Investigating the intracellular growth, cytotoxicity and apoptotic effects of solar irradiated Campylobacter jejuni in a murine macrophage cell line (RAW 264.7)

Abstract

Campylobacter jejuni is a leading cause of gastroenteritis worldwide. Solar disinfected (SODIS) water can reduce diarrhoeal incidences in communities where potable water is inaccessible. SODIS water has been associated with the reduction of diarrhoeal incidences amongst SODIS users. Thus solar attenuation of bacteria may have a significant impact on immune cells such as macrophages. When solar irradiated cells die the release antigenic materials such lipopolysaccharides and intracellular contents into the water. The solar irradiated bacterial may have an impact on the cytotoxicity and cell death of immune cells such as macrophages. This study assessed the effects of solar irradiation on the viability, metabolic activity and three virulence characteristics (invasion, cytotoxicity, and apoptosis) of C. jejuni on RAW 264.7 cells. Samples of C. jejuni were suspended in water and treated in the following ways (i) heat and chemically attenuated (1% formalin at 60°C for one hour), (ii)) exposure to solar ultraviolet radiation (SUVR) for 0, 4 and 8 and, (iii) non-exposure to SUVR. The metabolic activity was assayed spectrophotometrically using Alamar Blue. The C. jejuni samples were used to infect macrophage RAW264.7 cells and its intracellular growth was assessed by using the gentamicin protection assay. Cytotoxicity was assessed by using the Lactate Dehydrogenase Assay (LDH). Apoptosis of the treated macrophages was analysed by Flow Cytometry. The results showed that all the C. jejuni were not culturable. However, the non-solar irradiated Campylobacter jejuni retained its metabolic activity (40.2%±1%) whereas no metabolic activity was observed in the heat and chemically attenuated and solar irradiated bacteria. Intracellular growth of the bacteria in the RAW264.7 cells was not detected in all the treated samples. The non-irradiated C. jejuni showed higher cytotoxic and apoptotic effects on macrophages than the heat attenuated and solar irradiated samples. In conclusion, solar irradiation of C. jejuni eliminates its metabolic activity and also reduces its ability to induce cytotoxicity and apoptosis in the macrophages