Infection control of Staphylococcus aureus : spa typing to elucidate transmission

Sammanfattning: Staphylococcus aureus is a commensal of the human flora, primarily colonizing the anterior nares and throat, but it may also cause infections ranging from mild skin and soft tissue infections to severe diseases such as endocarditis and septicemia. S. aureus is also a major nosocomial problem increasing with the worldwide dissemination of methicillin-resistant S. aureus (MRSA). The main vector for bacterial cross-transmission in healthcare settings is the hands of healthcare workers (HCWs). No S. aureus was detected in the air in this thesis demonstrating that transmission through air is not important. Despite the fact that good compliance with hand hygiene is essential to prevent cross-transmission the compliance is generally less than 50 %. Gold standard to track bacterial transmission in healthcare settings has for long been pulsed-field gel electrophoresis (PFGE), a method that is labor- intensive, lacks consensus protocol and relies on semi-subjective analysis. Molecular typing by sequencing of the hypervariable part of the S. aureus protein A gene (spa typing) has overcome these problems and has shown promising results in epidemiological investigations.The aims of this thesis were to study bacterial transmission with S. aureus colonization of newborn infants as a model and to evaluate spa typing as a molecular tool. Additionally, the influence of compliance with hygiene guidelines on S. aureus transmission was assessed.Analysis of 280 MRSA isolates by spa typing revealed excellent typeability and epidemiological concordance and satisfactory discriminatory power. Additionally, spa typing was considered superior to PFGE thanks to its accessibility, ease of use and rapidity. Also, spa typing results are registered in a global database, facilitating inter-laboratory comparison.The prevalence of S. aureus ranged from 41 % to 66 % in the populations studied and males had the highest colonization rate. Throat was the premier colonization site for adults and transmission from individuals colonized in the throat only was documented, suggesting that throat cultures should be included in S. aureus screening programs. The umbilicus was the premier colonization site for newborn infants. Incubating the swabs in enrichment broth prior to plating increased the prevalence of S. aureus positive samples by 46 %, resulting in prevalence ranging from 51 % to 70 % in the populations studied. Thus enrichment prior to plating is necessary to determine more truthful S. aureus colonization rates. There were no indications of an institutional flora, as the colonization rates, spa type distribution and antibiotic resistance prevalence were similar among parents and HCWs.Direct observations and self-reporting by HCWs were both validated as tools for monitoring compliance with hygiene guidelines. The compliance with hygiene guidelines was significantly higher following a 10-point hygiene intervention as compared to baseline. The compliance was also higher three years after the intervention in three of four participating departments. These data show that it is possible to markedly improve the compliance with hygiene guidelines, but to achieve a long-term effect, continuous and varied reminders seems necessary.Both at baseline and following the intervention almost 60 % of the colonized infants were colonized with an S. aureus of the same spa type as isolated from their own family. At baseline approximately 25 % of the colonized infants received their S. aureus from non-family individuals, indicating transmission directly or indirectly from HCWs. Despite the improvement in compliance with barrier precautions from 41 % at baseline to 86 % following the hygiene intervention, the transmission from non-family did not decrease. This indicates that other factors may have a prominent impact on bacterial transmission. One factor might be the quality of hand hygiene technique which therefore needs to be studied further. However, to ensure patient safety it is still recommended that all HCWs comply with hygiene guidelines at all time.