2: Bacterial structure and taxonomy

Chapter 2 Bacterial structure and taxonomy

Classification of all living beings, including microbes has been attempted by many over centuries (Table 2.1). Traditionally, though they were all classified into two kingdoms, plants and animals, classification was arbitrary and based on morphological and growth characteristics. With the development of novel techniques, the latter classification was expanded to include five kingdoms: monera, protista, plantae, fungi and animalia. However, the current understanding based on their genetic relatedness is that all forms of life fall into three domains: Archaea, Bacteria and Eucarya. The main differences among Archaea, Bacteria and Eucarya are listed in Table 2.2. Note that taken together, Archaea and Bacteria are also known as prokaryotes (see below).

Table 2.2 Major differences among the three domains of life

Bacteria Archaea Eucarya
Organization of the genetic material and replication
DNA free in the cytoplasm DNA free in the cytoplasm DNA is contained with a membrane-bound nucleus. A nucleolus is also present
Only one chromosome Only one chromosome More than one chromosome. Two copies of each chromosome may be present (diploid)
DNA associated with histone-like proteins DNA associated with histone-like proteins DNA complexed with histone proteins
May contain extrachromosomal elements called plasmids Plasmids may be found Plasmids only found in yeast
Introns not found in mRNA Introns not found in most genes Introns found in all genes
Cell division by binary fission – asexual replication only Reproduce asexually and spores are not found Cells divide by mitosis
Transfer of genetic information occurs by conjugation, transduction and transformation (see Chapter 3) Processes similar to bacterial conjugation enables exchange of genetic material Exchange of genetic information occurs during sexual reproduction. Meiosis leads to the production of haploid cells (gametes), which can fuse
Cellular organization
Cytoplasmic membrane contains hopanoids Membranes contain isoprenes Cytoplasmic membrane contains sterols
Lipopolysaccharides and teichoic acids found No lipopolysaccharides or teichoic acids found  
Energy metabolism associated with the cytoplasmic membrane   Mitochondria present in most cases
Photosynthesis associated with membrane systems and vesicles in cytoplasm   Chloroplasts present in algal and plant cells
Internal membranes, endoplasmic reticulum and Golgi apparatus present associated with protein synthesis and targeting
Membrane vesicles such as lysosomes and peroxisomes present
Cytoskeleton of microtubules present
Flagella consist of one protein, flagellin Contains flagella that derive energy from proton pumps Flagella have a complex structure with 9 + 2 microtubular arrangement
Ribosomes – 70S Ribosomes behave more like eucarya when exposed to inhibitors Ribosomes – 80S (mitochondrial and chloroplast ribosomes are 70S)
Peptidoglycan cell walls Cell walls lack peptidoglycan Polysaccharide cell walls, where present, are generally either cellulose or chitin

Viruses are not included in this classification as they are unique, acellular, metabolically inert organisms and therefore replicate only within living cells. Other differences between viruses and cellular organisms include:

Morphology

Structure

The structure of a typical bacterium is shown in Figure 2.2. Bacteria have a rigid cell wall protecting a fluid protoplast comprising a cytoplasmic membrane and a variety of other components (described below).

Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 2: Bacterial structure and taxonomy
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