Bacterial Growth and Reproduction

• Microbial growth refers to the increase in cellular components and the number of bacterial cells.
• Bacteria typically reproduce through binary fission, where a single cell divides into two identical cells.

Generation Time and Bacterial Doubling

• Generation time is the time it takes for a bacterial cell to divide and its population to double.
• Bacterial doubling times can vary greatly.
• Escherichia coli can double in 20 minutes under optimal conditions.
• Mycobacterium tuberculosis can double in 15 to 20 hours.

Bacterial Growth Curve

• When bacteria are cultured in fluid media, their growth follows a definite course represented by a growth curve.
• The growth curve is plotted as the logarithmic number of viable bacteria against time in hours.
• The bacterial growth curve consists of four stages: the lag phase, the logarithmic phase, the stationary phase, and the decline phase.

Phases of the Growth Curve

• Lag Phase: characterized by no cell division and no increase in cell number, as bacteria prepare for active division.
• Logarithmic Phase: marked by division occurring at a maximum rate, with a constant and regular growth rate.
• Stationary Phase: the rate of division equals the rate of death, leading to a stationary number of living organisms while nutrients are depleted and wastes accumulate.
• Decline Phase: characterized by a higher rate of death than growth, eventually leading to all bacteria dying.

Nutritional Requirements of Bacteria

• Bacteria are classified into two main groups based on nutritional needs: autotrophic and heterotrophic.
• Autotrophic Bacteria: synthesize organic substances like proteins and carbohydrates from simple inorganic substances such as CO2 (carbon source) and ammonium (nitrogen source); typically free-living and of no medical significance.
• Heterotrophic Bacteria: require complex preformed organic substances like sugars and proteins from plant or animal sources; includes all bacteria of medical importance.

Gaseous Requirements of Bacteria

• Bacteria are classified into five groups based on oxygen requirements.
• Obligatory Aerobes: completely dependent on oxygen for growth and can only grow in the presence of O2.
• Facultative Anaerobes: can grow with or without oxygen, but grow better with oxygen.
• Aerotolerant Anaerobes: do not require oxygen and are not poisoned by it, growing equally well in presence or absence of O2.
• Obligatory Anaerobes: poisoned by oxygen and cannot grow in its presence
• Microaerophilic: require reduced levels of oxygen, as they are damaged by normal atmospheric levels of O2 (20%).

pH Requirements of Bacteria

• Acidophiles: have a growth optimum between pH 0 and 5.5.
• Neutrophiles: grow best between pH 6 and 8.0; most bacteria are neutrophiles.
• Alkalophiles: prefer a pH range of 8.5 to 11.5.
• Human blood and tissues have a pH of 7.2±0.2.

Temperature Requirements of Bacteria

• Psychrophiles: grow at 0 – 20 °C.
• Mesophiles: grow at 20 – 45 °C, with an optimum at 37°C (body temperature of warm-blooded animals); most pathogenic microorganisms and normal human microbiota are mesophiles.
• Thermophiles: grow at 50 – 80 °C.
• Hyperthermophiles: grow at temperatures more than 80 °C.