Food microbiology is an important issue we cover on this blog, because food safety is vital to prevent foodborne illness. The use of technologies like whole-genome sequencing help identify and pinpoint the source of microbial contaminants, but how do microbes become contaminants in the first place? A new Applied and Environmental Microbiology report suggests that Salmonella requires specific genes to become a foodborne contaminant. The bacterium, which causes approximately 1 million illnesses annually in the United States, is often found in produce (such as salad greens, tomatoes, and alfalfa sprouts).
Growth on plants allows bacteria to grow to higher numbers, increasing their likelihood of infecting plant eaters. The plant environment is much different than an animal environment and requires specific adhesion, metabolic, and stress response genes. The new report finds that Salmonella requires a specific pattern of gene expression to enable its persistent colonization on tomato plants.
The study, conducted by first author Marcos de Moraes and headed by Max Teplitski, used transposon mutagenesis to identify genes required for tomato plant colonization. While a number of studies have identified Salmonella genes involved in plant colonization, the authors hypothesized that the Salmonella genes necessary for growth on plants differ from those necessary for growth in animals, and that the required plant growth genes have not all been identified.
The scientific team used a pool of 280,000 independent colonies in an FDA-approved tomato wound model that best mimics the route of microorganism contamination. The relative abundance of the recovered inoculum was compared to that of the initial inoculum to determine mutations that decreased bacterial fitness. These results were compared to those from a similar screen in a murine model of systemic infection.
In some cases, plant colonization required different genetic expression patterns from animal or human colonization (see figure, right). Colonizing plants required amino acid biosynthetic genes, while colonizing animals required amino acid-scavenging genes. Previous research had revealed the importance of the urea cycle in systemic animal infection, but it wasn’t required for tomato infection.
Different tomato strains required different bacterial genetic expression patterns. Some bacterial mutants were even less fit (that is, grew worse) when grown on tomatoes carrying a mutation that inhibits ripening, despite these tomatoes accumulating amino acids to a higher concentration than wild-type tomatoes. The authors speculate that the tomato amino acids remain somehow inaccessible to the bacteria and thus not available for bacterial use.
Some genetic pathways were required regardless of plant or animal colonization, such as those for nucleotide and fatty acid biosynthesis. The ability to synthesize lipopolysaccharide, a component of the outer membrane of the gram-negative Salmonella, was also required for growth both on tomatoes and in mice. Overall, approximately 40-50% of identified bacterial genes overlap in their requirement for plant and animal growth.
Will understanding plant colonization by human pathogens help scientists better prevent these microbial contaminants from entering the food chain? This seems unlikely, since Salmonella reach ~107 cell forming units (CFU) on each tomato, but numbers as low as a few dozen ingested bacteria are predicted to cause disease in people. Nevertheless, learning these required genetic pathways for Salmonella growth on tomatoes gives researchers potential targets to decrease bacterial colonization of tomatoes, as well as target processes to investigate in other human enteric pathogens that successfully colonize plants.
Tomato photo credit