Heavy Metal Stress

1. Introduction

Bacteria have evolved several different mechanisms to enable them adapt successfully to their environment. All organisms have intrinsic genetic determinants that are expressed when challenged with different stress factors. Examples of such environmental stressors include antibiotics, chemicals, ionizing radiation, high salinity, extreme hydrostatic pressure, extreme temperatures and heavy metal stress, among others. When faced with single or multiple challenges, a series of transcriptional networks are upregulated to code for determinants that counteract such adverse conditions (1). One of the bacterial stress factors that is increasingly being studied is heavy metal stress. Several genetically encoded players have enabled bacteria

Although metals like zinc, iron, manganese and copper are essential for bacterial metabolism (3), their intracellular concentrations must be carefully regulated as their imbalances will be detrimental to cellular integrity. Metals like iron, arsenic, copper, chromium, cadmium, selenium, uranium and several other heavy metals pose significant toxicity to bacteria. For example, they promote oxidative stress and produce reactive oxygen species (ROS) that damage DNA causing mutations and maybe cell death, even in smaller concentrations (4, 5). To protect themselves from metal intoxication, bacteria rely on several mechanisms, including the use of efflux pumps that extrude metals, use of proteins that change the oxidation states of metals, and proteins that bind metals either intracellularly or extracellularly and reduce their toxic effects (6). In addition, mobile genetic elements also play an important function in bacterial response to different environmental challenges, including heavy metal stress (7,

It is considered a life-saving mineral by the united nations as it is both of biologic and public health importance. Natural levels of zinc without human influence typically ranges between 10 and 300mg/kg in soil, rock and rivers, and has an average of 70mg/kg in the Earth’s crust. Certain locations have high concentrations of zinc due to the natural process of erosion that transports zinc to such areas, as well as the influence of human activities that propagate its release to the environment www.zinc.org/environment/. For instance, activities like mining, the use of zinc in industry, and its application in agriculture and medicine (24). Environments that have either too low or very high concentrations of zinc have detrimental effects to living organisms. Living organisms are known to have evolved in the presence of natural levels of zinc and this plays an essential role in many biological pathways. For instance, many bacterial enzymes have zinc in their active or structurally important sites (25). They have evolved different mechanisms to maintain a constant desirable internal zinc concentration range, despite fluctuations in their external environment. This element that is important for all physiological processes has also for long been considered a drug. For example, it has long been used to boost the immune system in elderly