Hello. My name is Mette Burmølle and I am an associate Professor in Microbiology at University of Copenhagen. I will introduce you to this theme of the course, where you will hear about biofilms in, on and outside the human body As you know by now, chronic infections are composed of bacterial biofilms that can persist in or on our body for many years. But, as you may also know, biofilms are not constrained to chronic infections; they are present several places in the healthy human body and basically everywhere in our surroundings. The theme of this module is biofilms in, on and outside the human body. You will learn about their role and function in different environments and how they distinguish from the ones in chronic infections. The human body is composed of more bacterial cells than human cells living either in or on the body – these are described as the human microbiome. In fact the number of bacterial cells is 10 times higher than the number of human cells, and bacterial genes account for 99% of the total number of genes in the human body. These figures indicate that we carry around lots of bacteria with various functions – and that is in fact true. The majority of the microbiome is organized in biofilms and these are generally called commensal biofilms. The largest, most complex and most diverse part of the human microbiome is found in the digestive system, starting from the mouth over the stomach to the intestine. In each of these compartments, the bacteria have vital functions, protecting us from pathogens and facilitating nutrient generation and uptake. However imbalances and presence of specific organisms can also lead to unfavorable conditions: for example: on the teeth, bacterial activity may lead to caries, in the stomach specific bacteria can elicit stomach ulcers and in the intestine, many enteropathogenic bacteria can cause diarrhea and more severe diseases. A very different, but also complex bacterial community is found on our skin, mainly composed of bacteria tolerating variations in both water availability and salt concentration. You will learn much more about both the beneficial roles of the human microbiota and potential problems affecting health in this module. Also outside the human body, most bacteria live associated to surfaces in biofilms. Hereby they maintain themselves in a favorable niche and, as in the human body, they are more protected towards various sorts of stress compared to single, free living cells. These biofilms can cause problems in industrial settings – on ship wessels, in water pipes and in production facilities, but they also facilitate many vital processes including degradation of organic matter and facilitating plant growth. In our society, we are exploiting this ability in for example cleaning of waste water and removal of toxic compounds from soil or drinking water. The bacterial diversity in the commensal biofilms in our body and those found in natural settings are often much higher than that in chronic infections. Because of the up to billions of years of coexistence in commensal and natural biofilm, the bacteria have adapted to each other, and niches with different physical and chemical conditions are present. A complex succession takes place during the formation of these biofilms, which includes specific or random bacterial settlement of early colonizers followed by secondary attachment and niche differentiation resulting in very diverse and heterogeneous biofilms at the structural, resource, functional and taxonomical levels. In addition, the bacteria and the surrounding environment – including the host – depend on the presence and activities of each other, so in general the outside threads are less hostile compared to chronic infections. In fact, biofilm formation are facilitated by gut epithelium, plant roots and many other places, whereas in contrast most chronic infections form in locations in the human body where the host response acts to maintain sterility. So, not all biofilms are harmful to human health. Natural and commensal biofilms share some characteristics with those in chronic infections, but differ in others. You will learn much more about these biofilms in this module.