Biofilms contribute to survival of Salmonella

Salmonella bacteria survive better in biofilmsWhat is known about Salmonella biofilms and why are they a problem? The thesis Salmonella Biofilms of PhD graduate Greetje Castelijn (Wageningen University, project of TI Food and Nutrition) tells us all about it.

What are biofilms?

Biofilms are accumulations of bacterial cells attached to a surface. The cells can be embedded in a matrix of compounds produced by the bacteria, the extracellular matrix. Bacteria form such films on biological material, such as surfaces of plants, fruits and teeth, but also on man-made material, such as plastics and stainless steel.

Why are biofilms a problem?

Biofilms can be a threat to human health. Bacteria within biofilms are more protected to antimicrobial agents, such as disinfectants and antibiotics. This makes them harder to eradicate. Scientists estimate that up to 60% of all human bacterial infections are caused by biofilms.

In the food industry foodborne pathogens, such as Salmonella, can form biofilms on processing equipment. This way the bacteria may become a persistent source of contamination, resulting in food spoilage or foodborne illnesses.

Why are particularly Salmonella biofilms a problem?

The foodborne pathogen Salmonella causes salmonellosis, one of the most common foodborne diseases worldwide. The symptoms are diarrhoea, abdominal cramps, fever and nausea. It is a major public health burden, with millions of salmonellosis cases reported worldwide each year.

Salmonella is carried by animals such as pigs and chicken and via these carriers constantly introduced into food processing environments. A surveillance study of EFSA showed that at the point of slaughter, 10.3% of the pigs are Salmonella positive.  During the slaughter process cross-contamination due to contaminated equipment is the main route of Salmonella transmission.

Hygienic measures have been taken to ensure food safety and eradicate Salmonella from meat processing environments. However, several studies indicate that the bacteria are still able to survive inside slaughterhouses and have become part of the house flora because biofilms are so difficult to get rid of.

What can be done about Salmonella biofilms?

First of all more knowledge is necessary about the surface behaviour of Salmonella and the mechanisms of biofilm formation. Understanding this behaviour and the mechanisms will help to prevent cross-contamination and develop better strategies for Salmonella control, such as more effective cleaning and disinfection measures.

What was the PhD project Salmonella Biofilms about?

In her PhD project at Wageningen University dr. Greetje Castelijn analysed cell surface structures involved in Salmonella biofilm formation in different environmental conditions. She used S. Typhimurium as a model organism. She also looked into biofilm formation of different Salmonellaserovars relevant to the pork industry. Therefore biofilm formation on stainless steel and its contribution to the survival of Salmonella under conditions mimicking industrial settings was investigated. In the food industry processing equipment is disinfected on a regular basis to ensure food quality and safety. The effect of repeated disinfection on Salmonella cells was also examined.

What are the main findings and implications of this research?

The results indicate that biofilms contribute to the survival of Salmonella in food processing environments. Biofilm forming behaviour of Salmonella turns out to be largely affected by environmental conditions. Nutrient-rich conditions at both ambient and body temperature result in biofilms consisting of a monolayer of cells with little to no extracellular matrix, while nutrient-low conditions at ambient temperature result in biofilms consisting of cell clusters of which a subpopulation is completely encapsulated by the extracellular matrix. This extracellular matrix is largely responsible for the increased resistance of biofilms. Therefore different and/or more severe (combinations of) disinfection treatments might be needed to completely remove Salmonellabiofilms formed in different environmental conditions.

Another important finding was that the biofilm forming capacity differs between strains from different origin and even between and within Salmonella serovars.

An often used disinfectant in industry is benzalkonium chloride (BKC). Castelijn demonstrated that repeated exposure of S. Typhimurium to sub-lethal BKC concentrations rapidly selects for resistant variants that might become a part of the house flora in food processing environments.

The results implicate that cleaning and disinfection strategies for the eradication of Salmonella cells and biofilms from industrial equipment always should be validated and verified for the relevant conditions and also for a variety of relevant Salmonella serovars.

Furthermore the disinfection strategy should be validated and verified frequently, as the repeated use of the same disinfectant for a long period of time can result in the selection of variants with increased resistance.