The bacterial structures internal to cell wall includes cytoplasmic membrane, protoplast, spheroplast, spores, plasmid and others.
Cytoplasmic membrane: This is a thin structure that completely surrounds the cell. Its size is approximately 7.5 nm in thickness and is composed primarily of phospholipid (20 – 30 %), protein (60 – 70 %). The phospholipid form bilayered structure in which most of the protein are embedded tightly called integral protein. This protein can only be removed by destruction of the membrane with treatment such as with detergent. Other protein which are loosely attached are called peripheral protein. This peripheral protein can be easily removed mild treatment such as osmotic shock. The lipid matrix has fluidity so the cell component can move round and laterally. This fluidity is dependent on various factor such as temperature and on the proportion of unsaturated fatty acid to saturated fatty acid present in the phospholipid. The cytoplasmic membrane is a hydrophobic barrier to penetration by most water soluble molecules. However, specific protein in the membrane allows the passage of small molecules such as waste products and nutrients. Function of cytoplasmic membrane are as follows
1. Organic and inorganic nutrients are transported by enzymes present in plasma membrane called permease enzyme.
2. It consists of enzymes of biosynthesis pathways that synthesize different components of cell wall such as peptidoglycan, teichoic acid, polysaccharides, lipopolysaccharides and phospholipids.
3. It possesses the attachment sit for bacterial chromosome and plasmic DNA.
4. It provides permeability barrier and thus prevents the escape of cellular materials outside the cell and help to pass the selective components.
Protoplast: This refers to the entire cell containing protoplasm (cytoplasmic membrane and cell organelles bounded by it) from which cell wall has been removed. Protoplasm can be produced from gram positive bacteria by treating the cell with an enzyme such as lysosome.
Spheroplast: In gram negative bacteria having multilayered cell wall, when peptidoglycan layer is removed, some outer layer material can still be attached to cytoplasmic membrane called speheroplast.
Spores: Certain species of bacteria produces metabolically dormant structures called spores. They may be endospores (produced inside the cell) or exospore (produced external to the cell). After return of suitable condition, spore undergo germination and produce vegetative cells.
Endospore: These are made up of thick wall highly refractive bodies that are produced (one per cell) by Bacillus species, Clostridium species, Sporosarcina species, Thermoactinomyces species etc. They form endospore when there is lack of water of depletion of essential nutrients in the environment. Endospores are highly durable, extremely resistant to desiccation, staining, disinfecting chemicals, radiation and heat produced inside plasma membrane of the cell. Most endospore can resist temperature at 80°C for at least 10 minutes.
Endospore contains large amount of Dippicolinic acid which can account for 10 – 15 % of dry weight. It occurs in combination with large amount of calcium and is located in the core (in the central part of spore). The calcium DNA complex as well as dehydrated state may be an important factor for that resistance.
Exospores: Cells of Methylosinus species (methanol oxidizing bacteria) produce exospores through budding at one end of cell. The exospore do not contain dippicolinic acid so they are less resistant to heat and dessicaiton.
Bacterial Nucleus: A typical nucleolus and nuclear membrane is absent in bacterial cell. However, DNA is highly coiled into a region known as nucleoid. Bacterial cell contains only one chromosome and bacterial DNA lacks ‘introns’.
Plasmid: In addition to bacterial chromosome, bacterial cell consists of small circular shaped double stranded DNA molecule called plasmid. Like DNA, plasmid can also undergo replication. The number of plasmid ranges from 1 to 100 or more per bacterial cell. Plasmid contains 5 to 100 genes that determine several biological function.
Under certain circumstances, they provide special character to the bacterial cell and help them survive. On the basis of functions, plasmids are divided into several types. These are sex factor plasmid, resistance plasmid, heavy metal resistance plasmid, col plasmid, penicillinase plasmid etc.
The process of increasing population with new individuals having the same characteristics to that of parents is known as reproduction. In most prokaryotes, growth of an individual cells continues until the cell divides into two new cells by a process called ‘binary fission’. In this process, cells are elongated to approximately twice the length of an average cell and then form a partition that eventually separates the cells into two daughter cells. The partition is referred to as septum and is result of inward growth of the cytoplasmic membrane and cell wall from opposite direction until the two daughter cells are pinched off. During the growth cycle all cellular constituents are pinched off. During the growth cycle all cellular constituents including chromosomes, macromolecules, monomers and inorganic ions are divided.
The time required for complete growth cycle is highly variable and is dependent on a number of factors (nutritional and genetic). Under best nutritional condition, the bacteria E. coli can complete the cycle in about 20 minutes.
Some bacteria such as Rhodopseudomonas acidophilus reproduce by budding which is process in which a protuberance (bud) develops at one end of the cell. The bud gets enlarged and develops into new which eventually separates from parent cell.
Bacteria that produce extensive filamentous growth such as Nocardia species reproduces by fragmentation of filaments. Similarly, species of genus Streptomyces and related bacteria produce many spores per organisms by developing cross walls at the hyphal tips which ultimately gives rise to new organisms.
Reproduction in Bacteria
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.