All bacterial cells invariably have a cell membrane, cytoplasm, ribosome, and chromating bodies. The majority have a cell wall, which gives shape to the bacterial cell. Specific structures like capsule, slime, flagella, Pili, fimbriae and granules are not found in all bacteria.
Size
Bacteria range in size from about 0.1 to 600 µm over a single dimension. Bacteria vary in size as much as in shape. The smallest (e.g., some members of the genus Mycoplasma) are about 100 to 200 nm in diameter, approximately the size of the largest viruses (poxviruses) Escherichia coli, a bacillus of about average size, is 1.1 to 1.5 µm wide by 2.0 to 6.0 µm long. Some spirochetes occasionally reach 500 µm in length whereas staphylococci are 0.75 – 1.25µ in diameter.
| Recently a huge bacterium has been discovered in the intestine of the brown surgeonfish, Acanthurus nigrofuscus. Epulopiscium fishelsoni grows as large as 600 µm by 80 µm, a little smaller than a printed hyphen. It is now clear that a few bacteria are much larger than the average eukaryotic cell. |
Shape of bacteria
On the basis of general shape, bacteria are classified into three categories. These shapes are known as cocci, bacilli and spiral. Although most of the bacterial species have fairly constant characteristic cell shape, yet some sells are pleomorphic and they can exist in a variety of shapes.
| Exceptions to the above shapes are trachoma forming, sheathed, stalked, square, star-shaped, spindle-shaped, lobed and filamentous bacteria. |
The cocci are spherical or oval bacteria having one of several distinct arrangements based on their planes of division. If division is in one plane it will produce either a diplococcus or streptococcus arrangement is diplococcus, whereas when cocci form long chain of cells then arrangement is called as streptococci. When the division of cell is in two planes it will produce a tetrad arrangement. A tetrad is a suare of 4 cocci. Thirdly when the division is in three planes, it will produce a sarcina arrangement. Sarcina is a cube of 8 cocci. When division occurs in random planes, it will produce a staphylococcus arrangement in which cocci are arranged in irregular, often grape-like clusters. Diplocooccus pneumonia and staphylococcus aureus are some examples of cocci.
Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or diplobacillus. Bacillus is a single cell of bacteria. Streptobacillus is a chain of bacilli. When rod shaped bacteria occur in pairs then arrangement of cells is know as diplobacilli examples of rod shaped bacterial are Escherichia coli, bacillus subtilis. Pseudomonas.
The spiral shaped bacteria are spirally coiled. Spirals come in one of three forms, vibrio, a spirillum, or a spirochete. Vibrio is curved or comma-shaped rod. Spirillum is a thick, rigid spiral. Spirochete is a thin, flexible spiral. Examples of spiral shaped bacteria are Vibrio, hyphomicrobium.
Bacterial cell structure
Flagella and their factions: these are extremely thin, hair like appendages. They come out through cell wall and originate from basal body, structure just beneath the cell membrane in the cytoplasm. They are made up of protein Flagellin. On the basis of presence of flagella. Pattern of attachment of flagella and the number of flagella present bacteria are classified into different taxonomic groups.
Atrichous means bacteria are without any flagella. When single polar flagellum is present then condition is known as monotrichous. If tuft of flagella is present only at one pole of bacteria then these are lophotrichous flagella. Amphitrichous is a condition when tuft of flagella at each of two poles is present.
In peritrichous form, flagella surround the whole cell. Most of bacilli and spiral shaped bacteria have flagella. Cocci very rarely have flagella. Primary function of flagella is to help in motility. With the help of flagella, flagellate bacteria can also detect and move in response to chemical signals which is a type of behavior called as chemotaxis.
Pili and their functions
These are hollow, non helical, filamentous appendages. Pili are smaller than flagella and are not involved in motility. True pili are only present on gram-negative bacteria. They are made up of special protein called pilin. They are primarily involved in a mating process between cells called conjugation process. Some pili function as a means of attachment of bacteria to various surfaces.
The cell envelope: The outer wrapping of bacteria
Bacterial surface and walls are very diverse. Collectively complexes of layer external to the cell protoplasm are called as cell envelope and include capsule, slime and cell wall.
Capsule
Bacteria produce capsule, which is made up of repeating polysaccharide units, and of protein, or of both, capsule is tightly bound to the cell. It has a thicker, gummy nature that gives sticky characters to colonies of encapsulated bacteria.
Slime
Some bacteria are covered with loose, soluble shield of macromolecules which is called as slime capsule and slime provides greater pathogenicity to bacteria and protects them against phagocytosis.
Cell wall
Beneath the extracellular substances and external to cytoplasmic membrane cell wall is present. It is a rigid structure. It determines the shape of bacterium. Cell wall also protect the cells from osmotic lysis. Cell wall is only absent in mycoplasmas. Christian gram developed the technique of gram stain.
Bacteria could be divided into two groups based on their response to gram staining procedure. By this staining technique Gram-positive bacteria are stained purple ( retain the primary dye due to formation of CV-1 complex) and Gram-negative bacteria are stained pink (retain secondary dye) in colour.
There are many structural differences between two groups (table 6.1) which are the primary basis for difference in staining behavior.
Table 6.1: Comparison of Gram-positive and Gram-negative cell walls.
| Characteristics | Gram-positive | Gram-negative |
| Number of major layers Chemical makeup Overall thickness Outer membrane Periplasmic space permeability | 1 Peptidoglycan (50% of dry weight in some bacterial cells) Teichoic acid Lipoteichoic acid Lipids (1-4%) 20-80 nm No Present in some More permeable | 2 Lipopolysaccharides Lipoproteins Peptidoglycan 10% dry weight of some bacterial cells Lipids (11-12%) 8-11nm Yes Present in all Less permeable |
The cell walls of most bacteria have a unique macromolecule called as peptidoglycan. Its amount varies in different types of bacteria. It is composed of framework of long glycan chains cross-linked with peptide fragments.
The intact cell wall also contains chemical constituents such as sugar molecules, teichoic acid, lipoproteins and lipopolysaccharides, which are linked to peptidoglycan.
Several bacterial groups lack the cell wall structure characteristic of gram positive or gram negative bacteria, and some bacteria have no cell wall at all. Cell walls of archaebacteria are different from eubacteria. They do not contain peptidoglycan. Their cell walls are composed of proteins, glycoproteins and polysaccharides.
Cell Membrane
Just beneath the cell wall is the cell membrane or plasma membrane. It is very thin. Flexible and completely surrounds the cytoplasm. Plasma membrane is very delicate in nature any damage to it results in death of the organism. Bacterial membranes differ from eukaryotic membranes in lacking sterols such as cholesterol.
Cell membrane regulates the transport of proteins, nutrients, sugar and electrons or other metabolites. The plasma membranes of bacteria also contain enzymes for respiratory metabolism.
Cytoplasmic Matrix
The cytoplasm of prokaryotic cell lacks membrane bound organelles and cytoskeleton (microtubules). The cytoplasmic matrix is the substance present between the plasma membrane and the nucleoid. It has gel like consistency. Small molecules can move through it rapidly. The plasma membrane and everything present within it is known as protoplast.
Thus the cytoplasmic matrix is a major part of protoplast. Other large discrete structures such as chromatin/nuclear body, ribosomes, mesosomes and granules and nucleoid are present in this matrix.
Nucleoid
A bacterial cell unlike the cells of eukaryotic organisms lacks discrete chromosomes and nuclear membrane. The nuclear material or DNA in bacterial cells occupies a position near to the center of cell. This material is a single, circular and double stranded DNA molecule. It aggregates as an irregular shaped dense area called the nucleoid.
This chromatin body is actually an extremely long molecule of DNA that is tightly folded so as to fit inside the cell component. Since bacteria have a single chromosome, they are haploid.
Other names for nucleoid are nuclear body, chromatin body and nuclear region. It is visible in the light microscope after staining with Feulgen stain. Escherichia coli closed circle chromosome measures approximately 1,4000 um
Many bacteria contain plasmids in addition to chromosomes. These are the circular, double stranded DNA molecules. They are self-replicating and are not essential for bacterial growth and metabolism. They often contain drug resistant, heavy metals, disease and insect resistant genes on them.
plasmids are important vectors, in modern genetic engineering techniques
Ribosomes
Ribosomes consist of RNA and proteins. Some may also adhere loosely to plasma membranes. They are protein factories. There are thousands of ribosomes in each healthy growing cell. They are smaller than eukaryotic ribosomes.
Mesosomes
The cell membrane, invaginates into the cytoplasm forming structure called as mesosome. Mesosomes are in the form of vesicles, tubules or lamellae. Mesosomes play important role while DNA replication and cell division, where as some mesosomes are also involved in export of exocellular enzyme.
Granules and storage bodies
Since bacteria exist in a very competitive environment where nutrients are usually in short supply. They tend to store extra nutrients when possible. These may be glycogen, sulphur, fat and phosphate. In addition, cells contain waste materials that are subsequently excreted.
For example, common waste materials are alcohol, lactic acid and acetic acid.
Spores
Certain species of bacteria produce spores, either external to the vegetative cells (exospores) or within the vegetative cells (endospores). They are metabolically dormant bodies and these appears at a late stage of cell growth. Spores are resistant to adverse physical environmental conditions such as light, high temperature, desiccation, PH and chemical agents. Under favorable conditions they germinate and form vegetative cells.
Cysts
Cysts are dormant, thick-walled, desiccation resistant forms and develop during differentiation of vegetative cells which can germinate under suitable condition. They are not heat resistant.
Nutrition of Bacteria
Like other organisms bacteria. Need energy for their growth, maintenance and reproduction. Most bacteria are heterotrophic i.e., they cannot synthesize their organic compounds from simple inorganic substances. They live either as saprophytes or as parasites. Saprophytic bacteria get their food from dead organic matter. Soil is full of organic compounds in the form of humus.
Humus is the material resulting from the partial decay of plants and animals. Many soil inhabiting bacteria have very extensive enzyme system that breaks down the complex substances of humus to simpler compounds. The bacteria can then absorb and utilize these simpler substances as a source of energy. Parasitic bacteria for their nutrition are fully dependent on their host.
Some kinds of bacteria are autotrophic i.e., they can synthesize organic compounds which are necessary for their survival from inorganic substances. These bacteria may be separated into two groups: photosynthetic autotrophs and chemosynthetic autotrophs. Photosynthetic bacteria possess chlorophyll which differs from the chlorophyll in chloroplasts, bacterial chlorophyll is dispersed in the cytoplasm.
During photosynthesis the autotrophic bacteria utilize hydrogen sulphide (H2S) instead of water as a hydrogen source and liberate sulphur instead of oxygen. Nitrifying bacteria are chemosynthetic. Chemosynthetic bacteria oxidize inorganic compounds like ammonia, nitrate, nitrite, sulphur or ferrous iron and trap the energy thus released for their synthetic reactions. The overall reaction of photosynthesis in photosynthetic bacteria can be written as:
Green sulphur bacteria, purple sulphur bacteria and purple non-sulphur bacteria are photosynthetic bacteria.
Respiration in bacteria may be aerobic (requiring free oxygen) or anaerobic not requiring free oxygen. Bacteria, which are able to grow in the presence of oxygen, are called aerobic bacteria. While those which can grow in the absence of oxygen are known as anaerobic bacteria.
Some bacteria are neither aerobic nor anaerobic, but facultative. Facultative bacteria grow either in the presence or absence of oxygen. Some bacteria require a low concentration of oxygen for growth and are known as microaerophilic.
Pseudomonas is an aerobic bacterium.
Spirochete is an anaerobic bacterium.
E.coli is a facultative anaerobic bacterium.
Campylobacter is a microaerophilic bacterium.
Growth and Reproduction
Bacterial growth refers commonly to increase in number of bacterial cells. Bacteria increase in number by an asexual means of reproduction, called binary fission. In binary fission parent cell enlarges, its chromosome duplicates, and plasma membrane pinches inward at the center of the cell. When nuclear material has been evenly distributed, the cell wall grows inward to separate cell into two.
This sequence is repeated at intervals by each new daughter cell which in turn increases the population of cells. Once the division is complete, bacteria grow and develop their unique features.
The interval of time until the completion of next division is known as generation time. Pour distinct phases are recognized in bacterial growth curve.
Fig. 6.6 binary fission in bacteria
- Lag phase: It is phase of no growth. Bacteria prepare themselves for division.
- Log phase: It is phase of rapid growth. Bacteria divide at exponential rate.
- Stationary phase: bacterial death rate is equal to bacterial rate of reproduction and multiplication.
- Death/ decline phase: Bacteria start dying. Here the death rate is more than reproduction rate.
Bacteria lack traditional sexual reproduction and mitosis. However, some bacteria transfer genetic material from a donor bacterium to a recipient during a process called conjugation. Some conjugating bacteria use specialized sex pili to transfer genetic material. Conjugation produces new genetic combinations that may allow the resulting bacteria to survive under great variety of conditions.