Abstract
Anaerobic ammonium-oxidizing (anammox) planctomycetes oxidize ammonium in the absence of molecular oxygen with nitrite as the electron acceptor. Although planctomycetes are generally assumed to lack peptidoglycan in their cell walls, recent genome data imply that the anammox bacteria have the genes necessary to synthesize peptidoglycan-like cell wall structures. In this study, we investigated the effects of two antibacterial agents that target the integrity and synthesis of peptidoglycan (lysozyme and penicillin G) on the anammox bacterium Kuenenia stuttgartiensis. The effects of these compounds were determined in both short-term batch incubations and long-term (continuous-cultivation) growth experiments in membrane bioreactors. Lysozyme at 1 g/liter (20 mM EDTA) lysed anammox cells in less than 60 min, whereas penicillin G did not have any observable short-term effects on anammox activity. Penicillin G (0.5, 1, and 5 g/liter) reversibly inhibited the growth of anammox bacteria in continuous-culture experiments. Furthermore, transcriptome analyses of the penicillin G-treated reactor and the control reactor revealed that penicillin G treatment resulted in a 10-fold decrease in the ribosome levels of the cells. One of the cell division proteins (Kustd1438) was downregulated 25-fold. Our results suggested that anammox bacteria contain peptidoglycan-like components in their cell wall that can be targeted by lysozyme and penicillin G-sensitive proteins were involved in their synthesis. Finally, we showed that a continuous membrane reactor system with free-living planktonic cells was a very powerful tool to study the physiology of slow-growing microorganisms under physiological conditions.