Kwayoyin Nitrous nau'in kwayoyin cuta ne da ke da ikon aiwatar da tsarin nitrification, wanda ya haɗa da canza ammonia (NH3) zuwa nitrite (NO2-) da nitrate (NO3-). Bakteriya na nitrates suna da mahimmanci a cikin zagayowar nitrogen yayin da suke canza ammonia mai guba da rayayyun halittu ke fitarwa zuwa mafi ƙarancin nitrites da nitrates masu guba, waɗanda tsire-tsire za su iya amfani da su azaman tushen nitrogen. Manyan nau’ikan kwayoyin cuta guda biyu sune Nitrosomonas, wanda ke canza ammonia zuwa nitrite, da Nitrobacter, wanda ke canza nitrite zuwa nitrate.