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The ammonia oxidizing bacteria (AOB) community structure in a municipal wastewater treatment plant (WWTP) was monitored over the winter and summer periods. The AOB distribution in the full-scale WWTP was investigated using barcoded 454-pyrosequencing targeting the ammonia monooxygenase alpha subunit (amoA) gene. Using the quantitative polymerase chain reaction (qPCR) technique, the AOB population was quantified over 237 days. The plant operational parameters and its nitrification performance were also monitored. As revealed by pyrosequencing, majority of the identified AOB during the study were related to uncultured ammonia oxidizing bacteria, and Nitrosomonas oligotropha. Furthermore, it revealed higher AOB abundance during the summer which was 6 times more compared to winter. Substantial percentages of the reads from both seasons could not be assigned to any phylum, which suggests that vast population of novel, ecologically significant AOB species still inhabit the complex activated sludge communities unexploited. A significant seasonal variation in temperature (α = 0.05; P = < 0.0001) was recorded in the reactor with maximum temperature amplitude of 10.2 °C. The average nitrification rate recorded during the summer was 0.09 ± 0.03 g N-NH4+/g MLSS/d, whereas it was 0.04 ± 0.02 g N-NH4+/g MLSS/d during the winter. The nitrification rate of the plant had significant correlation with AOB population abundance (r= 0.70; P= 0.01) and temperature (α = 0.05; P= 0.0018). This study indicates that AOB diversity and population abundance were contributory factors to efficient nitrification in activated sludge system.

The emergence and spread of antibiotic resistance through insufficiently treated effluents from wastewater treatment systems are detrimental to the receiving environment and human health. Metagenomic and transcriptomic approaches were employed to assess the diversity and the removal of bacterial pathogens, antibiotic-resistant genes (ARGs) and mobile genetic elements (MGEs) in three wastewater treatment plants (WWTPs) in Durban, South Africa. In total, 23 pathogenic bacterial genera, including enteric and emerging opportunistic pathogens, were found abundant in the samples. Aeromonasand Acinetobacter sppwere the most dominant pathogens detected in the influent metagenomes, while the influent transcripts showed Escherichiaand Acinetobacterspp as the most dominant pathogens. Shannon-Wiener indices showed that the bacterial diversity increased from influents to final effluents in two selected treatment plants. ARG types, including those conferring resistance to aminoglycosides, beta-lactamases, tetracycline and sulfonamide were abundant in both influent and effluent samples. Results further exposed that MGE-ARGs associations were the main drivers of ARG persistence into final effluents. This included 5 plasmids: R338-R151 (sulI), pRH-1238 (strB), pPM91 (aadA), pRH-1238 (aadA4-5), pRH-1238 (sulII); two class 1 integrons (aadAand arr)and 1 transposon Tn4351(tetX). In transcripts, MGE-ARG association showed two plasmids: pRH-1238 (aadA) and pPM91(aadA) and one hybrid plasmid R338-R151 (sulI). Chlorination has little or no effect on MGE-ARG association was apparent across all WWTPs. Combined, this study has highlighted the presence of bacterial pathogens, ARGs and MGEs in treated effluents, which can encourage the propagation of antibiotic resistance and potential sharing of genes in the downstream environments.