F. Facilitated emergence of resistance in human pathogens
Horizontal Gene Transfer In The Alimentary Tract
For foodborne pathogens, the gastrointestinal tract is the most important environment for gene transfer. Referred to as “The Reservoir Hypothesis”, many believe that numerous species of intestinal bacteria have a significant role in storing and transmitting AMR genes. Several authors have also reported transfer of genes in the rumen, in foodstuffs and in biofilms present on food processing equipment30. Acquisition of resistance genes via conjugation or transformation in these environments may pose a serious health issue when a pathogen acquires resistance genes from the surrounding flora in the gastrointestinal tract.
Several findings in vitro and in vivo have demonstrated the occurrence of gene transfer in the alimentary tract. For example, tetracylcine and erythromycin resistance genes encoded on transposons were shown to be transferable from Enterococcus faecalis to E. coli and L. monocytogenes in the digestive tract of mice31. An epidemic R plasmid from Salmonella enteritidis moving to Escherichia coli of the normal human gut flora has also been observed32. Several epidemiologic and molecular studies involving antimicrobial resistance of human and animal pathogens also support this hypothesis.
Using mathematical models, Smith29 demonstrated that the use of animal agricultural antibiotics can hasten the appearance of AMR bacteria in humans, with the greatest impact occurring soon after the first emergence of resistance. Although it is true that such changes and adaptations can occur independently of antimicrobial use in animals, the existence of resistance genes in animal bacterial populations contributes a pool of resistant genes and resistant bacteria to the environment and reservoir hosts. This phenomena is illustrated in the resistance gene cycle depicted by Davies53 which shows how resistance gene acquisition by various microorganisms contribute to the environmental antibiotic resistance gene pool (seen at the top of the accompanying diagram) which then become a source of resistance genes for other types of bacteria.