Health hazards associated with dissemination of bacterial strains in waste water recycling

Abstract

Treated waste effluents with low levels of chemical and microbiological contents are used for domestic, industrial, agricultural and aquacultural purposes worldwide, and it is estimated that one tenth or more of the world’s population consume food produced through irrigation with wastewater. Treated hospital waste effluents may contain pathogenic and drug resistant bacteria, which constitutes the most dangerous single risk factor for dissemination of pathogenic and drug resistant organisms to the environment. This thesis focuses on the possibility of persistence and transmission of pathogenic and drug resistant bacteria like Aeromonas and Enterococcus from wastewater to environment and to humans, especially in relation to a sewage treatment process based on waste water recycling.

In one study (Paper I), we have shown that a prevalent ampicillin and ciprofloxacin resistant clonal lineage of Enterococcus faecium in Swedish hospital sewage water may be transmitted from hospital patients to hospital sewage water. Recycling of hospital sewage water may in the second step disseminate the diseasecausing organisms to the environment and in the third step to the human food chain. In other studies (Papers III, V), we have investigated the persistence of Aeromonas in a duckweed aquaculture based hospital sewage water treatment plant, where sewage grown duckweed is used as fish food. In this treatment system, the numbers of Aeromonas organisms, diversities and resistance patterns to different antibiotics were found to reasonably decrease through the purification process, with the exception of the fish culture pond where a high prevalence of resistance was observed.

We found that a phenotypically and genotypically stable clonal lineage of Aeromonas could survive in the treatment system for years. Both virulent and avirulent clonal lineages of Aeromonas could be transmitted from hospitalised children suffering from diarrhoea to the fish that will be consumed by human through recycling of hospital sewage water. It was also shown that transport vehicles such as duckweed and amoebas (if present) might play a role in this transmission. We were able to detect a gentamicin resistant (multiresistant) clonal lineage that was transmitted from hospital sewage water to fish. Furthermore, we found that a transmitted clonal lineage of Aeromonas had a high biofilm formation ability compared to other Aeromonas clones. As in other bacteria, we could show that transition between biofilm formation and motility of one strain of this clone is regulated by the cyclic di-guanosine-monophosphate concentration. We also characterized Aeromonas from fish with Epizootic Ulcerative Syndrome disease and showed that one pathogenic clonal group was associated with this fish disease in Bangladesh (Paper II).

To facilitate the monitoring of resistance in different sites of the waste treatment plant, we developed a user-friendly semi automated MIC determination method, the ScanMIC method, to screen a large number of isolates in a short time (Papers IV, V). We compared this method with a reference method and in both cases the ScanMIC method was found to meet the performance criteria guidelines for automated susceptibility testing.

In conclusion, the results indicate that sewage water recycling in aquaculture constitutes a potential health hazard through dissemination of pathogenic and drug resistant bacteria to the environment and/or to human.