Dominique Blanc Lab

Dominique Blanc, PhD, MER, PD (personnal web page)

Research topics

Molecular epidemiology of nosocomial infection

Nosocomial infections are an increasing problem in every countries. To prevent their occurrence, we need to understand their epidemiology, especially their source and mode of transmission. For that particular purpose, the use of molecular typing methods is required.

Our laboratory is involved in the development of new typing methods, their evaluation for epidemiological investigation and their application to specific topics.

• The last method that we developed was the Double Locus Sequence Typing (DLST) of Staphylococcus aureus. The method is based on the sequencing of 500 bps of two hyghly polymorphic genes, clfB and spa. We showed that DLST is as discriminant as Pulsed Field Gel Electrophoresis (PFGE) method, which so far has been considered as the gold standard. The other advantage of DLST are the high reproducibility, even between laboratories, and the unambiguous definition of type, allowing the creation of databases shared through Internet.
• One example of the application of molecular typing to nosocomial infections is the study of Pseudomonas aeruginosa. We showed that transmission of this opportunist pathogen was high in patients hospitalised in Intensive Care Unit (ICU), and that taps were involved as the source of contamination. Following the introduction of control measures, molecular typing allowed us also to conclude that the observed decrease of P.aeruginosa contamination in ICU patients was due to a decrease of transmission.

Identification of evolutionary processes responsible for the emergence of epidemic methicillin-resistant Staphylococcus aureus (EMRSA)

Horizontal DNA transfers, as a kind of parasexual mechanism, allow bacteria to rapidly adapt to new ecological niches. These transfers include lateral gene transfers as well as replacements of homologous genetic regions resulting in new allelic combinations. These new combinations of alleles play a major role in the diversification and adaptation of bacteria populations.

S. aureus population is clustered in clonal complexes (i.e. groups of closely related isolates) and its structure is thought to be highly clonal at the species level. However a restriction modification system that blocks horizontal transfer between S. aureus  isolates of different CCs was highlighted. In addition, recent results on adhesion gene markers obtained by our group suggest that genetic transfers occured preferentially between strains of the same CC.

To test this hypotheses and to understand how genetic variation is created within and among S. aureus CC, we are currently analyzing two datasets. S. aureus worldwide MLST database and our more detailed collection of four particular CC using six highly resolutive adhesion genes markers. Different methods for detecting recombination are handled and the results obtained between and within CC are currently compared.

More frequent genetic transfers within that between CC are expected. The genetic transfer at the CC level could be thus the mechanism of the emergence of S. aureus epidemics, virulent or resistant strains.

Patterns of acquisition of resistance to methicillin in predominant clones

Resistance to methicillin is conferred by the mecA gene situated on a mobile genetic element called the Staphylococcal Chromosome Cassette mec (SCCmec), which has likely been acquired by methicillin sensitive S. aureus from another bacterial species. To date, six SCCmec types (I–VI) have been distinguished but the overall genetic diversity of the SCCmec has not been addressed carefully.

The molecular epidemiology of more than 3000 MRSA isolates from Southern Europe, USA and South America showed that nearly 70% of them belonged to 5 major pandemic clones. Population genetic studies revealed that some isolates with identical sequence type had different SCCmec types, suggesting that the cassette must have been acquired on at least 20 occasions during S. aureus evolution. However, the overall pattern of acquistion of the SCCmec is not known. The sampling of identical clones in different countries proposes a single SCCmec acquisition, followed by international spread. Nevertheless, the contemporary observation of MSSA strains genetically identical to the predominant MRSA strains at regional level also suggests the possibility of multiple acquisition of the SCCmec at the local level.

To distinguish bewteen these two patterns, we are currently comparing strains belonging to the same clone, which were isolated from different countries and at different period of time, using recently developed highly discriminatory typing methods of both MRSA and SCCmec. Additionally, we also compare different isolates of the same clone collected in Western Switzerland.

Selected publications