Antioxidant proteins and metronidazole in Giardia lamblia

The single-cell parasite Giardia lamblia colonizes the small intestine of humans and causes hundreds of millions of episodes of diarrhoea and other forms of indigestion, commonly termed as giardiasis, every year across the globe. Although symptoms normally abate after few weeks, giardiasis can also persist can cause growth retardation in children. Moreover, giardiasis has been suggested to be a cause for post-acute irritable bowel syndrome. Due to its anaerobic metabolism, G. lamblia is susceptible to metronidazole which is standardly used in the treatment of giardiasis. Since the early 2000s, however, giardiasis cases refractory to metronidazole have been observed with increasing frequency, especially in travellers returning to Europe from India and other Asian countries. Surprisingly, G. lamblia isolates from refractory cases have not displayed any metronidazole resistance in in vitro tests so far. Prior to this project, our work group observed that some Giardia strains tolerate higher concentrations of metronidazole under microaerobic conditions than other strains. The degree of the observed tolerance was well within the margin of metronidazole concentrations attained in the serum of treated patients (approximately 16 g ml-1), which might explain why metronidazole therapy is unsuccessful in some giardiasis patients. Importantly, oxygen concentrations in the small intestine are microaerobic rather than anaerobic. Thus, metronidazole testing in G. lamblia under strictly anaerobic conditions, as commonly practised, might cause the oversight of metronidazole-tolerant Giardia strains. It will be the main goal of this project to elucidate the mechanisms behind the observed higher tolerance to metronidazole under microaerobic conditions. Our recent data suggest that this tolerance might be caused by the increased expression of antioxidant enzymes in certain strains. We will apply high-throughput proteomic methods in order to identify the antioxidant enzymes involved, followed by a thorough biochemical characterization and testing of the identified enzymes. The analyses will not only be performed in established laboratory Giardia strains, but also in clinical isolates kindly provided by Christian Klotz from the Robert Koch Institute in Berlin, in order to increase the significance of the findings.