Biology of Microbial Pathogens

Abstract

All organisms belong to one of the three domains of life: Bacteria, Archaea, and Eukarya. As foodborne pathogens, general properties such as the morphological and structural characteristics of bacteria, viruses, molds, parasites, and algae are reviewed in this chapter. The cell walls of Gram-positive bacteria contain a thick peptidoglycan (PGN), which is highly complex and dynamic containing a disaccharide N-acetyl-d-glucosamine (NAG) and N-acetylmuramic acid (NAM) and linked by β-1,4-glycosidic linkage (GlcNAc-(β1–4)-MurNAc) and pentapeptide. The PGN not only protects the bacterial cells against mechanical or physical damages but also hosts numerous structural and functional proteins for rigid exoskeleton and for functional attributes such as bacterial pathogenesis and induction of host immune response. The Gram-negative cell envelope consists of the outer membrane (OM) which carries LPS (an endotoxin), a thin PGN layer, and a cytoplasmic membrane. Due to the presence of OM, the protein secretion system in Gram-negative bacteria is mediated by several secretory pathways designated as type I–VI secretory pathways, of which type III secretion system is known as the molecular syringe that delivers bacterial virulence proteins directly to the interior of the host cell. Some bacteria produce endospores, which are essentially a long-term survival strategy for the bacteria.

Most foodborne viruses cause severe gastroenteritis and affect a large number of people every year. Foodborne enteric viruses are shed in large numbers (10⁹ particles per gram) from infected patients through feces or vomitus. Person-to-person or fecal–oral transmission is a common mechanism for the spread of such viral infection. Since viruses are highly infectious, only a small dose of 10–100 particles is required to infect a person. Protozoan parasites are increasingly becoming a major concern due to their spread through fresh vegetables and fruits. Immunocompromised people are highly susceptible to the intracellular protozoan parasites. Water and soil tainted with feces generally serve as the major contamination sources, and the presence of these pathogens indicates breaches in the hygienic or sanitary practices during food production and harvest. Mycotoxins are small molecules produced as secondary metabolites by the toxigenic molds. Mycotoxins may exert carcinogenic, mutagenic, or teratogenic activities. Seafood toxins are generally associated with fish and shellfish that acquire toxins from ingested algae or are due to the bacterial enzymatic activities on fish proteins.