Table 1. Comparison of immunohistochemical staining of epithelial, vascular and muscle tissues taken from TSA nu1 and from a contemporary specimen to show the preservation of immunoreactivity in the mummified skin. TSA nu1 Contemporary tissue a-SMC actin CD68 elastin collagen type I + 2 ++ ++ +++ +++ +++ +++ a-SMC actin = alpha smooth muscle cell actin; CD68 = Cluster of Differentiation 68. doi:10.1371/journal.pone.0002053.t001 induced by other bacteria and thus contributed to the exquisite tissue preservation. The heavy metals, present in such abundance in the hair of our specimen, are often found in rocks associated with sulphide minerals such as pyrite. Mining exposes these rocks to the weather and rain leaches these metals due to the action of sulphuric acid produced from the sulphides in the rock. Eventually these toxic substances find their way into surface waters where the acid releases more metals from exposed rocks. This process, known as acid mine drainage (AMD) is the most persistent form of water and soil pollution caused by mining [11]. South-America, especially Peru and Chile, have numerous mines that have been exploited for centuries. Gold, copper, lead, zinc, manganese, antimony, bismuth, molybdenum, tin and mercury mines have all contributed to AMD and consequent water and soil pollution. Thus it is not inconceivable that soil containing large amounts of heavy metals should be widely distributed in South-America. For bacteria to survive in such toxic surroundings they must evolve mechanisms to protect their metabolism. One such mechanism is concentrating the toxic elements in their bio-films or extra cellular polymeric substances (EPS) where they can do no harm. This mechanism of survival has been found for Ralstonia mettalidurans and for numerous other bacteria that thrive in highly toxic sludges concentrating for example lead and cadmium. EPS from pure bacterial cultures are composed of proteins and low quantities of polysaccharides and uronic acid [12]. But those extracted from the same bacteria living in activated sludges, for example, contain high levels of cadmium, lead and nickel [13]. The concentration of these elements varies with the complex- ation power of EPS [14,15]. Bacteria have also been used for the clean-up of toxic refuse and contaminated soils where their activity removes the toxic elements and stores the offending substances in their EPS where they can do no harm [13]. The extraordinary tissue preservation we found in our specimen is also consistent with a complementary explanation, however. The presence of a ‘wax-like’ substance filling the skull indicates that anthropogenic treatment of the head took place sometime after death. This substance, whose composition has not been ascertained, was poured into the skull and was also rubbed onto the skin and possibly hair for perhaps the specific purpose of preserving the tissues. Toxic elements of the kind present in the scalp hair (mainly lead) were identified, by micro-analysis, also in the deepest levels of the solidified ‘wax-like’ material within the skull. The combined actions of the embalming substances and of soil bacteria, which concentrated the toxic elements into their EPS, may account for the superb quality of mummification of the head. A South-American Head Figure 3. Scanning electron microscopy appearance of the ‘wax-like’ material filling the skull: (a, b) low power and higher power; (c) micro-analytical spectrum showing accumulation of barium sulphate and lead. doi:10.1371/journal.pone.0002053.g003 We conclude that, under certain circumstances, bacterial activity through the complexation of toxic elements in EPS may preserve tissues and we present supporting evidence for this conclusion. Our results have implications for forensic investigations. Hair is often subjected to careful analyses after death to exclude criminal intent. But consideration of bacterial activity as a cause of accumulation of toxic elements in the tissues has not, hitherto, been considered important. Our findings suggest that bacterial activity rather than metabolic concentration of toxic elements in hair may PLoS ONE | www.plosone.org 4 April 2008 | Volume 3 | Issue 4 | e2053