dissemination of the strain in question. We believe that, eventually, this study will allow us to establish bacterial spreading models within our institution.