Recent trends in microbial taxonomy
- Molecular method
- Numerical taxonomy
- Genetic method
- Serological method
- Taxonomy based on ecology
chemotaxonomy is the classification of bacteria based on chemical composition of cells and fermentation product which is analyzed by the various method or techniques like chromatography, electrophoresis.
Major chemical compounds are:-
- Cell wall composition
- Lipid composition
- Isoprenoid quiniones
- Cytochrome composition
Cell wall composition
- The variations in amino acids and sugar are the constituent of cell wall analysis, comes under the cell wall composition.
- The gram positive bacterial cell wall is composed of peptidoglycan, techoic acid and some bacteria like mycobacteria have mycolic acids.
- The gram negative bacterial cell consists- lipoprotein layer, outer membrane, lipopolysaccharides, periplasmic space, peptidoglycan.
- Lipids occur in the cytoplasmic membrane in all eubacteria.
- The eubacterial lipids belong to a number of different classes and atleast some of these lipids has taxonomic value. Like, mycolic acid have been found so far only in the taxa bacterionema, micropolyspora, nocardia etc.
- phospholipids are present in many bacteria but certain actinomycetes contain very characteristics phospholipids:-
- The phosphotidyl inositol mannosides.
- Phosphosphingolipids/glycolipids(gram negative)or glycosyl diacylglycerols(gram positive).
- It is a class of terpenoid lipids located in the cytoplasmic membrane of many bacteria.
- They play an important role in ETC, oxidative phosphorylation and active transport.this isoprenoid quiniones are of three types:-
Archebacteria contains menaquinones, cyanobacteria contains phyllaquinones, lactobacillus lack quiniones.
- cytochromes are membrane bound iron-containing hemeproteins that contain heme groups and carry out electron transport for generation of ATP and redox process.
- Five major type of cytochrome in bacteria are- a, b, c, d, o.
- gram positive bacteria contain cytochrome-bcaa3o, facultative anaerobic gram positive cytochrome is absent, clostridium lackd cytochromes, and gram negative bacteria-cytochrome bdoa1.
Study of proteins and nucleic acid has become increasingly important in prokaryotic taxonomy.
Some molecular methods are:-
- DNA homology
- DNA-RNA homology
- G+C ratio
- rRNA sequencing
- DNA homology experiments are generally used for detecting the similarity between closely related organisms.
- This experiment is designed to find out the similarity of the base sequence of this nucleic acid to those of other related organism.
- If the base sequences in the DNA of the organisms are sufficiently similar, hybridization of DNA is possible.
- If DNA molecules are very different in sequence; they will not form a stable, detectable hybrid, so more distantly organism can be compared by carring out DNA-RNA hybridization.
- Technique is similar to DNA hybridization, membrane bound DNA is incubated with radioactive rRNA, washed and counted.
- The percent of GC in DNA reflects the base sequence and varies with sequence changes.
- G+C is paired with 3-H bonds and therefore takes more heat to separate in single strandes DNA whereas A+T is paired with 2-H bond.
- GC content in prokaryotes is most variable ranging from around 25-80%.
- If the two organisms differ in their GC content by more than about 10% their genomes have quite different base sequence.
- It is the powerful and direct method for compairing genomes. Sequences of 16s and 18s ribosomal RNA are most often used in phylogenetic studies.
- Complete analysis of 16s rRNA sequence has revealed the presence of signature sequences short oligonucleotides unique to certain groups of organisms and useful in their identification.
- The development of computer has made possible the quantitative approach known as numerical taxonomy.
- The process begins with a determination of the presence or absence of selected characters in the group of organisms under study.
- Classification in numerical taxonomy is expressed in terms of a similarity coefficient found between two compared strains.
- Greater similarity coefficient, closer relatedness is inferred. Greater than ~70% and inference is that two compared bacteria are of the same species.
The genetic method used in bacterial classification are:-
- It is a mode of gene transfer in bacteria, in which a piece of free DNA is taken up by a bacterial cell and integrated into the recipient genome.
- Transformation of chromosomal DNA is a good indication of relatedness between different species and sub-species.
The form of gene transfer and recombination in bacteria that requires direct cell to cell contact.
The donor cell makes contacts with recipient cell conjugation can be done by three ways:-
- F- and F+ conjugation
- Hfr conjugation
- F’ conjugation
- Transduction is the transfer of bacterial genes by viruses. Bacterial genes are incorporated into a phage capsid.
- The virus containing these genes then injects then into another bacterium completing the transfer.
- The high specificity of serological reactions is determined by the chemical nature of the antigen, and it is possible to detect differences between complex molecules, particularly proteins, which cannot yet be distinguished by chemical analysis.
- Serology is therefore a delicate tool for comparing and contrasting antigenic components of the microbial cell, providing information of use both in identification and classification.
Taxonomy based on ecology
- Many properties are ecological in nature since they affect the relation of microorganisms to their environment. Very closely related microorganism can differ considerably with respect to ecological characteristics.
- Microorganisms living in various parts of the human body markedly differ from one another and form those growing in freshwater, terrestrial and marine environments.
Recent trends in microbial taxonomy