Humic acids (HAs) content of raw water is an important analytical parameter in water treatment facilities because HAs in the presence of chlorine may lead to the formation of dangerous by-products (e.g., trihalomethanes). The concentration of HAs in water is not directly accessible by common analytical methods due to their heterogeneous chemical structure. The aim of this study was to compare two methods to assess humic acids (HAs) in surface water namely absorbance of ultraviolet light at 254 nm (UV₂₅₄) and total organic carbon (TOC), as well as to evaluate the effects of calcium and magnesium concentrations, pH and sample filtration on the methods' results. An aqueous solution of a commercial HA with 10 mg L⁻¹ was used in the present work. Quantification of the HA was carried out by both UV₂₅₄ and TOC (combustion-infrared method) measurements. UV₂₅₄ results were converted to TOC using a calibration curve. The effects of calcium (0–136.3 mg L⁻¹) and magnesium (0–34.5 mg L⁻¹) concentrations, pH (4.0, 7.0 and 9.0) and sample filtration on UV₂₅₄ and TOC measurements of the HA suspension were evaluated. More accurate TOC values of HA suspensions were obtained by the combustion-infrared method than by the UV₂₅₄ absorbance method. The higher differences of TOC values between unfiltered and filtered samples were detected in the presence of calcium at pH 9.0 using the spectrophotometric method.

Buruli ulcer, caused by Mycobacterium ulcerans infections, is a necrotizing skin disease whose pathogenesis is associated with the exotoxin mycolactone. Despite the relevance of this emergent disease, little is known on the immune response against the pathogen. Following the recent demonstration of an intramacrophage growth phase for M. ulcerans, we investigated the biological relevance of IFN-γ and the antimycobacterial mechanisms activated by this cytokine in M. ulcerans-infected macrophages. Three M. ulcerans strains were tested: 5114 (mutant mycolactone-negative, avirulent strain); 94–1327 (intermediate virulence); and 98–912 (high virulence). We show in this study that IFN-γ is expressed in mouse-infected tissues and that IFN-γ–deficient mice display increased susceptibility to infection with strains 5114 and, to a lesser extent, 94–1327, but not with the highly virulent strain. Accordingly, IFN-γ–activated cultured macrophages controlled the proliferation of the avirulent and the intermediate virulent strains. Addition of mycolactone purified from strain 98–912 to cultures of IFN-γ–activated macrophages infected with the mycolactone-negative strain led to a dose-dependent inhibition of the IFN-γ–induced protective mechanisms, involving phagosome maturation/acidification and increased NO production, therefore resulting in increased bacterial burdens. Our findings suggest that the protection mediated by IFN-γ in M. ulcerans-infected macrophages is impaired by the local buildup of mycolactone.