Antibiotic resistance gene pool and bacterial adaptation to xenobiotics in the environment

Abstract

Extreme environments are habitats that experience steady or fluctuating exposure to one or more environmental factors, such as salinity, osmolarity, desiccation, UV radiation, barometric pressure, pH, temperature, heavy metals, xenobiotics and antibiotics. The evolutionary biologist studies the steps by which the adaptations have evolved. But the general nature of such adaptive steps is still unclear. Evolution is often thought to be random and dependent on unpredictable events. This chapter focuses on antibiotic and xenobiotic stress in the environment and the microbial adaptations along with the genetic regulation of these stresses. Bacteria have come up with sophisticated modes of cooperative behaviour to cope with adverse and varying environmental conditions. They developed intricate communication capabilities, including a broad repertoire of chemical signaling mechanisms, collective activation and deactivation of genes and even exchange of genetic materials. With these tools, they can communicate and self-organize their colonies into multicellular hierarchical aggregates, out of which new abilities emerge. Many examples of bacterial mechanisms are thought to be adaptations for survival in changing environments, some of which are the mutator phenotypes, sporulation, adaptive mutation, and phase variation. The most frequently proposed hypothesis is that genes involved in antibiotic resistance originated in antibiotic-producing organisms, as part of the cluster involved in antibiotic biogenesis, to prevent self-inhibition. Eventually, these genes may have moved to the neighboring bacterial organisms, which then became resistant. Alternatively, these neighboring organisms may have introduced changes in the DNA sequence of possibly duplicate genes involved in functions that were thus reoriented to antibiotic elimination or detoxification. Recent studies have revealed the existence of new types of xenobiotic catabolic mobile genetic elements, such as catabolic genomic islands, which integrate into the chromosome after transfer. Molecular analysis of the catabolic pathways of xenobiotic-degrading bacteria indicated that they might have adapted to the appearance of such compounds by expressing new functions to resist the potential toxic effects of the molecules or to use their beneficiary characteristics, for example, as an alternative source of essential nutrients, such as carbon, nitrogen or energy.