Bacteria are single-celled organisms falling within the Prokaryota grouping. The deterioration of concrete in contact with bacterial communities is principally through chemical reactions between cement and aggregate and substances produced by these organisms. However, it is of benefit to discuss the metabolism and reproduction of these organisms first, since these aspects have important implications with regards to interactions between bacteria and concrete. Throughout the process of growth bacteria will be dying, and a stationary phase is eventually entered in which the rate of reproduction equals the death rate. In evaluating the surface of concrete as a habitat for bacteria, it becomes evident that, whilst many aspects of concrete offer a welcoming environment, there are other aspects which present a challenge to colonization. One of the more extreme characteristics of concrete—at least in its early life—is the very high pH that water takes on when in contact with its surface or when present within its pores.