Helium ion microscopy image showing T4 phage infecting E. coli. Some of the attached phage have contracted tails indicating that they have injected their DNA into the host. The<unk> l cells are ~ 0.5 µm wide. Most<unk>  have a single circular chromosome that can range in size from only 160,000 base pairs in the endosymbiotic<unk>  Carsonella ruddii, to 12,200,000 base pairs (12.2<unk> p) in the soil-dwelling<unk>  Sorangium cellulosum. There are many exceptions to this, for example some Streptomyces and Borrelia species contain a single linear chromosome, while some Vibrio species contain more than one chromosome.<unk>  can also contain plasmids, small extra-chromosomal molecules of DNA that may contain genes for various useful functions such as antibiotic resistance, metabolic capabilities, or various virulence factors.<unk>  genomes usually encode a few hundred to a few thousand genes. The genes in<unk> l genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in<unk> , these are much rarer than in eukaryotes.<unk> , as asexual organisms, inherit an identical copy of the parent's genomes and are clonal. However, all<unk>  can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations come from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of<unk>  and even among different clones of a single species of<unk> . Genetic changes in<unk> l genomes come from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate. Some<unk>  also transfer genetic material between cells. This can occur in three main ways. First,<unk>  can take up exogenous DNA from their environment, in a process called transformation. Many<unk>  can naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA. The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells. The second way<unk>  transfer genetic material is by transduction, when the integration of a<unk> phage introduces foreign DNA into the chromosome. Many types of<unk> phage exist, some simply infect and lyse their host<unk> , while others insert into the<unk> l chromosome.<unk>  resist phage infection through restriction modification systems that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the<unk>  have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. The third method of gene transfer is conjugation, whereby DNA is transferred through direct cell contact. In ordinary circumstances, transduction, conjugation, and transformation involve transfer of DNA between individual<unk>  of the same species, but occasionally transfer may occur between individuals of different<unk> l species and this may have significant consequences, such as the transfer of antibiotic resistance. In such cases, gene acquisition from other<unk>  or the environment is called horizontal gene transfer and may be common under natural conditions.
Bacteria
Yeast
Virus
Enteric bacteria 
Chyme
Microgenia
Microorganism
Mold