MORPHOLOGY AND NATURE OF BACTERIA

Prepared By Dr. Mwakagile

Presented by Prof. Fred Mhalu with some minor changes

• The basic living material is called protoplasm, and is organised in units known as cells.

• The protoplasm within a cell is called protoplast, and is enclosed by a thin semi-permeable membrane, the cytoplasmic or plasma membrane.

• The plasma membrane is usually enclosed by a relatively rigid cell wall. The major part of the protoplast is differentiated into the cytoplasm and the nucleus.

• Microorganisms are generally regarded as living forms that are microscopical in size and relatively very simple, usually unicellular.

• The smallest body a naked eye can see is about 100 m (0.1 mm) in diameter.

• Most of the microorganisms are smaller than this and therefore a microscope is necessary for their visualisation.

• Cells are usually divided into two distinctions: the relatively primitive cells called prokaryotes and the relatively advanced ones called eukaryotes.

• Prokaryotes include the so-called “true" bacteria (eubacteria, or simply bacteria) and archaebacteria, or simply Archaea).

• Eubacteria include all bacteria that infect humans, while archaebacteria are bacteria that are free-living in the environment.

• All other cellular forms of life (including plants, animals, man, and fungi) are referred to as eukaryotes.

Differences between prokaryotes and eukaryotes:

• 1. Unlike eukaryotic cells, the prokaryotic cells are not compartmentalized. Nuclear membranes, mitochondria, endoplasmic reticulum, Golgi body, phagosomes and lysosomes are not present.

• 2. Prokaryotes have a nucleus that is simple: has no nuclear membrane, nor a nucleolus, nor a spindle. It has one chromosome. It is therefore called a nucleoid.

• 3. Prokaryotic ribosomes are 70S (S stands for Svedberg unit, a measure of size), whereas eukaryotic ribosomes are larger (80S). Prokaryotic ribosomal subunits are 30S and 50S (eukaryotic are larger).

• 4. Bacterial membranes generally do not contain sterols (e.g. cholesterol).

• 5. Most bacterial cells have cell walls

• Bacteria can be seen in various characteristic shapes: spherical (coccus), rod-shaped (bacillus), comma-shaped (vibrio), spiral (spirillum and spirochaetes) or filamentous (see diagrams).

• With respect to each other, cells can be seen in different arrangements:

:

FIGURE SHOWING The Typical shapes and arrangements

Components of bacterial cells include

THE CYTOPLASM

• It is the main part of the protoplast. It is a watery sap or soft gel packed with abundant small granules called ribosomes and a few convoluted membranous bodies called mesosomes.

• Also contains the nuclear body. It may also contain inclusion bodies or granules. It lacks mitochondria.

THE NUCLEUS

• The genetic information of a bacterial cell is contained in a single molecule of double stranded DNA that occurs in the form of a closed circle.

• This single molecule is referred to as the chromosome. It is about 1000 m (1 mm) long. There is no nuclear membrane or nucleolus.

• It replicates by growth and simple binary fission. The DNA can be transferred between bacteria in several different ways:

• transformation• transduction• conjugation

• Transformation involves the uptake of naked bacterial DNA across the cell wall. In transduction, DNA fragments are transferred from one bacterial cell to another by viruses called bacteriophages. Conjugation occurs when bacteria transfer DNA along a specialized hollow tube (‘sex pilus’) connecting the two cells.

THE RIBOSOMES

• These are smaller than those of eukaryotic cells (10-20 nm). Have a sedimentation coefficient of 70S (Svedberg units) being composed of a 30S and a 50S subunit (cf 80S: 40S +60S for eukaryotic cells).

• The code of the mRNA is translated into peptide sequences at the ribosomes. Therefore, ribosomes are the workbenches on which proteins are synthesized.

INCLUSION GRANULES

• Many species of bacteria have round granules in the cytoplasm. They are not permanent or essential structures of the cytoplasm.

• They are stores of nutrients. Volutin granules (metachromatic granules) are seen in Corynebacterium spp and contain polyphosphates.

• Lipid granules are seen in Bacillus spp. Polysaccharide granules may be found in some bacterial spp.

CYTOPLASMIC MEMBRANE

• This forms the enclosure of the protoplast. It is semi-permeable, permitting passive diffusion inwards and outwards of water and certain small molecules. It is the site of the respiratory enzymes and performs many of the functions carried out by membranous organelles in eukaryotes. Also called plasma or cell membrane. It is lipoprotein in nature.

THE CELL WALL

• This encases the protoplast and lies immediately external to the cytoplasmic membrane. It is thick, strong, rigid and porous. If the bacterium bursts because of weakening of the cell wall, it is said that the bacterium has undergone lysis. When the rupture occurs locally at some part of the cell wall, the process is called plasmoptysis. Plasmolysis is when a bacterium undergoes shrinkage because of being placed in a solution of very high concentration.

Functions:

• It supports the cytoplasmic membrane against the high internal osmotic pressure

• It maintains the characteristic shape of the bacterium.

• It plays an important role in cell division. A cross wall develops transversely across the bacterium separating two daughter cells.

• The chemical composition of the cell wall differs with bacterial spp, but in all spp the main strengthening component is a substance called mucopeptide or murein or peptidoglycan.

• Peptidoglycan is found only in bacterial cell walls and is synonymous to cellulose in plants and chitin in insects.

• The cell walls of most Gram-positive bacteria consist mainly of several layers ( about 40) of peptidoglycan. The layers are connected to each other by amino acid bridges. Apart from the layers of peptidoglycan, there are two more layers: teichoic acid and teichuronic acid.

• The cell walls of Gram-negative bacteria, on the other hand, contain a single or two layers of peptidoglycan.

• Therefore, cell walls of Gram-negative bacteria are weaker compared to those of Gram-positive bacteria.

• However, the cell walls of Gram-negative bacteria have additional layers of other substances located on the outside of the peptidoglycan layer.

• They include layers of lipopolysaccharide (LPS), lipoprotein, and phospholipids, in that order from the top. The lipid portion (Lipid A) of the LPS is called the endotoxin.

• Weakening, removal or defective formation of the cell wall results in the production of various abnormal forms called spheroplasts, free protoplasts and L-forms.

• Differences in the chemical composition of the cell wall determine the Gram staining properties of the cell and its susceptibility to some antibiotics.

• FIGURE SHOWING The Comparison of the thick cell wall of Gram-positive bacteria with the comparatively thin cell wall of Gram-negative bacteria. Note the complexity of the Gram-negative cell envelope (outer membrane, its hydrophobic lipoprotein anchor; periplasmic space).

THE CAPSULE

• Many bacteria are surrounded by a covering layer of gelatinous material that lies outside and immediately in contact with the cell wall.

• Capsules of most bacteria are polysaccharides but those of a few bacteria are polypeptides.

• A capsule is not essential to the life of the bacterium but is protective, mainly against phagocytosis.

• It is also used for attachment to surfaces. It therefore plays part in pathogenicity. Its presence is best demonstrated by negative stains.

THE FLAGELLA

• Motile strains of bacteria possess filamentous appendages known as flagella which act as organs of locomotion.

• Flagella are protein in nature. Depending on spp., there may be one or several flagella per cell.

• The arrangement of the flagella may be monotrichous, amphitrichous, peritrichous, kophotrichous or lophotrichous.

THE FIMBRIAE

• Certain Gram-negative bacilli possess filamentous or hair-like appendages known as fimbriae or pili.

• There are two types of fimbriae: common and sex pili. They are shorter and more numerous than flagella.

• The fimbriae may be lost without the bacterium dying. The common pili are organs of adhesion.

• Adhesion may be of value to the bacteria in holding on to nutritionally favourable areas. Fimbriae are also important in haemagglutination. The sex pili are involved in reproduction.

BACTERIAL SPORES• Certain bacterial spp., especially Bacillus and Clostridium spp., develop a

highly resistant resting phase or endospore, when subjected to harsh conditions like starvation or high temperatures. The spore forms inside the cell and contains the bacterial DNA, a small amount of cytoplasm, cell membrane, peptidoglycan, very little water, and very importantly, keratinlike coat that is responsible for the remarkable resistance of the spore to heat, dehydration, radiation, and chemicals. This resistance may be mediated by dipicolinic acid, a calcium ion chelator found only in spores. Once a spore has formed, it has no metabolic activity and can remain dormant for many years. The process of forming a spore is called sporulation or sporogenesis and does not involve multiplication. Before a bacterial cell has formed a spore it is called a vegetative cell. Reversion back to vegetative form is called germination, and is not a means of reproduction, since one spore germinates into one cell.

• The medical importance of spores lies in their extraordinary resistance to heat and chemicals. As a result of their resistance to heat, sterilization be achieved by boiling. Hence autoclaving is required. Spores are not usually seen in clinical specimens from patients infected by spore-forming bacteria because the supply of nutrients in the body is adequate.