**1. Introduction**

Water has a vital role in the world. In general, humans use it for their daily needs, for economic activities and recreation [1]. However, the different uses of water can become sources of pollution. The alteration of the physico-chemical and microbiological quality of water is sometimes the result of poor management by humans of waste and wastewater of domestic, agricultural and industrial origin [2]. The use of polluted water exposes populations to health risks. The medium- and long-term risks are linked to the poor chemical quality of the water, while the shortterm risks are biological in origin. The poor biological quality of water is partly due to the presence of protozoa, viruses or bacteria [3]. Several microbial communities

live in aquatic and soil environments, with physiologies that are sometimes similar in both types of biotope despite a significant difference in nutrient sources [4].

Bacteria are generally the most abundant microorganisms in nature and their unwanted presence in an environment can represent a health risk of varying degrees for humans. Indeed, the pathogenicity of normally harmless bacteria can occur as a result of the immunosuppression of a host. Bacteria are known to be responsible for water contamination in a community generally belong to the genera *Salmonella*, *Shigella*, *Escherichia*, *Yersinia*, *Vibrio* and *Campylobacter* [5, 6]. These bacteria can cause diarrhea, gastroenteritis and genitourinary infections in humans [7]. Their morphology and physiology in an aquatic environment vary with the general environmental conditions. The *E. coli* species belongs to the group of fecal coliforms or thermotolerant coliforms. The presence of these coliforms in water is generally a sign of the deterioration of its bacteriological quality, due to its contamination by other microorganisms that are strict pathogens or opportunistic pathogens [8–10]. For nearly a decade, numerous outbreaks attributed to pathogenic *E. coli* strains have been regularly reported worldwide [11–14]. In Ngoïla in the eastern region of Cameroon, from December 1997 to April 1998, 298 people came into contact with an epidemic of gastroenteritis caused by *E. coli* O157:H7 [15]. Contamination was by the oral route through ingestion of contaminated water or food.

In recent years, water disinfection methods using plant extracts have been proposed as a new alternative for household water treatment [16–18]. The use of plants for therapeutic purposes has been common practice for thousands of years [19]. However, little is known about the sensitivity of bacteria to these water extracts in the aquatic environment. There is still little information on the synergistic effect of the aqueous extract of *Eucalyptus microcorys* (*E. microcorys*) and light on planktonic cells of *E. coli* in the aquatic environment. Little information about the plant extracts of *Eucalyptus microcorys*, *Artemesia annua* and *Moringa oleifera* on *E. coli* bacterial cells is available. The present chapter summarizes the known effects of aqueous extract of the medicinal plants *Eucalyptus microcorys*, *Artemesia annua* and *Moringa oleifera* on the cultivability of *E. coli* in aquatic microcosm.
