Enhanced survival but not amplification of Francisella spp. in the presence of free-living amoebae

Abstract

Transmission of Francisella tularensis, the etiologic agent of tularemia, has been associated with various water sources. Survival of many waterborne pathogens within free-living amoeba (FLA) is well documented; however, the role of amoebae in the environmental persistence of F. tularensis is unclear. In this study, axenic FLA cultures of Acanthamoeba castellanii, Acanthamoeba polyphaga, and Vermamoeba vermiformis were each inoculated with virulent strains of F. tularensis (Types A and B), the attenuated live vaccine strain, and Francisella novicida. Experimental parameters included low and high multiplicity of infection and incubation temperatures of 25 and 30°C for 0-10 days. Francisella spp. survival was enhanced by the presence of FLA; however, bacterial growth and protozoa infectivity were not observed. In contrast, coinfections of A. polyphaga and Legionella pneumophila, used as an amoeba pathogen control, resulted in bacterial proliferation, cytopathic effects, and amoebal lysis. Collectively, even though short-term incubation with FLA was beneficial, the long-term effects on Francisella survival are unknown, especially given the expenditure of available amoebal derived nutrients and the fastidious nature of Francisella spp. These factors have clear implications for the role of FLA in Francisella environmental persistence.

Tularemia as a waterborne disease: a review

“…This mode of survival may apply to Francisella species, especially F. tularensis . Multiple experimental studies have focused on the interaction between Francisella species and several amoeba species [145–154]. Berdal et al demonstrated that F. tularensis could penetrate in the amoeba Acanthamoeba castellanii and be released from it [152].…”

“…tularensis , F. tularensis subsp. holarctica and F. novicida in co-culture with amoebae such as A. castellanii , A. polyphaga , Vermamoeba vermiformis or Ochromonas danica [145,147,153]. These authors described the presence of Francisella sp.…”

“…These authors described the presence of Francisella sp. inside and outside the amoeba in the co-culture model, suggesting that bacterial survival could be related to intra-amoebal replication, to favorable interaction between extracellular bacteria and amoeba, or both [145,147,153]. Interestingly, Gustafsson et al demonstrated that growth supernatant of A. palestinensis (without the presence of amoebae) increased multiplication of F. tularensis [154] .…”

Dual Role of Mechanisms Involved in Resistance to Predation by Protozoa and Virulence to Humans

“…Protozoa have been called the “Trojan horses of the microbial world” for their ability to promote the survival of pathogenic bacteria in the environment ( Barker and Brown, 1994 ). For example, pathogenic bacteria that have been shown to have increased survival in the environment after interactions with protozoa include Campylobacter jejuni ( Trigui et al, 2016 ; Reyes-Batlle et al, 2017 ), Francisella tularensis ( Buse et al, 2017 ), Listeria monocytogenes ( Fieseler et al, 2014 ), Legionella pneumophila ( Cervero-Arago et al, 2014 , 2015 ), Mycobacterium leprae ( Wheat et al, 2014 ), Stenotrophomonas maltophilia ( Cateau et al, 2014 ), and Yersinia enterocolitica ( Lambrecht et al, 2013 ). Although the mechanisms of increased bacterial survival are not fully understood, it has been suggested that bacteria can obtain nutrients from protozoa.…”

Parasite–bacteria interrelationship

“…Due to the widespread distribution of Acanthamoeba in the nature, it can be considered an environmental reservoir for these human microbes. Furthermore, some of these bacteria surviving inside the Acanthamoeba parasites showed reduced susceptibility to some antibiotics and to disinfectants as well (Marciano-Cabral and Cabral 2003;Huws et al 2008;Buse et al 2017). Moreover, Rickettsia was found to be a symbiont of Acanthamoeba (Rowbotham 1983) that may potentially exacerbate amoebic keratitis (Taylor et al 2012).…”