bacterial growth

Lec. 2 Dr. Fatima Malik

Growth of bacteria
Bacteria, like all living organisms, require nutrients for metabolic purposes and for cell division, and grow best in an environment. Chemically, bacteria are made up of polysaccharide, protein, lipid, nucleic acid and peptidoglycan, all of which must be manufactured for successful growth.
The meaning of growth
Growth is the orderly increase in the sum of all the components of an organism. The increase in size that results when a cell takes up water or deposits lipid or polysaccharide is not true growth. Cell multiplication is a consequence of binary fission that leads to an increase in the number of single bacteria making up a population, referred to as a culture.
Nutritional requirements
1-Oxygen and hydrogen: Both oxygen and hydrogen are obtained from water; hence, water is essential for bacterial growth. In addition, the correct oxygen tension is necessary for balanced growth. While the growth of aerobic bacteria is limited by availability of oxygen, anaerobic bacteria may be inhibited by low oxygen tension.
2-Carbon: Carbon is obtained by bacteria in two main ways:
A. Autotrophs, which are free-living, non-parasitic bacteria, use carbon dioxide as the carbon source.
B. Heterotrophs, which are parasitic bacteria, utilize complex organic substances such as sugars as their source of carbon dioxide and energy.
3-Inorganic ions: Nitrogen, sulphur, phosphate, magnesium, potassium and a number of trace elements are required for bacterial growth.
4-Organic nutrients
• Carbohydrates are used as an energy source and as an initial substrate for biosynthesis of many substances.
• Amino acids are crucial for growth of some bacteria.
• Vitamins, purines and pyrimidines in trace amounts are needed for growth.

- Extracellular factors that modify bacterial growth are:
• Temperature: the optimum is required for efficient activity of many bacterial enzymes, although bacteria can grow in a wide range of temperatures. bacteria can be classified as:
• mesophiles, which grow well between 25 and 40°C, comprising most medically important bacteria (that grow best at body temperature)
• thermophiles, which grow between 55 and 80°C .
• psychrophiles, which grow at temperatures below 20°C.
• pH: the hydrogen ion concentration of the environment should be around pH 7.2–7.4 (physiological pH) for optimal bacterial growth.


? according oxygen requirement bacteria can be classified as follows:
• obligate (strict) aerobes: which require oxygen to grow because their adenosine triphosphate (ATP)-generating system is dependent on oxygen as the hydrogen acceptor (e.g. M. tuberculosis)
• Facultative anaerobes: which use oxygen to generate energy by respiration if it is present, but can use the fermentation pathway to synthesize ATP in the absence of sufficient oxygen (e.g. oral bacteria such as mutans streptococci, E. coli)
• obligate (strict) anaerobes: which cannot grow in the presence of oxygen because they lack either superoxide dismutase or catalase, or both (Porphyromonas gingivalis)
• microaerophiles: that grow best at a low oxygen concentration \Campylobacter spp.).
Reproduction:
Bacteria reproduce by a process called binary fission, in which a parent cell divides to form a progeny of two cells. This results in a logarithmic growth rate – one bacterium will produce 16 bacteria after four generations. The doubling or mean generation time of bacteria may vary (e.g. 20 min for Escherichia coli, 24 h for Mycobacterium tuberculosis); the shorter the doubling time, the faster the multiplication rate. Other factors that affect the doubling time include the amount of nutrients, the temperature and the pH of the environment.









Bacterial growth cycle
The growth cycle of a bacterium has four main phases
1. Lag phase: may last for a few minutes or for many hours as bacteria do not divide immediately but undergo a period of adaptation with vigorous metabolic activity.
2. Log (logarithmic, exponential) phase: rapid cell division occurs, determined by the environmental conditions.
3. Stationary phase: this is reached when nutrient depletion or toxic products cause growth to slow until the number of new cells produced balances the number
of cells that die. The bacteria have now achieved their maximal cell density or yield.
4. Decline or death phase: this is marked by a decline in the number of live bacteria.



Bacterial growth curve

The bacterial chromosome
The bacterial chromosome contains the genetic information that defines all the characteristics of the organism. It is a single, continuous strand of DNA with a closed, circular structure attached to the cell membrane of the organism. The ‘average’ bacterial chromosome has a molecular weight of 2 × 109.

The Measurement of Microbial Concentrations

Microbial concentrations can be measured in terms of cell concentration (the number of viable cells per unit volume of culture) or of biomass concentration (dry weight of cells per unit volume of culture). These two parameters are not always equivalent because the average dry weight of the cell varies at different stages of a culture. For example, in studies of microbial genetics and the inactivation of microbes, cell concentration is the significant quantity; in studies on microbial biochemistry or nutrition, biomass concentration is the significant quantity.



A. Viable Cell Count
The viable cell count is typically considered the measure of cell concentration. For this, a 1-mL volume is removed from a bacterial suspension and serially diluted 10-fold followed by plating 0.1-mL on an agar medium. Each single invisible bacterium (or clump of bacteria) will grow into a visible colony that can be counted. For statistical purposes, plates containing between 30 and 300 colonies give the most accurate data. The plate count × the dilution × 10 will give the number of colony forming units (CFU)/mL in the undiluted bacterial suspension.
Using this method, dead bacteria within the suspension do not contribute to the final bacterial count.
B. Turbidity
The turbidity of a culture, measured by photoelectric means, can be related to the viable count using a standard curve. As an alternative a rough visual estimate is sometimes possible: For example, a barely turbid suspension of E. coli contains about 10 7 cells per milliliter, and fairly turbid suspension contains about 108 cells per milliliter. The correlation between turbidity and viable count can vary during the growth and death of a culture; cells may lose viability without producing a loss in turbidity of the culture.
C. Biomass Density
In principle, biomass can be measured directly by determining the dry weight of a microbial culture after it has been washed with distilled water. In practice, this procedure is cumbersome, and the investigator prepares a standard curve that correlates dry weight with viable cell count. Alternatively, the concentration of biomass can be estimated indirectly by measuring an important cellular component such as protein or by determining the volume occupied by cells that have settled out of suspension.