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Abstract
Listeria monocytogenes is able to form biofilms on various surfaces and this ability is thought to contribute to persistence in the environment and on contact surfaces in the food industry. Extracellular DNA (eDNA) is a component of the biofilm matrix of many bacterial species and was shown to play a role in biofilm establishment of L. monocytogenes. In the present study, the effect of DNaseI treatment on biofilm formation of L. monocytogenes EGD-e was investigated under static and dynamic conditions in normal or diluted complex medium at different temperatures. Biofilm formation was quantified by crystal violet staining or visualized by confocal laser scanning microscopy. Biomass of surface-attached L. monocytogenes varies depending on temperature and dilution of media. Interestingly, L. monocytogenes EGD-e forms DNase-sensitive biofilms in diluted medium whereas in full strength medium DNaseI treatment had no effect. In line with these observations, eDNA is present in the matrix of biofilms grown in diluted but not full strength medium and supernatants of biofilms grown in diluted medium contain chromosomal DNA. The DNase-sensitive phenotype could be clearly linked to reduced ionic strength in the environment since dilution of medium in PBS or saline abolished DNase sensitivity. Several other but not all species of the genus Listeria display DNase-sensitive and -resistant modes of biofilm formation. These results indicate that L. monocytogenes biofilms are DNase-sensitive especially at low ionic strength, which might favor bacterial lysis and release of chromosomal DNA. Since low nutrient concentrations with increased osmotic pressure are conditions frequently found in food processing environments, DNaseI treatment represents an option to prevent or remove Listeria biofilms in industrial settings.
Highlights
Listeria monocytogenes (Lm) is a ubiquitous saprophytic soil bacterium and an opportunistic foodborn human pathogen with a well characterized intracellular life-cycle (Vázquez-Boland et al, 2001; Hamon et al, 2006; Freitag et al, 2009)
Initial attachment of Lm to glass and plastic surfaces was shown previously to be dependent on Extracellular DNA (eDNA) and later stages of biofilm formation are sensitive to DNaseI treatment (Harmsen et al, 2010)
To characterize the role of eDNA in biofilm formation of Lm in more detail, biofilm assays were performed in polystyrene microtiter plates under static conditions at different temperatures in full strength or 0.1BHI (Figure 1A)
Summary
Listeria monocytogenes (Lm) is a ubiquitous saprophytic soil bacterium and an opportunistic foodborn human pathogen with a well characterized intracellular life-cycle (Vázquez-Boland et al, 2001; Hamon et al, 2006; Freitag et al, 2009). Severity of Lm infections and the symptoms of the associated disease (i.e., listeriosis) are dependent on the immune status of the patient eDNA in Listeria monocytogenes Biofilms (Hamon et al, 2006; Freitag et al, 2009). All outbreaks reported in recent years have been associated with consumption of contaminated food. A recent outbreak in Denmark caused by a traditional meat product has claimed 13 deaths amongst 28 cases (Ethelberg, 2014) and a nation-wide outbreak in the USA in 2011 with 147 patients and 33 deaths could be traced back to contaminated cantaloupe (McCollum et al, 2013). Since several smaller food-related outbreaks have been recorded in the USA (http://www.cdc.gov/ listeria/outbreaks/)
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