Reservoirs and transmission routes of methicillin-resistant Staphylococcus aureus and methicillin-susceptible Staphylococcus aureus in a tertiary hospital: one-year surveillance

Abstract number: p1334

Faria  N.A., Oliveira  D.C., Amorim  M.L., Calado  E., Vasconcelos  C., Castro  A.P., Amorim  J.M., de Lencastre  H.

Objectives: 

To decrease the burden of nosocomial S. aureus infections and prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in Hospital Geral de Santo Antonio, characterized by a high frequency of S. aureus infections and a MRSA prevalence of 40%. A systematic study was designed in order to: (i) identify the reservoirs of S. aureus; (ii) to characterize the transmission routes of S. aureus; (iii) to evaluate the prevalence of S. aureus colonization among outpatients; (iv) to make the monitorization of S. aureus clones.

Methods: 

To identify the S. aureus reservoirs, patients and healthcare personnel from the units with the highest risk of S. aureus infection, as well as diabetic foot outpatients, were screened for nasal colonization. To clarify the transmission routes, nasal swabs and infection samples were taken from patients with S. aureus infection. All strains were clustered in clonal types by pulsed-field gel electrophoresis. Clonal clusters were further characterized by spa typing and multilocus sequence typing (MLST). Staphylococcal chromosomal cassette (SCCmec) typing was also performed for all MRSA strains.

Results: 

In the vascular surgery and endocrinology units, active surveillance showed MRSA carriage rates of 11% and 9% among patients, and 8% and 0% among healthcare personnel, respectively. In the paediatric intensive care unit no MRSA carriers were found. Among diabetic foot outpatients, 9% were MRSA carriers. The percentage of methicillin-susceptible Staphylococcus aureus (MSSA) carriers was always higher than the MRSA carriers and was particularly high among health care personnel in the paediatrics intensive care and vascular surgery units (36% and 25%, respectively). Comparative analysis between infection and colonization isolates in the few available samples showed that the same strain was in both sites in 38% of the cases studied. Longitudinal analysis of carriage showed that most (72%) patients carried the same strain for long periods of time (<=6 months). Concerning the monitoring of MRSA clones, the previously dominant ST239-IIIA clone decrease its incidence to 16%, and clone ST22-IV has emerged with an incidence of 79%.

Conclusions: 

The vascular surgery and endocrinology units were shown to be the major reservoirs of MRSA among the hospital units studied. The infection control measures adopted by the paediatric intensive care unit were effective to curtail the MRSA transmission. Diabetic foot outpatients are important reservoirs of MRSA and may be vectors for MRSA dissemination between the community and the hospital.