Hidden in Plain Sight: How Dry Surface Biofilms Drive Environmental Persistence

Dry surface biofilms (DSBs) have been an area of interest for me for a number of years and I have been involved in several studies which examine the role of DSB in environmental persistence and survival. Ahead of the next IPC Partners Journal Club (which you can register for here) I have taken a look at the paper Dr Ruby Harsent will run through which provides some really interesting findings around the ability of Enterococcus spp., including vancomycin-resistant enterococci (VRE), to survive via DSBs in the healthcare environment.

This study examined whether different Enterococcus strains could form DSBs on materials found in healthcare and importantly, how long they could remain viable and culturable.

What do we mean by dry surface biofilms, and why do they matter?

DSBs are biofilms that survive under the dry conditions encountered on environmental surfaces. Unlike the hydrated biofilms traditionally associated with sinks, drains or medical devices, environmental DSBs can be thin and unevenly distributed, with microorganisms embedded within EPS.

DSBs containing multidrug-resistant organisms have previously been recovered from healthcare environments, including surfaces that have undergone routine cleaning. This makes understanding their role as a potential reservoir for healthcare pathogens increasingly important.

What the study examined

The researchers investigated multiple strains of Enterococcus faecium, Enterococcus faecalis and Enterococcus hirae, including clinical, epidemic and vancomycin-resistant isolates.

DSBs were established using repeated wet and dry cycles and examined across several healthcare-relevant materials, including stainless steel, PVC, ceramic, faux leather and a range of fabrics. The researchers also examined whether surface roughness and hydrophobicity influenced DSB formation.

Perhaps most interestingly, DSBs on stainless steel were stored at 20°C and 55% relative humidity for up to 84 weeks to determine how long enterococci remained culturable.

Key findings

All of the Enterococcus strains investigated could form DSBs on stainless steel, with approximately 7 log10 CFU/mL recovered from E. faecium DSBs.

Importantly, DSB formation was not restricted to stainless steel. Enterococcus spp. formed DSBs on all of the healthcare-relevant materials tested, including PVC, faux leather and different fabrics, with around 5–6 log10 CFU/mm² generally recovered.

There was no significant relationship between bacterial recovery and either surface roughness or hydrophobicity, suggesting that surface characteristics alone may not be a useful predictor of DSB formation.

The standout finding for me is the persistence data. Enterococci remained culturable at approximately 6 log10 CFU/mL for up to 84 weeks. For two epidemic E. faecium strains and the standard E. faecium strain, there was no significant reduction in culturability between week 0 and week 84.

Limitations

As with all laboratory biofilm models, the findings need to be interpreted carefully when applying them to clinical settings. The controlled wet and dry cycles cannot replicate all the variables encountered on healthcare surfaces.

The study also demonstrated persistence rather than transmission. It tells us that viable enterococci can remain within DSBs for prolonged periods, but does not establish the risk of transmission or infection associated with those reservoirs.

What this means for IPC

• DSBs may represent a long-term environmental reservoir. Enterococci formed DSBs across a range of healthcare-relevant surfaces and persisted at high concentrations for up to one year and eight months, creating the potential for prolonged environmental contamination.

• Detecting DSBs remains a challenge. Routine environmental sampling may provide false reassurance. Moist cotton swabbing is ineffective for detecting DSBs, while foam-tipped swabs have been shown to recover only around 30% of DSBs.

• Mechanical cleaning remains critical. With limited evidence for the effectiveness of antimicrobial surfaces or automated technologies such as UV against established DSBs, mechanical removal combined with an effective disinfectant remains the best-supported approach for hard surfaces.

• Enhanced cleaning and disinfection may therefore be particularly important where persistent environmental reservoirs are suspected.

• Textiles present a particular challenge. Where textiles are contaminated with VRE DSBs, the authors suggest that disposal may currently be the most appropriate approach until decontamination methods such as steam have been shown to be effective against DSBs.

Conclusion

This study provides important evidence that Enterococcus spp., including VRE, can form DSBs across a range of healthcare-relevant materials and remain culturable for remarkably long periods.

For IPC, the practical message is that effective environmental decontamination needs to combine physical cleaning, mechanical removal and appropriate disinfection. Perhaps the bigger question is whether our current approaches to cleaning, environmental monitoring and outbreak investigation adequately account for microorganisms existing within persistent biofilm communities rather than simply as transient surface contamination.