Bacterial physiology consists in studying the nutrition, metabolism and growth of bacteria according to variations (natural or controlled) in the environment in which they live.
it is the analysis of the elementary, energetic and specific needs necessary for the functioning and growth of the bacterium, as well as the physico-chemical factors likely to influence them.
It is the set of chemical transformations (anabolism or biosynthesis and catabolism or degradation) that ensure the development of bacterial constituents and their functioning.
In an optimal environment, the bacterial cell, thanks to its highly developed enzymatic system, will give birth in a short time (20 minutes for the majority of bacteria in the environment), to two daughter bacteria that are identical to it: We are talking about bacterial growth. It is manifested by a numerical increase in bacterial cells and not an increase in size as in higher organisms (Human, animal, plant).
To survive and multiply, bacteria need a greater or lesser quantity of mineral and organic substances called food substances or nutrients.
The degradation of these foods, which are made available to them in culture media, will provide them with simple elements (Carbon, Nitrogen, Minerals) and energy, which they will reuse to synthesize their own structural and enzymatic constituents.
Bacteria all have a certain number of common needs such as: water, a source of energy, a source of Carbon, a source of Nitrogen and mineral elements. Moreover, by examining the chemical composition of the bacterial cell, we can guess its nutritional needs:
❖ Therefore, the bacteria will need 3 types of nutrients:
1-a- Basic needs:
❖ Water: A major need, it is part of the composition of all growing media.
❖ Carbon: This is one of the most abundant elements in bacteria: it must be provided in sufficient quantity. The simplest carbon compound is CO2 or carbon dioxide. Bacteria are classified into 2 categories:
❖ Nitrogen: Nitrogenous substances are part of the composition of bacterial proteins. Nitrogen can be fixed by bacteria :
❖ Phosphorus and Sulfur: They occupy a place of choice.
❖ Phosphorus :
It enters into the composition of nucleic acids, many coenzymes and ATP. It is incorporated into the bacteria in the form of inorganic phosphate. It allows the recovery, accumulation and distribution of energy in the cell.
Il entre dans la composition des acides aminés et des proteins (thiol group). It is incorporated into the cell as sulfate, organic sulfur compounds, rarely as reduced sulfur.
❖O2 and H2: are brought by water and atmospheric air. The elements mentioned must be provided in sufficient quantities.
❖ In smaller quantities are provided the mineral elements :
❖ In trace amounts, often brought by water: These are the trace elements because they are essential in minute quantities: These are Ca, Mg, Co, Cu, Mn….
It should be noted that many metal ions can be toxic for certain cells (ex: copper salts) whereas other ions are essential at precise concentrations, either for the synthesis of a metabolite (ex: 0.14 mg of Fe/ l for the synthesis of diphtheria toxin), or for the synthesis of pigment (eg: Iron + Magnesium for the production of Prodigiosine in Serratia marcescens).
1-b- Energy needs
❖ They cover the expenses incurred in the biosynthetic processes. Bacteria can use as an energy source:
❖ Phototrophic bacteria use mineral or organic compounds as sources of electrons.
❖ Chemotrophic bacteria, which draw their energy from redox reactions, use mineral or organic compounds as "hydrogen or electron donors" or "electron acceptors".
❖ These are essential metabolites that the bacterium is not able to synthesize by enzymatic defect and that must therefore be provided to allow its development. They are called Growth Factors.
❖ Bacteria are therefore classified into 2 categories:
❖ They play an essential role in obtaining an optimal culture. Indeed, the nutrients must be provided to the bacterium in the environmental conditions that suit it, otherwise, they can inhibit it.
a- Temperature: Depending on the behavior of the bacteria with respect to temperature, we distinguish :
b- The pH : Bacteria prefer a neutral or slightly alkaline pH (7 –7.5) but the limits are very wide :
c- Oxygen : Depending on the respiratory type, we distinguish :
d- Osmotic pressure: Thanks to a specific wall of prokaryotes (Murein) which gives them rigidity and resistance to shocks, bacteria tolerate variations in ionic concentrations. Some bacteria tolerate high salt concentrations eg. Enterococcus (6.5%Nacl) Staphylococcus aureus (7.5%Nacl).
It is the orderly growth of all the components of an organism. If, in higher organisms, it results in an increase in size, in unicellular organisms (bacteria, yeasts), it results in an increase in the number of individuals.
There is therefore a multiplication, approximately every 30 minutes, of a bacterium, giving rise by division to 2 new identical bacteria.
Generation time is defined as the time required for a doubling of the number of bacteria. ex. E.coli: TG = 20mn, M.tuberculosis: TG = 20h. Growth rate is also defined as the number of divisions per unit time (eg 3 for E.coli).
During growth, the environment becomes depleted in available nutrients and enriched in products of catabolism, which are often toxic. Changes occur there, affecting the pH, the Redox potential, the osmotic pressure, etc.
a- Direct counting of bacteria : bacteria are considered as Particles that are counted fresh or after staining.
a-1- Total count: for this we use :
a-2- Viable cell count :
- The membrane filtration technique can be used, which concentrates the bacteria present in small quantities in a liquid sample (e.g. Colimetry of water).
- A sample was filtered using a membrane filter that does not allow bacterial cells to pass. The filter was then placed on culture medium and incubated. The resulting colonies are easily counted using the grid. The resulting value is used to determine the colony forming units in the sample.
b- Biomass measurement :
b-1- determination of dry weight :
- Techniques for measuring changes in cell mass can also be used to determine population size. One approach is to determine the microbial dry weight. The cells growing in a liquid medium are collected by centrifugation, washed, dried in an oven and weighed. This is a particularly useful technique for measuring the growth of filamentous fungi
- Disadvantage: All cell mass is measured. Moreover, it is a long and delicate technique.
b-2- Measurement of Optical Density (OD) :
The OD of the growth medium is evaluated as a function of time, at a given wavelength. By using the BEER-LAMBERT Law which defines the relationships between the intensities of a light beam before and after passing through a bacterial culture, bacterial growth can be evaluated by determining the Optical Density of the bacterial culture.
The measurement of the O.D. is done at a wavelength ranging from 450 to 550nm and the bacterial cultures are diluted so as to obtain an O.D. of less than 0.4 (the O.D.s change linearly with the cell concentration).
b-3- Flow cytometry technique :
It consists of measuring one or more specific parameters of an isolated cell, driven by a liquid flow. This technique is commonly applied in hematology. It is still being evaluated in microbiology.
c- Chemical markers
- This is the determination of proteins, DNA, ATP, peptidoglycan..
The study of bacterial growth over time or growth kinetics can be represented on a graph by plotting :
The growth curve obtained then shows 6 phases:
a- Continuous growth:
The growth curve is that obtained for a bacterial culture in non-renewed medium. It is possible with tricks to obtain a bacterial culture maintained for a very long time in the exponential growth phase: This is called continuous growth, obtained by a regular supply of new nutrient medium and extraction of an equivalent quantity of old environment. These methods are commonly used in industry to obtain bacterial bodies of the same age (preparation of bacterial vaccines), or bacterial metabolites (vitamins), bacterial toxins (preparation of toxoids) in large quantities.
A chemostat is constructed such that the rate at which sterile medium is introduced into a culture vessel is the same as the rate at which medium containing microorganisms is removed
b- The diauxie:
In a synthetic medium, when the bacteria are provided with 2 carbonaceous substrates (limiting foods), 2 growth modes can be ensured:
Cause: during growth, one of the substrates is used first until exhaustion before the second substrate is assimilated in turn.