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2.2 Human pathogens that can infect plants

Our human-centric view of disease often leads to sweeping assumptions that human pathogenic bacteria are devoted to human hosts. Species of Salmonella, Serratia, Enterobacter, and Enterococcus are commonly considered problematic human pathogens that are frequently found in the nosocomial environment, and which cause food poisoning, general infections, and septicaemia. Actually, other bacterial species that cause skin, wound, and urinary tract infections (e.g. Bacillus cereus, Proteus vulgaris) can also be found in rhizosphere environments.

Relatively recent studies, however, have begun to uncover that these human-pathogenic bacterial species are also capable of colonizing and causing disease in a wide variety of plant hosts. Various studies have indicated that human pathogenic bacteria enter the root tissue at sites of lateral root emergence. Notably, many of these studies have been conducted under laboratory conditions, providing evidence for the phytopathogenic potential of these bacterial species. However, the incidence of plant disease caused by many of these human pathogens in the natural environment remains unknown.

he occurrence of human pathogenic bacteria in the rhizosphere has been ascribed to several factors, including the high nutritional content, protection from ultraviolet (UV) radiation, and the availability of water films for dispersal and for preventing desiccation. Others have argued that the abundant and highly diverse indigenous rhizosphere microbial communities provide a strong barrier against the invasion of human pathogens. For example, the growth of S. enterica and E. coli O157:H7 on roots of Arabidopsis thaliana was strongly inhibited by a plant-associated strain of Enterobacter asburiae. Nevertheless, many human pathogenic bacteria can be highly competitive for nutrients and produce various antimicrobial metabolites allowing them to colonize and proliferate on plant surfaces in the presence of the indigenous microbial communities. Interestingly, the mechanisms involved in rhizosphere colonization and antimicrobial activity of human pathogenic bacteria appear to be similar to those involved in virulence and colonization of human tissues.

Next, we will discuss the biology and pathogenesis of some of the most common causative agents of infection associated with the consumption of plant foods.