Immunotoxic responses induced by Streptomyces californicus and Stachybotrys chartarum - the role of microbial interactions

Adverse health effects have been associated with dampness and microbial exposure in buildings, but the possible pathophysiological mechanisms behind these effects are still poorly understood. Although previous studies have shown that certain microbes and microbial components have clear inflammatory and cytotoxic potentials, the complex mixture of microbial species, their spores, metabolites and components in indoor air inevitably leads to interactions which may change the toxic characteristics of the microbes. However, little is known about the importance of microbial interactions in the activation of the cellular mechanisms which may cause varying health outcomes in different exposure situations. The present study assessed interactions between two microbes isolated from moisture damaged buildings, the actinobacterium Streptomyces californicus and the fungus Stachybotrys chartarum, during co-exposure or co-cultivation. The main interest was to study how these microbial interactions affect the ability of their spores to activate important cellular mechanisms i.e. cytotoxicity, inflammation, genotoxicity and oxidative stress in mouse RAW264.7 macrophages. The results of these studies indicated that the spores of S. californicus have cytotoxic, cytostatic, genotoxic and inflammogenic properties, whereas the spores of S. chartarum caused significant cytotoxicity only at relatively high concentrations, but no cytostatic, genotoxic or inflammogenic activity was observed in macrophages. In simultaneous exposure, the mutual proportion of these microbes influenced the nature of cellular responses, leading to increased or suppressed inflammatory response in macrophages. Interestingly, the microbial interactions during co-cultivation were capable of stimulating or potentiating the production of highly toxic compound(s), and thus the spores of co-cultivated microbes evoked stronger immunotoxic responses in macrophages than the respective spore-mixture of separately cultivated microbes. Compound(s) produced during co-cultivation had strong cytotoxic, cytostatic and genotoxic properties, and the mechanism of cell death resembled the triggering of the apoptotic pathway by the cytostatic drugs, doxorubicin and actinomycin D, which both originate from streptomycetes. Furthermore, simultaneous exposure to an antioxidant, N-acetyl-L-cysteine, with the spores of co-cultivated microbes inhibited these responses indicating that oxidative stress was involved in the cascade leading to the detected cellular damages caused by the co-culture. In conclusion, the present findings showed clearly that the toxic mechanisms activated in macrophages during microbial exposure include cytotoxicity, oxidative stress, genotoxicity and inflammation associated injury. In addition, microbial interactions may significantly change the immunotoxic characteristics of the inhaled particles and this may explain, at least in part, the adverse health effects observed in damp indoor environments where there may be relatively low microbial concentrations. These kinds of interactions should be carefully considered when evaluating the health effects experienced by occupants of moisture-damaged buildings.