Acanthamoeba and the bacterial pathogen interactions
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The present study investigates Acanthamoeba-bacteria interaction and how this relation can influence human health aiming at the influence of bacteria on Acanthamoeba in terms of their isolation and diversity, and the effect of Acanthamoeba on bacteria focusing on two emerging human bacterial pathogens Arcobacter butzleri and Rhodococcus equi. To first objective was investigated by the test question “can the presence of a particular type of bacteria play role in the diversity of Acanthamoeba by masking and/or favouring certain genotypes of Acanthamoeba?” To answer this, two different bacteria the Gram+ve Enterococcus the Gram-ve Arcobacter were used as food source for isolation of Acanthamoeba from 102 soil samples while E. coli was used as control. It was found that the presence of different bacteria could affect the isolation of genotypes specially the subgroups and subtypes of Acanthamoeba as manifested by greater diversity of 18S rRNA sequences of Acanthamoeba isolated from environmental samples on Arcobacter (Arc) and Enterococcus (Ent) than those isolated on E. coli (Eco). The Eco isolates consisted of only T4>T11=T13 compared to Ent isolates with T4>T16>T13/16 and the Arc isolates which comprised of T4>T2>T2/6=T13>T13/16. The T13/16 were the intermediate sequence types with no match to any T types. There were also considerable differences among the T4 subgroups; the Eco isolates consisted of T4-A>T4-B>T4-N>T4- E>T4-D>T4-C while Ent isolates comprised of T4-A>T4-C=T4-D=T4-E=T4-N>T4-B and the Arc isolates had only T4-E>T4-A>T4-B>T4-N. In both Eco and Ent isolates 11 subtypes were recovered with T4-36 being the most abundant, however, in Arc isolates eight subtypes were recovered with T4-12 as the most abundant. The non-Eco isolates were also different in their bacterial endosymiotic profile from Eco isolates with Arc isolates having the greatest proportion of bacterial endosymbionts (15.7%) as compared to 7.8% of Eco and 12.9% of Ent isolates. Together these results indicate a prominent role of prey bacteria on favouring certain genotypes and thus compelling consideration for use of different types of bacteria for isolation of Acanthamoeba to help surface the masked populations as well for more realistic prevalence that will help in better designing of prevention and control strategies. The influence of Acanthamoeba on bacteria was investigated for A. butzleri and R. equi both of which appeared to exploit the former as an environmental reservoir and for modulation of their pathogenic potential. A. butzleri which are closely related to Campylobacter, appeared to have a smooth interaction with Acanthamoeba. They were shown to be easily located through chemotaxis, readily attached and internalized using monosaccharide receptors and a complex phagocytic process, and could survive/proliferate in Acanthamoeba by defying the intra-vacuolar killing processes. Intracellular survival in Acanthamoeba did play a role in promoting the pathogenicity of these bacteria enabling them to survive more than three times longer. Co-culturing of the two organisms also seemed to benefit the bacteria but not Acanthamoeba. A. butzleri were found to be able to sense the environmental changes and thus modulate their virulence, a feature that together with selection pressure for intracellular survival in Acanthamoeba can cause rapid adaptation to intra-amoebal environment and enhance the pathogenic potential of these bacteria for humans and animals. Exploitation of Acanthamoeba for survival was also found to be exhibited by the Mycobacterium-resembling Gram+ve R. equi by utilizing similar strategies for survival/proliferation as used for macrophages, which involved the definite presence of virulence plasmid and its activation at higher temperatures. Moreover, similar genes (vapA, vapC and vapF) were found to play role in intracellular survival in both the macrophages and amoeba cells. The intra-amoebal survival/proliferation capabilities of A. butzleri and R. equi appear to support the notion that free living protists like Acanthamoeba act as environmental reservoirs/virulence trait selectors and are strong candidates for the “missing link” between the ecology and pathology of these emerging pathogenic pathogens. Overall, the observations made in this study explore the vital role of Acanthamoeba-bacteria interaction not only mutually on each other but as a consequence the impact on human health either as a result of masked genotypes in clinical diagnosis of Acanthamoeba or due to environmental reservoir role of Acanthamoeba in selecting virulence traits of bacteria, can pose serious challenges leaving ample opportunities for more emerging bacterial pathogens. These observations call for revising the protocols for Acanthamoeba prevalence, eradication and control strategies.