1. Basic Bacteriology Part-2 Bacterial Growth
(Updated, June 13th 2017)
Bacterial Growth
(First Semester )

2. Bacterial Growth
Bacterial cell division: Bacteria reproduce by binary fission, a process by which one bacterial cell divides to form two cells.
Bacterial growth: it is the increase in the number of bacterial population.
Bacterial cell division Bacterial growth

3. During bacterial growth, the cell division of one bacterial cells gives rise to two cells. Accordingly, during growth of a bacterial population, bacteria is said to have exponential growth (logarithmic growth)
Generation time (doubling time): is the time needed to complete one division cycle. In other words, it is the time that is needed to double the number of bacterial population only one time (under optimal physical and chemical conditions) Examples of Doubling (Generation) time:
17-20 minutes for Escherichia coli
12-18 hour for Mycobacterium tuberculosis
12 to 13 days for Mycobacterium leprae (in mouse foot pad).
E. coli, which has a generation time of about 20 min, will produce over 1000 bacterial cell in about 3 hours and over 1 million cell in about 7 hours ( under optimal physical and chemical conditions).

6. In reality, the doubling time (Generation time) can be affected by several factors such as:
Bacterial species
Nutrient’s availability
Water availability
Temperature
Oxygen availability pH
Generation time and infectious disease incubation period:
The incubation period is the time between infection and appearance and clinical symptoms
Bacterial pathogens that have long generation time have long incubation periods and vise-versa. Why????

7. Bacterial Growth Curve: Can be determined by using a batch culture, (a closed system)
If a small number of bacteria are inoculated into a liquid nutrient medium (broth) and the bacterial cells are counted at frequent intervals, the typical phases of a standard growth curve can be demonstrated (under optimal physical and chemical conditions):

8. 1- The lag phase:
During which, vigorous metabolic activity occurs to prepare bacterial cells for division(Bacteria cells do not divide at this phase).
This phage can be as short as few minutes and up to many hours depending on the tested bacterial species.
2- The log (logarithmic) phase: is when bacteria start replicating (dividing) in constant rate causing bacterial population to increase in a logarithmic manner.

9. 3. The stationary phase: occurs when nutrient depletion or toxic products accumulation cause bacterial growth to slow until the number of new cells produced balances the number of cells that die resulting in a steady state in the number of bacterial population ( or during this this phage, bacteria stop replicating).
4. The death phase: is marked by a decline in the number of viable bacteria.

10. The stationary phase:
Once bacteria enter into the stationary phase, they undergo certain structural and physiological alterations to enhance their tolerance to stressful conditions such as nutrients deficiency , pH alterations, accumulation of toxic metabolic byproducts ( in other words, to enhance their survival strategy under stressful conditions):
Examples:
Increase in peptidoglycan cross-bridging
Increase in the nucleoid density
Increase expression of virulence factors (example: toxins)
Increase expression of heat-shock proteins, which are chaperones that preserve the folding of important proteins and enzymes
In spore-forming bacteria, spore formation starts at during this phase)
Note: Many of these alterations are mediated by the transcription factor RpoS, which is expressed at the stationary phage to alter gene expression, so that expression of genes that are needed of stressing conditions become more favorable.

11. The death phase:
By the end of the stationary phase, complete depletion of nutrients, accumulation of large amounts of toxic metabolic byproducts, complete depletion of O2, most bacteria start dying.
Viable But Not Culturable Stage
However, it has been found that certain types of bacteria (under the above mentioned conditions) are able to enter into a deep dormancy rather than dying , so that it is not possible to revive them upon providing them with fresh nutrients or try to culture them on a fresh artificial media. (in this case, such bacterial cells are said to be viable but not culturable).
Viable but not culturable bacteria can be revived upon inoculating these bacteria in animals.
What would be the impact on viable but not culturable bacteria on food microbiology??

12. Open Bacterial Growth Systems:
In industrial microbiology, bacterial population is maintained at a certain growth phase during which, the cultured bacteria is producing a particular substance of interest such as an enzyme or an antibiotic during that particular phase
In medical microbiology, bacterial population is maintained at a certain growth phase when the bacteria is producing a substance of interest such as a toxin or a capsule that can be used as vaccines.
Open Bacterial Growth System

13. In addition, certain bacterial pathogens express important virulence factors that are involved in pathogenesis during a particular growth phase ( mostly the stationary phase) . Upon, upon conducting pathogenesis experiments in animals, it is important to take certain bacterial pathogens during a particular growth stage.

14. Physical Conditions That Affect Bacterial Growth:
Oxygen (O2)
For most organisms, an adequate supply of oxygen enhances metabolism and growth.
During aerobic respiration, Oxygen acts as the hydrogen acceptor in the final steps of the electron transport chain.
Oxygen utilization or exposure to O2 generates toxic molecules known as free radicals such as:
Hydrogen peroxide (H2O2)
Superoxide free radical (O2)
These molecules are highly reactive and can result in major cell damage that can lead to cell death.

15. In case of oxygen utilization, or toleration of exposure to O2, bacteria (and other cells) need to have certain mechanism to eliminate free radicals. Two enzymes play major role regarding this issue:
The first is superoxide dismutase, which catalyzes the reaction
The second is catalase or peroxidase , which catalyze the following reactions:

16. Based on Oxygen utilization, bacteria are classified into:
Obligate aerobes: They require oxygen to grow because their ATP-generating system is dependent on oxygen as the hydrogen acceptor.
Facultative anaerobes : they utilize oxygen-if it is present- to generate energy by aerobic respiration, but they can use the fermentation pathway to synthesize ATP in the absence of sufficient oxygen.
Microaerophilic: needs O2 in low concentrations.
Obligate anaerobes: they cannot grow or get killed in the presence of oxygen because they lack either superoxide dismutase or catalase, or both. Such bacteria produce energy by fermentation or anaerobic respiration.
Aerotolerant: these are anaerobic bacteria that do not utilize O2 but can tolerate its presence and are not killed by O2.
All of the above mentioned bacteria have enzymes that can eliminate free radicals except Obligate anaerobes

17. Temperature:
According to their optimal growth temperature, bacteria can be classified as
Psycrophiles
Mesophiles (most pathogenic bacteria are mesophiles)
Thermophiles
Hyperthemophiles

18. pH:
According to their optimal growth pH, bacteria can be classified as:
Acidophiles
Neutrophiles
Alkaliphiles
Regardless to the above mentioned classes of bacteria, the pH of the bacterial cell cytoplasm is almost always neutral. Accordingly, bacterial cells utilizes certain mechanism to adjust and maintain the neutral pH of their cytoplasm (such as proton pumps, K/H+ counter transport)
The importance of that why acidophilic bacteria must live in an acidic environment and an alkaliphilic bacteria must live in alkaline environment is attribute to the stability of their cell walls and membranes, which are exposed to the surrounding environments.
The stability of cell walls and membranes of these bacteria have evolved according to the surrounding environment. However, the pH of their cytoplasm is almost neutral as mentioned above.

19. Salt Concentration:
According to their optimal growth concentration of NaCl, bacteria can be classified as:
Nonhalophiles
Halotolerant
Moderate Halophiles
Extreme Halophiles
In solutions, NaCl is found as Na and Cl ions. The concentration of these ions make affect stability of bacterial enzyme and other structural components such as cell wall and cell membrane.

20. Iron Availability:
Iron is an essential nutrient for most living organisms.
It is necessary for the activity of many enzymes that mediate important cellular functions ranging from respiration and basic metabolism to DNA replication.
Iron is very abundant metal in environment. However, because, it is a very reactive metal, consequently, iron is mostly found reacted with other elements to generate water- insoluble ferric compounds. Accordingly, free iron is found in a very limited amounts. In the surrounding environment .
In human body, iron is either found as part of certain molecule such as heme or as a cofactor for certain enzymes. In addition, in the blood stream, iron is bound to iron-binding protein. In other words, and similar to the situation of free iron in the environment, in our bodies, free iron is found in a very limited amounts.
This implies that for bacteria, obtaining free iron (which is found in a very limited amount as mentioned above) represents a serious challenge whether the bacterial cell is found in the environment or in our bodies. That why it was an essential an a crucial issue that bacteria develop efficient iron uptake system that enable them to obtain free iron that is available in a very small amounts. 

21. Iron Uptake by Bacteria:
To obtain free iron that is needed for their growth, bacteria produce low molecular weight, ferric ion-chelating agents known as siderophores, which have a very high binding-affinity for iron.

22. Bacteria secrete siderophores into the surrounding environment
Bacteria secrete siderophores into the surrounding environment. Once a siderophores molecule binds to any available free iron, the bacteria internalize it into its cytoplasm, where iron is released to be utilized in many important cellular functions that are important for the survival and growth of bacteria.