Host Defenses and Bacterial Assaults: A Delicate Balance

In health, a symbiotic relationship exists between the human body and the bacteria which colonize its surfaces. Under normal circumstances, products of bacterial metabolism, such as short-chain fatty acids or vitamins, are critical for human well being. A single cell layer of mucosal epithelium with a complex underlying immunologic system and various innate nonspecific mechanisms allow absorption of nutrients while maintaining an effective barrier against the 1-1.5 kg of bacteria and other toxic factors. In times of illness, this balance is upset. Changes in gut permeability, inability to ingest nutrients, antibiotic pressure and many other factors alter the normal checks and balances between coexistence and absorption of toxic intraluminal products and sepsis. While two decades of research have focused on epithelial changes in permeability to toxins and bacteria, recent investigations of intestinal immunity and responses of bacteria to altered body defensives are modifying concepts of host defenses during critical illness. These new concepts in immunology and bacterial responses are the focus of this review.