Mycobacterium bovis hosted by free‐living‐amoebae permits their long‐term persistence survival outside of host mammalian cells and remain capable of transmitting disease to mice

Abstract

Bovine tuberculosis (TB) is a zoonotic disease caused by Mycobacterium bovis. Despite intensive TB control campaigns, there are sporadic outbreaks of bovine TB in regions declared TB free. It is unclear how M. bovis is able to survive in the environment for long periods of time. We hypothesized that Free-living amoebae (FLA), as ubiquitous inhabitants of soil and water, may act as long-term reservoirs of M. bovis in the environment. In our model, M. bovis would be taken up by amoebal trophozoites, which are the actively feeding, replicating and mobile form of FLA. Upon exposure to hostile environmental conditions, infected FLA will encyst and provide an intracellular niche allowing their M. bovis cargo to persist for extended periods of time. Here, we show that five FLA species (Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba lenticulata, Vermamoeba vermiformis and Dictyostellium discoideum) are permissive to M. bovis infection and that the M. bovis bacilli may survive within the cysts of four of these species for over 60 days. We further show that exposure of M. bovis-infected trophozoites and cysts to Balb/c mice leads to pulmonary TB. This work describes for the first time that FLA carrying M. bovis can transmit TB.

Mycobacterium bovis uses the ESX-1 Type VII secretion system to escape predation by the soil-dwelling amoeba Dictyostelium discoideum

“…One school of thought is that amoebae may provide a protective or replicative niche for the mycobacterium. Indeed, M. bovis has been shown to survive within the trophozoites of Acanthamoeba polyphaga and A. castellanii, [18,73,74] where a proportion become encased in a protective cyst and maintain the ability to cause infection in mice [73]. However, the majority of M. bovis bacilli bypass the amoeba cyst stage, possibly by exiting the protozoan via exocytosis or non-lytic ejection described in D. discoideum [17,18,74].…”

Animal tuberculosis: impact of disease heterogeneity in transmission, diagnosis, and control

“…Mycobacterium bovis has been found in the free‐living amoeba that may work as long‐term reservoirs of these bacteria in environmental samples, particularly when present in cystic forms, retaining their ability to transmit bacteria and cause disease in mice (Sanchez‐Hidalgo, Obregon‐Henao, Wheat, Jackson, & Gonzalez‐Juarrero, 2017). Moreover, earthworms were found to be able to disseminate M. bovis from contaminated animal faeces to the surrounding soil through casting egestion (Barbier, Chantemesse, et al, 2016).…”

“…Besides, earthworms can also shed bacteria for four days after the initial ingestion of contaminated faeces (Barbier, Chantemesse, et al, 2016). Both organisms act as environmental reservoirs of M. bovis that could lead to cattle and wildlife infection (Barbier, Chantemesse, et al, 2016; Sanchez‐Hidalgo et al, 2017) and may be associated with the re‐emergence of animal TB in areas where no infected animal was present or no infected animal was introduced.…”

The Badgers Have Moved the Goalposts!

“…Scientists are looking for-and finding-the microbe in places where they had thought it was absent: in the bodies of cows cleared by tuberculin testing, in soil and slurry, and even within the bodies of single-celled amoebae. 12 Scientists and public health bodies-most significantly the World Health Organization (WHO)-have started renaming bTB from 'bovine TB' to 'zoonotic TB', flagging the capacity of M. bovis to pass beyond livestock into many other mammals, including humans and wildlife. 13 Because these new testing technologies often measure the presence of microbes rather than the presence of disease, their findings are likely to further disintegrate the twentieth-century livestock disease of bTB, recognising the complexities of an infection that passes across and between humans, other animals and wider environmental systems.…”