Dutch scientist Antonie van Leeuwenhoek first observed bacteria in 1676, using an ordinary microscope. In 1838, the German naturalist Christian Gottfried Ehrenberg called them ‘bacteria’, from the Greek word ‘baktḗria’, meaning ‘little stick’. Later, Robert Koch’s research, famously dubbed ‘Koch’s postulates’, demonstrated that microorganisms such as bacteria cause infectious diseases.
Shapes of Bacteria
Based on their shape and arrangement, bacteria can be classified into the following four main types:
Cocci: spherical, round or oval-shaped, example – Staphylococcus groupBacilli: rod-like or cylindrical shaped, example – Bacillus group,Spirilla: spiral or coil-shaped, example – Spirillum groupVibrios: curved or comma-shaped, example – Vibrio group
Parts and Their Morphology
Lag Phase: This is the initial phase during which the cells acclimatize to the new environment. During this phase, the cells increase in size due to the accumulation of proteins and other molecules necessary for cell division, without any increase in cell number. Log Phase: Also known as the exponential phase, this is when the metabolic activity of the cell is high, and the cell undergoes cell division to rapidly increase in number. Stationary Phase: Eventually, the population growth declines due to the depletion of available nutrients and the accumulation of waste products in culture. During this phase, bacterial cell growth stops and reaches a plateau since the number of dividing cells equals the number of dying cells. Death Phase: As nutrients become less available and waste products start to accumulate, the number of dying cells continues to rise. In the death phase, the number of living cells decreases exponentially, and population growth experiences a sharp decline. Being a diverse group, bacteria are also sometimes classified based on their growth parameters such as temperature, pressure, modes of nutrition, respiration, pH, and osmotic pressure. a) Binary Fission: It is the most common mode of reproduction in bacteria. Under favorable conditions of growth, the parent cell grows in size and duplicates its genetic material, which then undergoes division to form two identical daughter cells. Bacteria can grow and divide extremely rapidly, and a fixed bacterial population doubles itself in just 9.8 minutes. b) Budding: A small bud forms at one end of the mother cell, gradually growing in size while the mother cell remains the same. When the bud is about the same size as the mother cell, it separates to form an independent bacterial cell. c) Sporulation: Under unfavorable conditions of growth such as adverse temperature or shortage of nutrients, some bacterial cells convert into spores and cysts by forming a thick-walled covering around the cell. This is basically an adaptation to survive the harsh environmental conditions. On return of favorable conditions, the spores’ returns back into reproductive cells. Exchanging Genetic Material Although bacteria do not have an obligate sexual reproductive stage in their life cycle, they can be very active in exchanging their genetic material, the DNA. It occurs by three different processes: a) Conjugation: Transfer of DNA through direct contact between two bacterial cells. Conjugation occurs with the help of sex pili that forms a bridge between the two participating cells. b) Transformation: Bacteria cell taking up free fragments of DNA that are floating in the medium. c) Transduction: Transfer of DNA from one bacterium to the other using viruses that are capable of infecting a bacterial cell.
