21 July 2014
Infections acquired during contact with healthcare facilities or services, particularly those that have become resistant to the antibiotics most commonly used to treat them, are a significant threat to our health. Concerted, government-mandated action by the NHS and public health authorities to improve infection control measures in our hospitals and other health care settings has however dramatically reduced the incidence of infections such as methicillin resistant Staphylococcus aureus (MRSA) and Clostridium difficile (C. diff). In just 15 years, cases of MRSA bloodstream infections have been reduced by 90%, testament to the effectiveness of the steps taken to reduce the incidence of this devastating and potentially fatal infection.
Despite the significant reductions in incidence of HCAIs, there were still nearly 900 cases of MRSA bacteraemia in England in 2013, and the rate of decline in incidence of MRSA has slowed significantly in the last few years. This raises the question of whether or not it will ever be possible to prevent all cases of HCAIs, whether so-called ‘zero tolerance’ of these infections is realistic. A new study from researchers at the University of Cambridge and the Wellcome Trust Sanger centre has attempted to tackle this question using the power of bacterial whole genome sequencing to help understand why cases of MRSA infection occur, and whether they can all be prevented.
Torok and colleagues investigated a suspected outbreak of MRSA amongst a group of five patients with advanced liver disease to see whether there was indeed transmission between patients that could have been avoided with better infection control, and to understand whether it is ever realistic to expect to avoid all cases of MRSA where patients are subject to multiple medical interventions, in multiple healthcare settings.
Whilst epidemiological and standard molecular testing of the MRSA bacteria causing the infections suggested that they could well have been transmitted from patient to patient on the hepatology ward, the superior resolution of bacterial whole genome sequencing revealed that each patient had in fact become infected by MRSA bacteria they were already carrying (without symptoms) on their own skin and each patient’s strain was too dissimilar from that of the other patients to support the idea that there was transmission between them. Furthermore, by sequencing MRSA isolated from a large cohort of patients who had also spent time on the hepatology ward, the researchers were additionally able to show that this ward was not itself a recurrent source of new MRSA infections, and that they were instead being imported by patients from other parts of the hospital network.
The question remained, however, could any of the original five patients’ infections have been avoided? The authors of this study suspect not, and argue instead that while robust infection control measures should remain in place, and be strengthened where necessary, there will always remain a significant population of patients whose underlying medical conditions will render them susceptible to HCAIs and thus aiming for ‘zero tolerance’ of these infections is futile.
This study is an excellent demonstration of the positive role that whole genome sequencing of pathogens can play in improving public health and individual patient care. Whilst MRSA and C. diff are declining in incidence in our hospitals, other HCAIs such as carbapenem resistant Enterobacteriacae are becoming more common, and access to genomics will allow affected hospitals to identify (or rule out) outbreaks and identify where they may need to improve infection control.
As demonstrated in the current study, HCAIs may be acquired in a wide range of different health care settings, not just in tertiary referral centres, and so access to genomics-based surveillance and outbreak investigation is likely to be required far beyond the small network of university-associated teaching hospitals currently pioneering these techniques. The increased use of whole genome sequencing across the NHS has the potential to enable the sources and patterns of transmission of HCAIs and other infections to be elucidated, with genomic data becoming a vital tool not only for informing local infection control measures within health care settings but also for ensuring national targets and sanctions used to drive adherence to good infection control are set rationally and reali stically.