These naturally occurring DNA molecules are circular, and they can replicate inside a bacterium independent of the bacterial chromosome . Such pathogens are usually diagnosed by the detection of specific antibodies in conjunction with the assessment of clinical symptoms or the molecular detection of specific DNA sequences. Pathogenicity islands are gene clusters incorporated in the genome, chromosomally or extrachromosomally, of pathogenic organisms, but are usually absent from those nonpathogenic organisms of the same or closely related species.

The accessory virulence factor neuraminidase is encoded by VPI-2. Neuraminidase is secreted and converts oligosaccharides by cleavage of sialic acid. This reaction results in a larger number of ganglioside GM1-like molecules on host cell surfaces, which serve as binding sites for the cholera toxin. However, this process is not absolutely required for the action of cholera toxin . The SaPI family of Staphylococcus aureus pathogenicity islands, mobile genetic elements, encode superantigens, including the gene for toxic shock syndrome toxin, and are mobilized at high frequencies by specific bacteriophages. Pathogenic bacteria have adapted both to saprophytic or free-living states, possibly environments outside the body, and to the human host.

Four of these orfs demonstrated low homology to hypothetic proteins from Haemophilus influenzae; the other four orfs have no known function. Flexneri 2a chromosomal island are from the left end of Mu (98% identity) and total 2.9 kb of the 36.7-kb Mu genome. This region contains the transposase gene and the repressor c gene, which is responsible for repressing the lytic cycle, thus allowing the phage to integrate into the host chromosome. Given that only a small part of the Mu genome is present in the S. Flexneri 2a island, and there are numerous IS elements in this region, significant rearrangement appears to have taken place after the insertion of Mu.

Indication of chromosomal integration of horizontally acquired DNA, sequence analysis of 280 kb of genomic islands coding for important fitness factors, and comparison of Nissle 1917 genome content with that of other E. Coli strains by DNA-DNA hybridization. backdrop curtains PCR-based screening of 324 nonpathogenic and pathogenic E. Coli isolates of different origins revealed that some chromosomal regions are frequently detectable in nonpathogenic E. Coli and also among extraintestinal and intestinal pathogenic strains.

Although the majority of the genes of the flexible gene pool confer selective advantages to their bacterial recipients, a few represent selfish DNA only promoting their own spread. The latter elements are insertion elements, particularly prophages and restriction/modification systems. PAI are part of that flexible gene pool.

Presumably, motility is not important in the pathogenesis of the diseases caused by these bacteria. Y enterocolitica is motile when grown at 25°C but not when grown at 37°C. Similarly, Listeria is motile when grown at 25°C and not motile or minimally motile when grown at 37°C. Expression of virulence genes of B pertussis is enhanced when the bacteria are grown at 37°C and suppressed when they are grown at lower temperatures or in the presence of high concentrations of magnesium sulfate or nicotinic acid. Many bacteria are transmitted from one person to another on hands.

In this regard, the designation “islet” (e.g., pathogenicity islet or genomic islet) has been used for virulence gene clusters not fully complying with the PAI definition because of being less than 10 kb . Nevertheless, low-G+C content, remnants of bacteriophage genes, or association with mobile genetic elements or tRNA genes may identify them as PAI or as ancestral PAI which have undergone genetic modification and immobilization. For example, resistance islands refer to genomic islands that provide bacterial resistance to antibiotics, while metabolic islands confer the ability to use new carbon sources. Thus, a pathogenicity island enables bacteria to induce disease. Coli from the intestinal tract of healthy people can be considered commensal, as some isolates showed up to five pathogenicity islands.