In　this　study,　Candidatus　Kuenenia　stuttgartiensis　were　subjected　to　distinct　nitrite　shocks　(66　(control),　200,　300,　400,　and　500　mg　NIL),　and　the　responses　of　mRNA　levels　of　cytochrome　cd(1)　nitrite/nitric　oxide　oxidoreductase　(nirS),　hydrazine　synthase　(hzsA),　and　hydrazine　dehydrogenase　(hdh)　were　assessed.　Changes　in　the　hydrazine　dehydrogenase　(HDH)　protein　level　were　monitored.　At　200　mg　NO2--N/L,　the　normalized　specific　anaerobic　ammonium-oxidizing　activity　(nSAA)　slightly　increased　relative　to　the　control　despite　a　significant　decrease　in　nirS,　hzsA,　and　hdh　mRNA　levels.　When　nitrite　increased　to　300　and　400　mg　N/L,　increased　nirS,　hzsA,　and　hdh　mRNA　levels　were　observed,　but　the　nSAA　decreased,　relative　to　the　200　mg　NO2--N/L　exposure.　HDH　protein　detection　revealed　that　Candidatus　Kuenenia　stuttgartiensis　attempted　to　yield　high　enzyme　levels　by　stimulating　mRNA　synthesis　to　resist　the　nitrite-induced　stress.　On　500　mg　NO2--N/L　shock,　the　nirS,　hzsA,　and　hdh　mRNA　levels　decreased,　alongside　decreased　nSAA　and　HDH　levels.　Although　the　mRNA　levels　did　not　always　coincide　with　activities,　our　findings　advance　understanding　of　the　mechanisms　that　anammox　bacteria　use　to　cope　with　nitrite　inhibition　at　the　transcriptional　and　translational　levels,　which　will　improve　the　diagnostic　accuracy　of　bioreactor　failures　when　nitrite　accumulation　occurs.