Biofilms: Bacteria Under Cover

Bacteria have adapted to a "biofilmed" state to survive unfavourable environmental conditions such as those which are poorly oxygenated, have extremes in temperature or are lacking in nutrients. In the natural environment, biofilms act as a reservoir for microbial species and guarantee ecological balance. But in animal production, they constitute a source of permanent contamination that is particularly difficult to eliminate.

Biofilms in the breeding house

A livestock rearing house is a closed space with a dynamic microbial ecosystem due to the high concentrations of organic matter, high temperatures, and high humidity levels. The characteristics of the microbial ecosystem are determined by the microbiota of each animal and that of the herd. 

Animal excrement regularly enriches the microbiota of the building, especially with strains of Enterococcus, coliforms, lactobacilli, and Bacillus which are all found in the digestive tract of the animal. The presence of animals in the barn causes a rise in the temperature and humidity of the ambient air. This warm air rises to the top of the building, carrying with it many microorganisms in the form of bioaerosols. Thus, all surfaces of the building become contaminated and biofilms are formed (Figure 1). 

Figure 1. Bioaerosols and biofilm formation in livestock buildings

Formation and evolution of a biofilm

Biofilms are complex structures that can reach several centimeters in thickness, constituting a considerable bacterial reserve. They are formed on surfaces through the accumulation of stacked bacteria which secrete a polysaccharide or extracellular polymeric substance (EPS) during the maturation phase. This mucous matrix is excreted through a network of channels in which the medium can circulate.

Figure 2. Sectional diagram of biofilm formation

The thickness of the biofilm does not increase indefinitely. The curve of establishment of a biofilm reaches a plateau when a balance is reached between the rate of accumulation and the rate of detachment of contaminating cells to the environment (air then surfaces; Figure 2). The detachment of parts of the biofilm is partly due to variations in temperature and humidity inside the building. These detachments contribute to the contamination of the air in the building and cause the infectious pressure to increase.

Properties of biofilms

Bacteria in a biofilm are organised within the mucous matrix through a network of pores and circulation channels which allow communication and exchange between cells (Figure 3). 

Figure 3. The structure of a biofilm

The cells at the outer part of the biofilm, those at the interface with the air or water, have a larger amount of nutrients, oxygen and water and their metabolism is active. The bacteria in the deeper part of the biofilm, those at the interface with the support, are in micro-aerophilic or anaerobic conditions and their metabolism is inactive or dormant.

The biofilm is therefore a mosaic of micro-niches containing different species but also different phenotypes of the same bacterial species. The cohesion of this microbial community relies on synergistic interactions and homeostatic mechanisms. The architecture and organisation of a biofilm is similar to that found in a tissue, highlighting their remarkable evolutionary importance.