Explain how changes in a bacterial genome could cause a nonpathogenic bacterium

Question

Explain how changes in a bacterial genome could cause a nonpathogenic bacterium to develop into a pathogen. 49. 50. Autoclaves use high temperatures and steam to sterilize surgical instruments. When instruments are sterilized they do not contain any bacteria. Explain how Bacillus stearothermophilus, a bacterium within the phylum Firmicutes, can be used to test sterilization within autoclaves. 51. Describe at least one similarity and one difference between the causative agents of syphilis and yaws.

Explanation / Answer

49.

Horizontal gene transfer is a process which refers to the incorporation of genetic elements transferred from a donor organism directly into the genome of the recipient organism, where they form genomic islands—that is, blocks of DNA which contain mobile genetic elements. Genomic islands may contain large blocks of virulence determinants (adhesins, invasins, toxins, protein secretion systems, antibiotic resistance mechanisms, etc), and thus are referred to as pathogenicity islands. Pathogenicity islands were first described in pathogenic species of E coli, but have since been found in the genomes of numerous bacterial pathogens of humans, animals, and plants (Salmonella, Vibrio, Shigella, Yersinia, Listeria, S aureus, etc).1,74 Pathogenicity islands consist of large regions of genomic DNA (approximately 10–200 kilobases) that are present in pathogenic bacterial strains but absent from the genomes of non-pathogenic members of the same or related species. Pathogenicity islands are believed to have been acquired as a block by horizontal gene transfer because (a) their G+C content is significantly different from that of the genomes of the host micro-organism(b) they are often flanked by direct repeats; (c) they are often associated with tRNA genes; (d) they are associated with integrase determinants and other mobility loci; and (e) they exhibit genetic instability.

It is important to note that, in addition to pathogenicity islands, plasmids and bacteriophages can also be transferred horizontally. Indeed, all three mechanisms for genetic exchange or transfer between bacteria (that is, transformation, transduction, and conjugation) appear to be important for the evolution of pathogenic species. The determination and analysis of the complete genomic sequences of several important bacterial pathogens have led to the revelation that horizontal gene transfer may be much more extensive than previously appreciated. Accordingly, a more comprehensive understanding of the evolution of bacterial pathogens will be required to elucidate the virulence mechanisms of emerging and re-emerging infectious diseases, as well as changes in virulence and drug resistance associated with these infections, so that effective diagnostic and therapeutic strategies may be developed.

50.

51. Syphillis is caused by Treponema pallidum subspecies pallidum. Yaws is caused by Treponema pallidum pertenue. Both are spiro chaete bacteria. The treponemes have a cytoplasmic and an outer membrane. Using light microscopy, treponemes are visible only by using dark field illumination. They are Gram negative, but some regard them as too thin to be Gram stained.

•           All autoclaves used for infected waste decontamination should be regularly monitored for effectiveness in three areas: mechanical, thermal, and biological performance

•           Autoclave indicator tape indicates that the outside of the container came to temperature, it does not reflect time or conditions inside the load and does not prove effective decontamination

•           General mechanical maintenance will be conducted on an annual basis by Facilities Services or as recommended by the manufacture

•           Research autoclaves should be tested quarterly with a spore indicator for biological effectiveness and the results documented by user (See Autoclave Validation Policy).

o          Spore indicators should be placed at the slowest point of heating (e.g. the center of the load)

o          For researchers who autoclave infectious waste, it is suggested that a simulated load of typically autoclaved materials which is not infectious/contaminated with a spore indicator be run each quarter as a validity check. This load can be safely dissembled after autoclaving to check the spore indicator without risk of infectious exposure.

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