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Amoeba and Bacteria, Newfound Friends?
Antimicrobial resistance is an increasing problem in today’s world particularly in clinical settings. One genus of bacteria, Pseudomonas, is of huge concern due to its ability to linger in hospitals and cause a significant number of infections, particularly in those who are immunocompromised. The most familiar species of Pseudomonas is P. aeruginosa; however, P. putida is another species that has been implicated in human infections.
When a person is infected with a Pseudomonas species, they are typically treated with an antibiotic, ciprofloxacin. However, resistance to ciprofloxacin is increasing leading to persistent Pseudomonas infection (Soares et al.). This persistence can also occur on surfaces as Pseudomonas has been becoming increasingly resistant to many surface cleaners as well (Sanchez et al.).
Investigators in this study sought to characterize an increasingly worrying method by which Pseudomonas resist antimicrobials. This method occurs through hiding within human immune cells known as macrophages which help fight bacterial infection. Numerous studies on macrophages have been performed, but there are complexities and complications involved with using them. As a result, different methodologies to understand phagocytic resistance have begun to be explored, one of which is the use of a microbe known as Acanthamoeba.
Acanthamoeba exists as a free-living amoeba commonly found in soil and water. They survive through feeding off of bacteria and fungi through a process known as phagocytosis, which is when the amoeba engulfs the bacteria and uses enzymes to digest it.
The authors of this study hypothesized that bacteria such as Pseudomonas can resist this phagocytic digestion and survive within the amoeba. This would shield the bacteria from the environmental stressors such as antibiotics or the chemicals in common cleaners, allowing the bacteria to replicate. This phagocytic process is similar to the process used by human macrophages, which suggests that acanthamoeba can be used in the laboratory to study the process of phagocytosis and subsequent resistance.
To investigate this, researchers explored the survival rates of P. putida when incubated with a species of Acanthamoeba versus P. putida by itself in the presence of ciprofloxacin. Multiple different concentrations of ciprofloxacin were used, starting at levels 4 times higher than the minimum inhibitory concentration (MIC), which is the lowest concentration of antibiotic that can inhibit growth of an organism (Figure below). What researchers found was that when the lowest level of ciprofloxacin was used (2 ug/ml), growth of P. putida by itself was completely inhibited. However, when incubated with Acanthamoeba, P. putida could be isolated and grown, indicating that Acanthamoeba helped the bacteria survive.
The researchers observed similar results when using a greater amount of antibiotic than would normally be used by physicians. To completely inhibit the P. putida when incubated with Acanthamoeba, the investigators had to increase the concentration of ciprofloxacin by 40x the normal MIC. This is concerning because not only does it show increased resistance to the drug, but that higher and higher doses are needed which can increase toxicity.
The fight against antimicrobial resistance is very well underway worldwide. The development of antimicrobial properties is multifactorial and complex, and we need increasing tools and methods at our disposal. This study demonstrates a new way to investigate mechanisms of resistance due to the similarities of Acanthamoeba phagocytosis and human macrophage phagocytosis. Additional studies are needed to determine if Pseudomonas can survive in the macrophage, but this new method shows that mechanisms of resistance can be explored at a cellular level without the need to use the common mammalian-host model that can present challenges within itself.
Link to the original post: Giammarini, E., Mooney, R., Mui, E., & Henriquez, F. L. (2025). Preliminary insights into the potential role of acanthamoeba–pseudomonas interactions in the development of antibiotic resistance. Access Microbiology, 7(6). https://doi.org/10.1099/acmi.0.000999.v3
Additional Sources:
- Sanchez CA, Vargas-Cuebas GG, Michaud ME, Allen RA, Morrison-Lewis KR, Siddiqui S, Minbiole KPC, Wuest WM. Highly Effective Biocides against Pseudomonas aeruginosa Reveal New Mechanistic Insights Across Gram-Negative Bacteria. ACS Infect Dis. 2024 Nov 8;10(11):3868-3879. doi: 10.1021/acsinfecdis.4c00433. Epub 2024 Oct 23. PMID: 39440866; PMCID: PMC11555683.
- Soares A, Roussel V, Pestel-Caron M, Barreau M, Caron F, Bouffartigues E, Chevalier S, Etienne M. Understanding Ciprofloxacin Failure in Pseudomonas aeruginosa Biofilm: Persister Cells Survive Matrix Disruption. Front Microbiol. 2019 Nov 13;10:2603. doi: 10.3389/fmicb.2019.02603. PMID: 31798554; PMCID: PMC6864029.
Featured image: Scanning electron microscopy image of Pseudomonas aeruginosa. | Credit: Janice Haney Carr via Wikimedia Commons
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