Host Defense Against Foodborne Pathogens

Abstract

To understand the pathogenic mechanism of microbial infection, one has to have a clear knowledge of immune response, both innate and adaptive, that are in place to protect the host against infective agents. Microbial dominance results in the disease, while successful host response averts the full-blown infection. Foodborne pathogens affect primarily the digestive system; therefore, the natural immunity of the gastrointestinal tract is the most important protective immune response. Moreover, the onset of symptoms for some diseases (i.e., intoxication) is very fast – appearing within 30 min to an hour. Thus, protection by adaptive immune response would have little impact since it is slow to develop, typically requiring 4–7 days. However, the adaptive immune response is essential for foodborne pathogens that have a prolonged incubation time and are responsible for systemic infection such as Listeria monocytogenes, Shigella spp., Campylobacter spp., Salmonella enterica, Yersinia spp., hepatitis A virus, Toxoplasma gondii, Trichinella species, and so forth. In innate immunity, gastric acids, bile, antimicrobial peptides, mucus, natural microflora, macrophages, neutrophils, NK cells, interferons, and complement proteins play a critical role. In the adaptive immune response, T and B lymphocytes produce cytokines and antibodies, respectively. CD4⁺ T cells are most important for protection against extracellular bacterial infections and their exotoxins, while CD8⁺ T cells are involved in the elimination of intracellular bacterial, viral, and parasitic infective agents. Antibodies neutralize pathogens or toxins by preventing them from binding to the host cell receptors, so they become the target for elimination by macrophages and neutrophils. An antibody–antigen complex also activates complement for inactivation of pathogens via opsonization. However, the immune system sometimes fails to protect the host. It is because pathogens have developed strategies to overcome immune defense by producing virulence factors that ensure their invasion, survival, replication, and spread inside the tissues. It is important to recognize that the pathogenic action by foodborne exotoxins is very quick and the host has literally no time to mount any immune response; thus, a majority suffers from this form of food poisoning (intoxication) irrespective of their health status or immune response.