‘MRSA is becoming more virulent, so alertness remains necessary’

The virulence factor Panton-Valentine leukocidin (PVL) is more common in methicillin-resistant Staphylococcus aureus (MRSA), which makes MRSA more likely to cause infections. In addition, PVL-positive live-stock associated MRSA (LA-MRSA) is increasing, even in people without a relationship with animals. These are some conclusions from an extensive MRSA study by the RIVM. Dr. Antoni Hendrickx, molecular microbiologist and coordinator of national surveillance, discusses the research.

The RIVM has been responsible for monitoring MRSA since 1989. The Typeringsnetwerk-Nederland (Type-Ned) started in 2008 as an initiative of a number of medical microbiology laboratories. There is now national coverage, with 50 affiliated laboratories. Type-Ned is an online laboratory network that monitors potentially pathogenic micro-organisms at molecular level. “The laboratories together send approximately 5,000 MRSA isolates to the RIVM every year,” says Hendrickx. “We characterize these at molecular level, after which we feed the data back to the laboratories via the website. Until now, the MLVA (multiple-locus variable number tandem repeat analysis) typing method has been used for kinship and outbreak analyses. That method is sufficient, but outdated. We are now increasingly using whole-genome sequencing (WGS) to obtain all molecular properties of a MRSA, and WGS-based whole-genome multilocus sequence typing (wgMLST) for kinship and outbreak analyses. This way we can monitor changes in MRSA in the Netherlands better and more accurately.”

A lot of data

The RIVM collection now contains several tens of thousands of MRSA isolates, some of which have been sequenced. These data were used for the study, which is one of the largest studies in this area worldwide.¹ “The size is exceptional,” says Hendrickx. “The publication, in Nature Communications Medicine, contains a huge amount of data that makes many analyzes possible. For now we have therefore made choices, including by looking at PVL. That toxin is produced by S. aureus which can kill white blood cells. This makes the bacteria more virulent. MRSA has even more virulence factors, which may make it easier for the bacteria to cause infections. This is an increasing trend that appears to be going slowly, but which we must continue to monitor closely.”

Changes

The study used 43,321 isolates from 36,520 people, collected between 2008 and 2019. The isolates were all typed with both MLVA and PCR (polymerase chain reaction) for detection of PVL. WGS was applied to 4,991 isolates from 4,798 people for the identification of resistance and virulence genes. Changes in the MRSA population can be seen over the years. The proportion of PVL-positive isolates increased from 15% in 2008-2010 to 25% in 2017-2019. In human LA-MRSA isolates, PVL positivity increased to 6% in 2017-2019, with isolates mainly from regions with few pig farms. Using wgMLST, 35 different genogroups were identified with specific gene profiles for resistance and virulence.

The conclusion from the study is that molecular genetic changes occur in the MRSA population at a national level, with a growing proportion of PVL-positive MRSA, including LA-MRSA.

Data not yet published appear to indicate that MRSA is more common and causes infections more often. “This may be related to the increase in PVL-positive MRSA,” says Hendrickx. “We see another development with PVL-positive LA-MRSA. Previously it was thought that they were not that dangerous. Around 2008-2010 they amounted to less than half a percent of all isolates, but in 2017-2019 they were already 6%. And the most recent data is already at 10%. This MRSA genotype now also occurs in people in the Randstad without association with pigs or other animals. So we have to keep a close eye on that. Also because we have the resistance gene cf emerge, which codes for the so-called PhLOPSA phenotype. This has resistance to 5 classes of antibiotics, including last-resort antibiotics. We don’t know where the gene comes from and this MRSA will be virtually untreatable. Fortunately, it is not a major problem yet, but it is already present.”

Ongoing attention

Of the annual 5,000 MRSA isolates, RIVM characterizes approximately 500 per year with WGS. These data are online and anyone can use them for further research, for example into possible new targets for antibiotic development. Hendrickx: “Another advantage of sequencing is that you know exactly which resistance genes a MRSA contains. This may be important in the future when determining the appropriate antibiotic treatment.”

Hendrickx emphasizes that alertness about MRSA remains necessary. “The subject is no longer in the news that often, but that is actually unjustified. MRSA is still a serious pathogen that can cause an infection in anyone without any reason. In the Netherlands we see healthy people with abscesses due to MRSA. The feeling among medical microbiologists is that more MRSA infections occur in hospitals. MRSA deserves continued attention.”

Come and go

The study also shows that certain genogroups of MRSA emerge and disappear. Hendrickx does not yet know the exact meaning of this. “We see connections with these trends with Denmark, among others, where a lot of sequencing also takes place. We do not yet know whether there are connections with the rest of Europe. But because people travel a lot and there is now a lot of migration from war zones, I can imagine that new MRSA genotypes will come along. We can see them in our surveillance and they can also disappear again. We are now using a newer method that allows us to sequence even faster and cheaper. We share WGS data not only with laboratories in the Netherlands, but also with the European Center for Disease Prevention and Control (ECDC) for monitoring trends in antimicrobial resistance.”

‘MRSA still under control in the Netherlands’

Internist-infectiologist/acute medicine Dr. Heidi Ammerlaan (Catharina Hospital, Eindhoven) occasionally deals with MRSA in the hospital. “This happens both in the clinic for people who come to the ER with sepsis, and in the outpatient clinic for a skin infection that does not respond to standard antibiotics. But we still find MRSA most often during screening, for example in the case of a ring test surrounding a MRSA-positive patient, after a stay in a foreign hospital, or in the case of living or working in a livestock farm. This usually does not indicate an active infection, but only carrier status. The virulence factor PVL often plays a role in skin infections. It is common practice to also test housemates of the patient prior to eradication treatment. This regularly shows that there are multiple carriers who sometimes also have skin infections, but I don’t know whether that happens more often than before.”

Stay alert

Ammerlaan also believes it is necessary to remain alert to MRSA and, in the event of a positive finding, to follow the protocols to prevent its spread (the so-called ‘search & destroy’ policy). The Netherlands is still doing well in that area compared to many other countries. “We use antibiotics sparingly and try to treat infections in a very targeted manner. And in the case of MRSA, we try to eradicate it to prevent spread as much as possible. The PVL factor makes the bacteria more virulent, but not more resistant. It is therefore not necessarily more difficult to treat MRSA, there are still antibiotics that we can use against it. It is worrying that the RIVM data shows that the originally less virulent LA-MRSA strains become more virulent due to an increasing number of PVL-positive strains. An increase in the resistance gene cf within this PVL-positive MRSA would significantly reduce the treatment options for infections.”

According to Ammerlaan, healthcare providers continue to pay attention to MRSA in hospitals: “There are protocols to check whether MRSA carrier status exists in risk situations. Hygiene measures are taken if suspected. I recently participated in the Infection Prevention Guidelines Partnership to update the existing MRSA infection prevention guideline, which came online in November. Fortunately, the situation regarding MRSA is still under control in the Netherlands.”

Reference

1. Schouls LM, Witteveen S, van Santen-Verheuvel M, et al. Molecular characterization of MRSA collected during national surveillance between 2008 and 2019 in the Netherlands. Nature Comm Med, 2023;3: article no. 123.