Microbes at the host surface

The human body encounters pathogenic bacteria every day although establishment of infections after such contacts is rare. The skin and mucosal surfaces, which comprise the barrier between the body and the external milieu, are the first lines of defense, and colonization of these surfaces is normally the first step in bacterial disease. As a physical barrier, the skin is reinforced by dry, slightly acidic conditions, sloughing of cells and a resident microflora. The non-keratinized mucosal surfaces have functions such as secretion of digestive enzymes and absorption of nutrients that require the barrier to be ‘semi-permeable’ with ensuing demands on their design. In addition to a resident microflora and sloughing of cells, the mucosal surfaces are protected by a mucus layer. This gel-like layer is formed by highly glycosylated proteins referred to as mucins and is efficient in trapping microorganisms. The underlying cells relay signals to activate the immune system and mobilization of tissue and blood defense mechanisms is an important aspect of host defense. However, such mechanisms may be accompanied by tissue damage, and infectious diseases are often aggravated by an excessive host response to the invading pathogen. Invading pathogens also have weapons to fight the host with, for example mucindegrading enzymes to disrupt mucus, toxins to disrupt epithelial integrity, and structures to adhere to the host and facilitate invasion. This review will focus on the interplay between the host and bacteria at the mucosal surfaces, which forms a barrier between the inside and the outside of the body.