Bacterial Host Specificity

Bacterial Host Specificity

The bacterial host range of phage is generally narrower than that found in the antibiotics thathave been selected for clinical applications. Mostphage are specific for one species of bacteria andmany are only able to lyse specific strains within a species. This limited host range can be advantageous, in principle, as phage therapy results in less harm to the normal body flora and ecology than commonly used antibiotics, which often disrupt the normal gastrointestinal flora and result in opportunistic secondary infections by organisms such as Clostridium difficile. The potential clinical disadvantages associated with the narrow host range of most phage strains is addressed through the development of a large collection of well-characterized phage for a broad range of pathogens, and methods to rapidly determine which of the phage strains in the collection will be effective for any given infection.

Advantages over Antibiotics

Phage therapy can be very effective in certainconditions and has some unique advantages overantibiotics. Bacteria also develop resistance tophages, but it is incomparably easier to developnew phage than new antibiotic. A few weeks versus years are needed to obtain new phage for new strain of resistant bacteria. As bacteria evolve resistance, the relevant phages naturally evolve alongside. When super bacterium appears, the super phage already attacks it. We just need to derive it from the same environment. Phages have special advantage for localized use,

Because they penetrate deeper as long as theinfection is present, rather than decrease rapidlyin concentration below the surface like antibiotics. The phages stop reproducing once as the specific bacteria they target are destroyed. Phages do not develop secondary resistance, which is quite often in antibiotics. With the increasing incidence of antibiotic resistant bacteria and a deficit in the development of new classes of antibiotics to counteract them, there is a need to apply phages in a range of infections.

Lytic phages are similar to antibiotics in thatthey have remarkable antibacterial activity. However, therapeutic phages have some advantages over antibiotics, and phages have been reported to be more effective than antibiotics in treating certain infections in humans and experimentally infected animals. For example, in one study, Staphylococcus aureus phages were used to treat patients having purulent disease of the lungs and pleura. The patients were divided into two groups; the patients in group A (223 individuals) received phages, and the patients in group B (117 individuals) received antibiotics. Also, this clinical trial is one of the few studies using phage administration (48 patients in group A received phages by injection). The results were evaluated based on the following criteria: general condition of the patients, X-ray examination, reduction of purulence, and Microbiological analysis of blood and sputum. No side effects were observed in any of the patients, including those who received phages intravenously. Overall, complete recovery was observed in 82% of the patients in the phage-treated group as opposed to 64% of the patients in the antibiotic-treated group. Interestingly, the percent recovery in the group receiving phages intravenously was even higher (95%) than the 82% recovery rate observed with all 223 phage-treated patients.

Therapeutic Use of Phages and Antibiotics

Antibiotics target both pathogenic microorganismsand normal micro flora. This affects the microbialbalance in the patient, which may lead to serioussecondary infections.

High specificity may be considered to be adisadvantage of phages because the disease-causing bacterium must be identified before phage therapy can be successfully initiated. Antibiotics have a higher probability of being effective than phages when the identity of the etiologic agent has not been determined. Replicate at the site of infection and are thus available where they are most needed.

They are metabolized and eliminated from thebody and do not necessarily concentrate at thesite of infection. The “exponential growth” of phages at the site of infection may require less frequent phage administration in order to achieve the optimal therapeutic effect. No serious side effects have been described. Multiple side effects, including intestinal disorders, allergies, and secondary infections (e.g., yeast infections) have been reported.

A few minor side effects reported fortherapeutic phages may have been due to the

liberation of endotoxins from bacteria lysed invivo by the phages. Such effects also may be observed when antibiotics are used. Phage-resistant bacteria remain susceptible to other phages having a similar target range.

Resistance to antibiotics is not limited totargeted bacteria. Because of their more broad-spectrum activity, antibiotics select for many resistant bacterial species, not just for resistant mutants of the targeted bacteria. Selecting new phages (e.g., against phage-resistant bacteria) is a relatively rapid process that can frequently be accomplished in days or weeks. Developing a new antibiotic (e.g., against antibiotic-resistant bacteria) is a time-consuming process and may take several years (16, 51).Evolutionary arguments support the idea thatactive phages can be selected against every antibiotic-resistant or phage-resistant bacterium bythe ever-ongoing process of natural selection.

(2)     PROTOPLASMIC FUSION

Somatic fusion, also called protoplastfusion, is a type of genetic modification in plants by which two distinct species of plants are fused together to form a new hybrid plant with the characteristics of both, a somatic hybrid. Hybrids have been produced either between different varieties of the same species (e.g. between non-flowering potato plants and flowering potato plants) or between two different species (e.g. between wheat triticum and rye secale to produce Triticale).

Uses of somatic fusion include

Making potato plants resistant to potato leaf roll disease. Through somatic fusion, the crop potato plant Solanum tuberosum – the yield of which is severely reduced by a viral disease transmitted on by the aphid vector – is fused with the wild, non-tuber-bearing potato Solanum brevidens, which is resistant to the disease. The resulting hybrid has the chromosomes of both plants and is thus similar to polyploid plants. Somatic Hybridization was first introduced by Carlson in Nicotiana ‘glauea’ Process for plant cells

The somatic fusion process occurs in four steps:

·        The removal of the cell wall of one cell of each type of plant using cellulose enzyme to produce a somatic cell called a protoplast

·        The cells are then fused using electric shock (electrofusion) or chemical treatment to join the cells and fuse together the nuclei. The resulting fused nucleus is called heterokaryon.

·        The somatic hybrid cell then has its cell wall induced to form using hormones

·        The cells are then grown into calluses which then are further grown to plantlets and finally to a full plant, known as a somatic hybrid. Different from the procedure for seed plants describe above, fusion of moss protoplasts can be initiated without electric shock but by the use of polyethylene glycol (PEG). Further, moss protoplasts do not need phytohormones for regeneration , and they do not form a callus.

Instead, regenerating moss protoplasts behave likegerminating moss spores. Of further note sodium nitrate and calcium ion at high pH can be used, although results are variable depending on the organism.

Applications in animal cells

Somatic cells of different types can be fusedto obtain hybrid cells. Hybrid cells are useful ina variety of ways, e.g.,

Ø to study the control of cell division andgene expression,

Ø to investigate malignant transformations ,

Ø to obtain viral replication,

Ø for gene or chromosome mapping and for

Ø Production of monoclonal antibodies byproducing hybridoma (hybrid cells between an immortalized cell and an antibody producing lymphocyte), etc.

(3)     MITOCHONDRIAL INHERITANCE IN YEAST

Mitochondria are a central platform for diverse cellular functions including respiration, metabolite biosynthesis, ion homeostasis and apoptosis. Mitochondrial DNA (mtDNA), which encodes subunits of the oxidative phos-phorylation (OXPHOS) complexes, is pivotal for ensuring functional mitochondria and has an active role in determining phenotypic diversity and fitness in animals and budding yeasts owing to sophisticated interactions between the mitochondrial and chromosomal genomes.

Mitochondrial partitioning during mitosis in yeast (Saccharomyces cerevisiae).Extra-nuclear inheritance or cytoplasmic inheritance is the transmission of genes that occur outside the nucleus . It is found in most eukaryotes and is commonly known to occur in cytoplasmic organelles such as mitochondria and chloroplasts or from cellular parasites like viruses or bacteria.Mitochondria are organelles which function to produce energy as a result of cellular respiration. Chloroplasts are organelles which function to produce sugars via photosynthesis in plants and algae. The genes located in mitochondria and chloroplasts are very important for proper cellular function, yet the genomes replicate independently of the DNA located in the nucleus, which is typically arranged in chromosomes that only replicate one time preceding cellular division. The extranuclear genomes of mitochondria and chloroplasts however replicate independently of cell division. They replicate in response to a cell’s increasing energy needs which adjust during that cell’s lifespan. Since they replicate independently, genomic recombination of these genomes is rarely found in offspring contrary to nuclear genomes, in which recombination is common. Mitochondrial disease are received from the mother, sperm does not contribute for it.

Three general types of extranuclear inheritance exist.

·        Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell’s organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells.

·        Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the maternal inheritance of human mitochondria. The mother’s mitochondria are transmitted to the offspring at fertilization via the egg. The father’s mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father’s mitochondrial genome is found in offspring. Chloroplast genes can also inherit uniparentally during Sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. Inheritance from species to species differs greatly and is quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species.

·        Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the yeast Saccharomyces cerevisiae. When two haploid cells of opposite mating type fuse they can both contribute mitochondria to the resulting diploid offspring.

References

Birky W. Jr. (1994). “Relaxed and stringent genomes: why cytoplasmic genes don’t obey Mendel’s laws”. Journal of Heredity 85 (5): 355–366.

Helgeson JP; Hunt GJ; Haberlach GT and Austin S (1986). “Somatic hybrids between Solanum brevidens and Solanum tuberosum: expression of a late blight resistance gene and potato leaf roll resistance”. Plant Cell Rep. 5 (3): 212–214. doi:10.1007/BF00269122 .

Patrick Duff (1996). “HIV infection in women”. Primary Care Update for OB/GYNS 3 (2): 45–49. doi :10.1016/S1068-607X(95)00062-N.

Sangeeta Jain, Nima Goharkhay, George Saade, Gary D. Hankins & Garland D. Anderson(2007). “Hepatitis C in pregnancy”.American Journal of Perinatology 24(4): 251–256. doi:10.1055/s-2007970181.