CONtrol of COmmunity-acquired MRSA: Rationale and Development of counteractions
Grant agreement number: 222718 FP7 HEALTH
Project context and objectives
Methicillin-resistant S. aureus emerged in the early 1960s following the introduction of semi-synthetic, beta-lactamase stable penicillins such as methicillin. For nearly 30 years, MRSA were mainly restricted to hospitals where they had a selective advantage compared to drug-susceptible wild-type strains. Outside healthcare settings, MRSA were not able to spread, probably due to a fitness cost associated with the acquisition of the staphylococcal chromosome cassette carrying mecA (SCCmec). In the 1990s MRSA were found in the community, in particular in (healthy) individuals who had no direct or indirect link with healthcare settings. The first epidemics of MRSA in patients without prior history of hospitalisations were described in the 1990s first in Australia and some years later in the United States. Since the beginning of this century community-acquired MRSA (CA-MRSA) is increasing at an alarming rate and even has reached countries, which have not had major problems with healthcare-associated (HA) MRSA infections, such as Denmark and The Netherlands. Early this century MRSA emerged on farms (livestock-associated or LA MRSA).
Meta-analysis of mortality associated with MRSA suggests an increased mortality expressed as odds ratio in the range of 2. The Strategic Council on Resistance in Europe (SCORE) in which the University Medical Center Utrecht participated calculated that blood stream infections caused by MRSA in Europe were annually responsible for an estimated 1300 additional deaths per year in the EU and €120 millions in additional healthcare costs.
The reasons for the emergence of CA-MRSA and LA-MRSA are not understood. In addition, no measures are available to curb the spread of CA- and LA-MRSA.
CONCORD is aimed at explaining the ecological success in the community and the farm environment of CA- and LA-MRSA in contrast to HA-MRSA in order to facilitate the rationale and development of effective strategies against CA- and LA-MRSA. An integrated approach was used involving epidemiology, genomics, phenotypic analysis, interventions and mathematical modeling of spread and the effect of interventions.
To control and prevent infections by CA-MRSA, there is an urgent need to improve our understanding of the ecological success of CA-MRSA in the community in contrast to HA-MRSA and to develop counter-measures to control this community spread. This understanding is essential in order to develop effective strategies against CA-MRSA, for both human and farm-related MRSA (CA-MRSA and LA-MRSA, respectively). A distinction between CA-MRSA and LA-MRSA (and also CA-methicillin-susceptible S. aureus (MSSA) and LA-MSSA) is critical, because the reservoirs do not completely overlap and the farm-associated S. aureus strains may also show specific adaptations for their respective hosts.
The CONCORD consortium therefore addressed the following issues:
- Collect and characterize recent CA- and LA-MRSA from EU member countries in order to obtain a comprehensive picture of CA-MRSA in Europe.
- Unravel the ecological success of MRSA outside the hospital by investigating genetic and transcriptional differences between HA-, CA- and LA-MRSA.
- Determine the physiological role of gene products that contribute to the ecological success of CA-MRSA.
- Develop mathematical modeling tools in order to quantify potential transmission control measures.
- Develop effective intervention strategies for CA-MRSA and LA-MRSA.
The emergence of methicillin-resistant Staphylococcus aureus (MRSA) is driven by the acquisition of the staphylococcal cassette chromosome mec (SCCmec), which carries the mecA gene. Our results showed a low frequency of SCC elements in community-associated methicillin-susceptible S. aureus (CA-MSSA) (7%), indicating that these isolates are not reservoirs of SCC elements. The data further suggested that SCC elements from CA-MSSA (methicillin-susceptible S. aureus) may be involved in SCCmec evolution either by the acquisition or loss of mecA.
The population structure of S. aureus from community and farms was compared to understand the extent of dissemination of MRSA and MSSA strains. Only a small proportion of genotypes (6%) were in common, suggesting that the extent of dissemination is not frequent. Also our results suggest that dissemination probably occurs in the two directions. Additionally, we observed that SCCmec is more diverse among humans than animals and that different animal species could be reservoirs for different SCCmec types.
Coagulase-negative staphylococci are the most probable donors of SCCmec to S. aureus. This was never proven for MRSA of animal origin, but our results suggest that S. haemolyticusis the most probable donor of SCCmec V for livestock-associated (LA) MRSA.
To enable a comparative genomic hybridization study, a high resolution S. aureus microarray was constructed. Analysis indicated four correlated groups of genomes being hospital-acquired (HA)-MSSA, HA-MRSA, CA-MRSA and LA-MRSA. The sporadic isolates could be assigned to either HA-MRSA or CA-MRSA isolates. Most toxin genes were absent from LA-MRSA, as well as a class of proteases. The presence of other virulence factors was variable. Apparently, different combinations of virulence factors can lead to a successful CA- or LA-MRSA.
Three sets of isolates from patients who were first sampled as carrier and afterwards developed an infection were sequenced. One set was not a pair, the second set showed only one phage difference. The third pair did not show any clear differences. For two LA-MRSA from one farm no clear differences were present. However, a small difference in the pleiotropic regulator MgrA was noted compared to the first sequenced ST398 isolate (S0385) which leads to increased biofilm formation. Further analysis showed that Integrative Conjugative Elements and ß-toxin which is usually intact in LA S. aureus, but usually disrupted in human derived strains contributes to biofilm formation. Disruption of the ß-toxin gene led to lower colonization in a pig nasal explant model.
A comparative study gene expression study using two genetically similar strains showed remarkable differences between the two strains as well as between the two conditions (blood and CO2). The expression of genes of LA-MRSA during interactions with pig nose tissue showed only a few genes regulated compared to medium.
We have used transcriptome data from microarray analysis to predict sRNA candidates. A total of 115 putative sRNAs were identified. For two sRNAs a regulation could be shown.
In conclusion, MRSA (and CA-MRSA and LA-MRSA) show a whole range of adaptations to their host. Some adaptation appear to be necessary for some type of infections, but preliminary results presented here suggest that an adaptation to become an infectious isolate from a carrier isolate are not always necessary. These differences are frequently on the level of gene regulation, but the discovery of a large number of sRNAs highly complicated analysis of the pathways involved.
We have developed and employed mathematical modelling tools to determine the transmission potential (as measured by the basic and single admission reproduction ratios) of CA-, HA-, and LA-MRSA. Transmissibility has been determined within the hospital environment (for all MRSA strains) and between farms (for LA-MRSA). The effect of control measures on prevalence and persistence of LA-MRSA has been assessed at the between-farm level studying the effect of decreasing farm recovery times, and within-farm (i.e. animal) prevalence. The between-hospital model is being used to study hospital-based surveillance strategies.
It is likely that MRSA originated by its acquisition of the SCCmec from coagulase-negative staphylococci (CNS). So, the presence of a potential reservoir for transfer of SCCmec to S. aureus on pig farms was investigated. Based on the distribution of SCCmec types conclude that staphylococci on pig farms act as a reservoir of heterogeneous SCCmec elements, and that these staphylococci may act as source for transfer of SCCmec to S. aureus.
The potential impact of the project can be summarized in the following points:
- Novolytics is using the data from the phage experiments to optimize the applications they are currently developing.
- The impact of the mathematical models is their reusability beyond CONCORD
activities: as long as basic assumptions are verified and there exist suitable data sets, they can be employed to perform, in other countries, similar analyses on MRSA transmission potential in hospitals, health-care networks, or different livestock industries. The models can also be combined with other mathematical tools to better test control strategies and their cost-effectiveness. An example is the possibility of merging the between-farm transmission model with a farm-to-human model focused on the spill-over of LA-MRSA from a farrow-to-finish farm to the surrounding community, developed by members of the FP7 Pilgrim consortium."
In addition, two results may have some impact in the design of future control strategies.
- Results obtained with the between-farm model show that because of the high volume and frequency of between-farm trade, there are currently no practical, low-cost, farm-based control strategies that will effectively tackle endemicity in the pig-industry.
- Results obtained with the between-hospital model show that infection control strategies must at least have a regional scope, and control measures may have to be targeted to individual hospitals to effectively tackle the spread of HCAI. In particular, resource allocation to large, complex hospitals will be needed for both early detection of novel pathogens, and their elimination from the health-care network after successful spread.
- The ex vivo pig nasal explant model will be used to study colonization and decontamination measures.
- Increased knowledge of LA-MRSA clones in pigs and veal calves in Europe and their dissemination among animals in European Countries.
- Construction of a strain collection and data collection which could be used within the project and which can be available for future studies.
- Contribute to a better understanding on the relationships between LA-MRSA as potential reservoir for CA-MRSA.
- Contribute to the knowledge about potential factors implied in the emergence and spread of MRSA in livestock and potential targets for interventions.
- Conribute to the scientific experience in the field of young scientists involved.
- Contribute to improve the capacity of researchers for advising policy makers regarding MRSA in Europe.
- Contribute to the knowledge that coagulase-negative staphylococci on farms are a potential source of SCCmec.
The potential impact of the project for CA-MRSA can be summarized in the following points:
- A tremendously high level of genetic diversity among CA-MRSA from humans in Europe was discovered impacting on diagnostics, surveillance and control.
- A close watch of exchange of MRSA between different reservoirs is required.
- Contribute to the knowledge of the S. aureus population structure by defining subpopulation that impact on vaccine development.
- Contribute novel genes that define host specificity.
- The establishment of different MRSA subpopulations with different virulence genes, e.g., toxins and immune evasion molecules will influence vaccine design.
- The discovery that TraG also contributes to biofilm formation will offer a new/additional target for the prevention of biofilm formation.
- Knowledge that gene regulation in one strain is not predictive for closely related strains will influence the evaluation of gene expression for vaccine development.
- The discovery that sRNA also can regulate virulence genes will influence our thinking how MRSA can adapt to new ecological niches.
- The finding that HA-MRSA adapted to the hospital will influence our thinking of MRSA evolution, but also that of other antibiotic-resistant pathogens.
- PVL receptor
- The data obtained in the project is currently being used by the UMCU to prepare a grant application from the Foundation of Applied Sciences (Stichting Technische Wetenschappen – STW).
So, CONCORD will impact on various aspects of our knowledge how CA- and LA-MRSA adapted to their ecological niche. The mathematical tools can be used when new data become available and Novolytics applies the knowledge obtained to improve their applications for the treatment of both humans and animals.
CONCORD-PILGRIM PRESS RELEASE
A European alliance of scientists confronts the spread of dangerous MRSA strains in the community and livestock
Hospital-acquired MRSA is so well adapted to the hospital that it has difficulty to survive outside the hospital. This is one of the conclusions of a European research project involving an alliance of European scientists.
The alliance conducted this research in collaboration with 8 European research groups from six countries – Denmark, France, Poland, Portugal, the Netherlands, and the UK - in the CONCORD project. The European Union funded the project in 2009 with 3 million euros for 3 years.
MRSA is present in hospitals since the 1960s and since the 1990s also became prevalent in the community and the last years among livestock. MRSA prevalence varies among European hospitals but can reach more than 50% in some intensive care units. The bacterium however did not escape from the hospitals but emerged, amongst others through antibiotic and zinc use, among live-stock. The eradication of MRSA among live-stock is practically impossible to achieve. Mathematical models show that the intensive contacts between farms maintains the contamination even when only low levels of the bacterium are present.
The MRSA variant present in hospitals is adapted to the hospital. It is more resistant to antibiotics, but as a consequence grows more slowly. This means that it is difficult to get rid of the bacterium in the hospital, but that it will hardly spread outside the hospital. In contrast the variant in the community is less resistant to antibiotics and grows normally. The research project also showed that MRSA found among livestock differs from both the hospital and community variant.
The bacterium is genetically flexible and has been shown to adapt rather easily to new circumstances. Therefore, MRSA remains a health care threat. A solution is not yet available and prudent use of antibiotics is currently the only option to limit spread as much as possible.
On November 28 and 29 the researchers will present their results on a symposium in Brussels. During that symposium also scientist from the PILGRIM project will present their results. They searched for ways to prevent the spread of MRSA.