Classification of Microbes and Pathogenicity
By: Pallavi Dikshit
Tags: Microbiology, Cells, Biology, Immunology, Pharmaceutical

To begin with, Microbes are diverse groups of organisms which not only include small microorganism but it has the widest range of the organism. For instance, it includes :

Unicellular groups(Archae, Eubacteria, Protista, some fungi and Chlorophyta).

Larger fungi (small organisms with multicellular structure)

And Viruses.

Microbial lineage and prokaryotic systematics

There are three main lineages of the microbial world namely-

Eubacteria

Archea

Eukarya

Hence, the Eubacteria and Archea combined considered as prokaryotes while the Eukarya or eukaryotes include viruses, protists, fungi, Chlorophyta.

Linnean system

Classification of organism into a suitable hierarchical group from higher to lower group is known as Linneaen classification or taxonomical. Thus the following is the classification where each level is described as a taxon(taxa) namely:

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

HOMOLOGS, ORTHOLOGS, PARALOGS AND ANALOGS

Homology: Homologous that means the same origin. Therefore sequence homology is the biological homology between DNA, RNA and protein sequences defined in terms of shared ancestry in the evolutionary history of life. It usually means sequence similarity which is given in terms of per cent homology.

Orthology: Homologous sequences are orthologous if they descend from the same ancestry separated by a speciation event. They are species from a single gene from the last common ancestor.

Paralogy: Paralogous genes related to a duplication event. Thereby the genes which are the results from the mutation of duplicated genes and are completely products of separate speciation events.

Analogy: Analogous means having a different origin but the same function.

PROKARYOTES

Prokaryotes mean pro that is old and similarly karyon that means nucleus. Thus nucleus without a membrane and that is free-floating in the cytoplasm. Also, chromosomes are haploid, single and circular, complexed with histone-like protein. Since Ribosomes are smaller with 70s Svedberg’s unit. The cell division in prokaryotes is by binary fission or budding and the transfer of the genetic material might occur by any of the below methods such as conjugation, transformation, transduction.

Also, Flagella in prokaryotes are made of a single protein.

Cytoplasmic membranes- The prokaryotic cytoplasmic membrane is present between the cytoplasm and cell wall. It is a lipid bilayer made up of phospholipids. Some proteins do not pass through the membrane, some selective proteins sometimes pass through it and some proteins are embedded in the membrane. Although the cytoplasmic membrane does provide any kind of strength and rigidity to the cell, the membrane does some checks in the entry of a few particles into and out of the cell. The passage of gases is mostly through simple diffusion, however, water-soluble ions like sodium ion and potassium ion and water pass through the membrane by osmosis but some molecules move into and out of the cell only through an expenditure of energy by active transport. Thus the bacterial cell is reinforced with sterols instead of hopanoids that stabilise the eukaryotic cell.

Cytoplasm- Only 20% of the cytoplasm is constituting protein, lipids, nucleic acid, carbohydrate, inorganic ions etc and then the major remaining part is completely water. Since a prokaryotic is not the advanced version of the cell, it does not have endoplasmic reticulum and therefore ribosomes are also are found free in the cytoplasm. Furthermore, ribosomes are 70s Svedberg’s unit when undergoes sedimentation.

Therefore bacterial cell consists of some other components floating in the cytoplasm such as :

Inclusion bodies Carboxysomes Magnetosomes Phycobilism Mesosomes

Inclusion bodies :

Small concentrated stores of a certain substance in the cytoplasm, some of them are:

Volutin granules

Poly Beta-hydroxybutyrate granules

Glycogen granules

Sulphur granules

The genetic material in a prokaryotic cell is a free-floating genome uncovered, which includes two types of DNA present such as-

->Chromosomal DNA

->Extra chromosomal DNA

Structure of the cell

Cell wall-Hard outer covering of the cell, residing beneath a capsule or sheath and flagella. It maintains the cell shape and rigidity as well as also prevents any mechanical shock to the cell any also prevents it from bursting under hypotonic conditions.

External Structure

Flagella: Flagella are thin, hair-like helical protruding appendages responsible mostly for movement. Consists of three parts; Basal body, hook and filament.

Pilli or Fimbria: Pilli is non-helical, thinner and shorter, more in number on the surface of the cell. Motility is not their function but some pilus like the F. pilus is a sex pilli used for the exchange of genetic material during mating.

Slime layer or capsule: Capsules surround the cell wall and the chemical composition includes; homopolysaccharide, heteropolysaccharide and polypeptide (sometimes).

There are very vital functions of the capsule layer-

Prevent desiccation Blocks bacteriophage attachment Antiphagocytic Promotes attachment to the bacterial surface.

Spores or Cysts

Spores or cysts are thick-walled, highly refractile bodies as a defence mechanism in a harsh or extreme environment(heat, chemicals, radiation). Two types of spores formed mainly such as- Exospores and endospores.

Classification of bacteria on a morphological basis:

Based on shape:

Coccus- If the shape of the bacteria is spherical.

Diplococcus- cells divide in a plane and always pair up.

Streptococcus- cells divide in a plane and forms a chain.

Tetracoccus- cells divide into two planes and stays in a group of four.

Staphylococcus- cells divide in three places and form non-uniform bunches.

Sarcinae- cells divide into three planes and form a cubical arrangement.

2. Bacilli- Rod-shaped bacteria

Diplobacilli- cells divide into one plane and form pairs

Streptobaccili- cells divide in a plane and remain in chains

Palisade arrangement- cells divide into two planes and line up side by side.

3. Spirilla- Helical shaped bacteria.

4. Pleomorphic- Bacteria exhibiting different shapes.

Based on flagella arrangement on the surface of the cell.

Monorichous- If flagella are present at one end. Amphitrichous- If flagella are present at both ends. Petritrichous- If flagella are present all over the surface of the bacterial cell. Lipotrichous- If flagella grow from a single point in a cluster.

Growth

Growth is mostly by binary fission or budding. In addition without the presence of mitotic spindle, mesosomes help in duplication of a cell. As an example growth in bacteria shown by the increase in number than in size.

Therefore various phases of growth in bacterial cell -

Lag phase Log phase Stationary phase Death or decline phase

Lag phase — In culture, there is no immediate increase in population and the growth is stationary.

Log phase — The growth curve increases simultaneously giving rise in the population.

Stationary phase — The nutrient is getting exhausted hence the number of deaths is in equilibrium with that of the number of cells added to the population.

Death phase — The population drops due to exhaustion of nutrients and increase in toxic by-products of metabolism.