Inactivation of microorganisms by sunlight. The effect of solid particles

GUTIERREZ CACCIABUE, DOLORES; CID ALICIA; CRUZ, MERCEDES CECILIA; RAJAL, VERONICA

Torremolinos

Conferencia; IV International Conference on environmental, industrial and applied microbiology, BioMicroWorld 2011; 2011

Various diseases such as cholera, gastroenteritis, and different hepatitis, can be transmitted by contaminated water.Many microorganisms including bacteria, viruses, and protozoa are usually present in aquatic environments; some of them, pathogens, may be harmful for human health. They reach the water through feces, organic and inorganic particles, wastewater from industries, and other wastes produced by infected people. Numerous methods are used for treatment and disinfection of contaminated water to preserve people health and improve life quality. Disinfection by sunlight is a well known process that affects the survival of microorganisms in aquatic environments. However, this inactivation may not lead to a total removal of microorganisms due to different factors. Two main seasons: Wet Season (December-March) and Dry Season (April-November) can be differentiated in the Province of Salta, located in the northwest of Argentina. Rainfall during summer (Wet Season) increases the flow and the turbulence of rivers, resuspending small particles like clays that enhance the turbidity of the water. Turbidity refers to cloudiness of water and has no health effects itself but may indicate the presence of disease-causing organisms (EPA). These particles can interfere with sunlight disinfection providing a medium for microbial growth, increasing their persistence in the water column and becoming a potential health hazard because of the subsequent human ingestion.The aim of the present study was to evaluate the effect of solid particles on the inactivation of microorganisms when exposed to natural sunlight.Sediments from two aquatic environments (Campo Alegre Reservoir and Vaqueros River) were collected, dried and classified in different sizes in the laboratory. Water from the Wierna River was used to prepare two matrixes: one with a high concentration (5000 mg/l) of solid particles, diameter < 44 μm, (maximum turbidity in NTU) and another with no sediments (no turbidity in NTU). Each matrix was spiked with 106 UFC/ml of two bacteria: Escherichia coli (ATCC 25922) and Enterococcus faecalis (ATCC 29212). Cellulose dialysis tubing (M.W. 12,400 SIGMA) was cut into 10 and 20 cm-length pieces, which tied at the ends, were meant to contain the microcosms to simulate the aquatic environment. These “bags” where filled with both matrix and deposited into two glass containers (50×30×40 cm) containing also water from the Wierna River. One of the containers was exposed to the sunlight while the other was not; the latest was covered with black bags. Water from the glass containers was renewed every three days. Solar radiation was measured at every hour. During one month, samples were removed in duplicates from each container and taken to the laboratory for microbiological analysis. Escherichia coli (Method 1103.1-EPA, 2002) and Enterococcus (Method 1106.1-EPA, 2002), were determined by the Membrane Filtration Method using a three branch stainless steel manifold system (Sartorius, USA). Decay rates were calculated using a first order decay model (Ct=C0*e-kt).Results indicate that microorganisms, with and without particles, decay faster when they are exposed to sunlight compared to those who were shielded from it. Also in both cases, with and without sunlight, microorganisms survived longer in the presence of solid particles than in water without them. Finally, it was observed that Escherichia coli was more persistent than Enterococcus faecalis in the presence of solid particles, maybe because Escherichia coli has a better adherence to particles in freshwater than Enterococcus faecalis, which seems have more affinity with seawater.