Investigations of the localisation of bacterial activity on sandstone fromancient monuments

Methods were investigated for the determination of activity levels of bacte ria on sandstone using the reduction of 2-(4-iodophenyl)-3-(4 nitrophenyl)- 5-phenyl tetrazolium chloride (INT) to INT-formazan as a measure of dehydro genase activity. A microscopy technique, based on use of acridine orange wi th bright-field illumination, was developed which gave a good visual image of bacterial cells, including those containing INT-formazan. Surveys at two monuments, Portchester Castle and Tintern Abbey, were carried out using th is method which showed that between 20.7 and 51.9% of bacterial cells prese nt in situ were active. Extraction of INT-formazan directly from the stone using methanol indicated that bacteria were tightly bound to stone particle s and that microscopic methods would underestimate counts of bacteria. Surv eys of five monuments using the extraction method showed that microbial pop ulations on sandstone in situ were active but activity could not be related to decay state of the stone. (C) 2001 Elsevier Science Ltd. All rights res erved.