**12.** *Acanthamoeba* **and bacteria**

The association of *Acanthamoeba* spp. with other microorganisms is most significant from the ophthalmological point of view. This species can generate corneal injuries that remain for months or years and are difficult to treat with diamine and biguanide drugs in developing countries (chlorhexidine and polyhexamethylene biguanide, respectively) [33].

Some of the bacteria mechanisms within *Acanthamoeba* spp. have been described. Scheid *et. al*., using an *in vitro* model with electronic microscopy, showed the cycle of a coccoid-like organism in the free-living amoebae *Vannella* spp. The coccoid microorganism is a phagocyte and is transported by phagocytic vacuoles through the cytosol until reaching the amoeba nucleus where the microorganism proliferates and is released by rupture of the host mem‐ brane. In the beginning of the life cycle, coccoid microorganisms are phagocytes for other amoeba [34]. In addition, endosymbiont bacteria can replicate only into the amoeba cytosol, can break the cells, and can be ingested by neighboring amoeba [35]. However, the intrusion of bacteria into *Acanthamoeba polyphaga* has consequences for both microorganisms. A protein bellows the amoeba that adheres to the surface of *Legionella pneumophila*. The authors discussed the possibility that the liberation of the bacteria from the amoeba integrates amoeba antigens in its membrane. However, *Pseudomonas aeruginosa* and its liberated products kill *Acanthamoe‐ ba* spp. [30, 32, 36]. Another study showed that endosymbiont bacteria favor the growth of different species of *Acanthamoeba*, and all microorganisms isolated from contact lens care solutions contained numerous trophozoites [37].

*Acanthamoeba* spp. by itself causes severe inflammation in the cornea. Aggressive keratolysis or sclerokeratitis is a common complication. The most frequently reported symptoms are pain, photophobia, and tearing [38]. The association of *Acanthamoeba* spp. with bacteria or a fungus presents a coinfection that can mimic bacterial, fungal, or herpetic keratitis, which can delay the time to diagnosis and increase the pathogenicity of the *Acanthamoeba* spp. infection.

Several molecular methods and electronic microscopy have facilitated the observation of bacteria within *Acanthamoeba*, including *Pseudomonas aeruginosa*, *Listeria monocytogenes*, *Mycobacterium* spp., *Escherichia coli*, *Legionella pneumophila*, *Chlamydia* spp., and *Aeromonas* spp. They are also capable of producing keratitis themselves [30, 32, 39, 40, 41]. In addition, there are many reports in the literature of cases of *Acanthamoeba* and bacteria coinfection isolated from patients with or without contact lens-associated keratitis [39, 42, 43, 44].

## **13.** *Acanthamoeba* **and fungi**

This type of coinfection is uncommon. Similar to bacteria, fungi have a complex relationship with *Acanthamoeba* spp. and generate lesions that can be confused with bacterial or herpetic keratitis; the prognosis is discouraging. In all cases reported, penetrating keratoplasty was performed. These coinfections require immediate and aggressive treatment with antifungal and antiamoebic drugs [45, 46, 47, 48, 49].
