Sliding on

We studied slime for 19 years!

Professor John D Brooks FNZIFST

John Brooks' view of the food world through the lens of a microbiologist.

I was tempted to write this month about food safety after flooding events but realised that most people reading this column would already know about the hazards of contamination and spoilage. This column is on the subject of biofilms in the food industry. It might be said that the subject has been done to death, but it is clear that biofilms are still causing problems and researchers are looking for novel ways to control them.

The term ‘biofilm’ was coined by Bill Costerton in 1978 and is defined as an aggregation of microbial cells and their extracellular polymeric substances actively attaching to and growing on a solid-liquid interface. Not only do the cells multiply within the film, but they also can be sloughed off the surface into any liquid flowing over the film. The film may appear to be slime on the surface of equipment, or food such as bacon.

Biofilm of Streptococcus on 316 stainless steel

For a long time, it was thought that thermophilic spore-forming bacilli, such as Bacillus stearothermophilus (now called Geobacillus stearothermophilus) were growing in the milk as it flowed through the equipment. Some microbiologists had doubts about this as the microbiological quality of the milk entering the plant was high and the residence time in the plant was too short. Nevertheless, around the world, it was recognised that milk powder could contain high levels of thermophilic spores. I remember sitting in a meeting listening to the arguments and doing a calculation on the back of an envelope, taking into account the growth rate of thermophiles and the surface area inside the separators and evaporators and coming to the conclusion that growth in the liquid milk could not explain the levels in the finished powder. The only other explanation was that the bacteria were growing in biofilms on the surface of the equipment. My PhD student and I received funding from the NZ Dairy Board to investigate biofilm formation within dairy plant and we were able to collaborate with other researchers and post-doctoral fellows in New Zealand, most notably Dr Phil Bremer who already had experience in biofilm research.

One of the characteristics of bacterial biofilms is that the cells are somewhat protected from heat and cleaning solutions, so, unless the equipment is cleaned very thoroughly, the film is able to reinoculate the plant during the next run.

It became very important to understand the properties of the stainless steel in contact with the milk and the properties of the bacterial cell surface that influenced the initial adhesion of the cells so that steps might be taken to limit the development of biofilms. These procedures included “intelligent plant operation” such as modifying the separation temperature, chemical surface modification of the stainless steel, reduction of surface roughness to limit attachment, and cleaning protocols to ensure removal of biofilms. Disrupting the cell growth pattern has also been investigated by cycling the temperature in plate heat exchangers and evaporators. This is a fairly drastic intervention but has been shown to reduce biofilm growth. Planktonic cells also respond to quorum-sensing molecules released by biofilms, so this too became an area for research. It should be noted that similar studies were initiated in New Zealand research teams, as well as overseas. It is clear that bacterial cell attachment to surfaces is a very complicated process, affected by other molecules attracted to the surface, the surface charge of the substrate, cell surface charge and surface proteins, hydrophobicity and pH of the suspending medium (1).

Of course, biofilms affect other food processes besides the manufacture of milk powder. One complication of studying biofilm growth in the laboratory is that the films usually consist of a single species, whereas in food and water processing plants, the films are often multi-species and the nature of the initial colonisers may influence the attachment of further species. Thus, Campylobacter jejuni cells in biofilms are more hydrophobic than those grown in planktonic mode, while Enterococcus faecium readily forms biofilms on stainless steel and may attract C. jejuni to attach to the film and the lethal effect of heat on C. jejuni is reduced. Biofilms can form on leafy vegetables. We now know that the biofilm matrix can significantly increase the settlement of marine larvae, Perna canaliculus, which is of importance in commercial shellfish farming.

Examples of current biofilm research topics:

• Influence of culture conditions on biofilm formation by Escherichia coli O157:H7

• Development and control of bacterial biofilms on dairy processing membranes.

(1) Palmer, J., Flint, S., & Brooks, JD. (2007) Bacterial cell attachment, the beginning of a biofilm. DOI 10.1007/s10295-007-0234-4