Bacteria within phagocytic vacuole
Acanthamoeba sp.
Pseudomonas sp.
Bacteria bound to pseudopodia
Figure 1. Predation of Pseudomonas aeruginosa by Acanthamoeba castellanii trophozoites. Bacteria are taken into the intracellular region of the amoeba via phagocytosis. Ordinarily, these bacteria are degraded and used as a food source; however, many species have evolved survival strategies that permit intracellular survival. Ronnie Mooney
in cooling tower disinfection are significantly less effective in killing bacteria when cells are co-cultured with amoebae. Heat treatment is also less effective against intra-amoebic bacteria; L. pneumophila can survive in temperatures as high as 90°C when internalised, posing a significant risk to facilities using heat treatment as a disinfection strategy. Commonly used contact lens biocides are woefully ineffective against certain amoebic species and as such can often fail to prevent corneal infections resulting from contact lens wear. The increased tolerance of bacteria within amoebae is something that requires consideration when implementing effective disinfection strategies and, while unexplored at present, it could be speculated that antimicrobial exposure at sub-inhibitory concentrations within amoebae could select for increasingly resistant bacteria, which in turn might be a contributing factor to rising levels of AMR. The amoebae ‘switch’ for VBNC bacteria Amoebae can also trigger physiological changes within bacteria that can increase resistance and limit detection. In times of environmental stress, bacteria can transition to a quiescent life stage better suited to the extracellular pressures they are exposed to. These bacteria are termed viable but non-culturable (VBNC) and have demonstrated an increased tolerance to many disinfectants as well as being able to evade culture-based detection. For reasons yet unknown, the conversion between the vegetative and VBNC stage can be triggered in both directions upon ingestion by FLA. In many instances, this intra-amoebic switch causes bacteria
Understanding these interactions will improve the efficacy of mitigation strategies and will be an essential aspect of combatting AMR globally. The microbial Trojan Horse Resistance to antimicrobials is not only facilitated by the increased likelihood of gene transfer events; the ability to resist phagocytosis and remain viable within the amoebae also directly limits exposure of the bacteria to the external environment and, as a result, exposure to antimicrobial therapies. Clinically, the implications of increased tolerability to antimicrobials within FLA is significant, particularly within high-risk areas such as artificial water systems or during food processing treatments. Amoeba can function as a microbial Trojan Horse, shielding pathogenic bacteria from the external environment and delivering them to a vulnerable human host. Indeed, Legionella pneumophila , Pseudomonas spp., Helicobacter spp., Mycobacterium spp., Aeromonas spp., Salmonella spp. and Escherichia coli have all been shown to survive within FLA after exposure to antimicrobial treatments. Many amoebae have an increased tolerance to commonly used disinfection strategies such as chlorine or heat treatment and as such can shield intracellular bacteria from the effects. Sodium hypochlorite, for example, is a commonly used disinfectant within hospitals and is often used in the treatment of potable water; research has shown, however, that killing intra-amoebic bacteria requires concentrations four times higher than is required for extracellular bacteria. Similarly, broad-spectrum biocides such as quaternary ammonium compounds used
77 Microbiology Today October 2022 | microbiologysociety.org
Made with FlippingBook Online document maker