microbial nutrition, growth and survival Flashcards
(16 cards)
in order to grow all organisms require:
chemical elements for synthesis of cell constituents.
energy generating system. autotrophs- light or oxidation of inorganic compounds. heterotrophs- organic compounds.
bacteria are nutritionally the most versatile.
major nutritional types:
chemotrophs-
heterotrophs- oxidation of organic compounds. all animals, fungi, protozoa and most bacteria.
autotrophs- oxidation of inorganic compounds. co2. certain bacteria.
phototrophs-
autotrophs- light. co2. plants algae and photosynthetic bacteria.
most important elements- C, H, O, N, S, P.
others in small amounts- K, S, Mg, Ca, Fe. some specific elements in very low concentrations- vitamins.
bacterial growth:
a typical bacterium reproduces by binary fission.
growth defined as orderly increase in all major constituents of an organism.
involved synthesis of cell structures and components from nutrients obtained from outside the cell.
population growth:
in batch culture (closed culture) the solid
or liquid culture medium is in a state of
constant change – nutrients decrease
and waste products accumulate.
In batch culture, if single cells are inoculated into the culture medium and incubated under suitable conditions, they will
continue to multiply until one necessary nutrient approaches
exhaustion and becomes growth-limiting or toxic
substances accumulate.
population growth:
Growth of a batch cell culture can be depicted graphically by plotting the logarithms of cell numbers against time.
A typical growth curve of this kind is sigmoidal in shape and can be divided into a number of growth phases, which are regularly present but may vary in extent.
These are the lag phase,
exponential or logarithmic phase,
stationary phase and
death phase
microbial growth:
in exponential/log phase, cells divide at constant intervals
Time taken for cells to divide or the population to double in size (cell number) is the generation time (g).
Escherichia coli g = 17 minutes
Bacillus subtilis g = 35 minutes
Mycobacterium tuberculosis g = 792 minutes
factors affecting microbial growth:
nutrient availability.
temperature.
pH.
aeration- o2/co2.
moisture content.
presence of inhibitory substances.
temperature:
temperature and microbial activity
Microbial diversity –
psychrophiles (optimum temp. < 20oC)
mesophiles (optimum temp. 20-45oC)
thermophiles (optimum temp. > 45oC)
Majority of micro-organisms are mesophilic
Psychrophilic and thermophilic micro-organisms can be isolated from most environments
pH and microbial activity:
pH effects on micro-organisms
- extracellular enzymes – pH optima
Optimum pH for most micro-organisms is pH 7.0 i.e. they are neutrophilic
Bacterial growth very slow or inhibited at or below pH 5.0 but some bacteria (acidophilic types) grow best at low pH (pH 3-4).
Fungi as a group tend to be more acid-tolerant than bacteria. Many fungi grow optimally at pH 5.0 and a few
grow well at pH values as low as pH 2.0
aeration and microbial activity:
Microbial diversity – anaerobes, aerobes, (may be obligate or facultative) and microaerophiles
CO2 in atmosphere is an important factor in itself :-
- pH changes
- may be inhibitory at high concentrations
moisture content and microbial activity:
Growth rates of micro-organisms normally
increase with increasing water content of
substrates
Responses vary – microbial diversity
environmental stress conditions:
Desiccation –high salt conditions
Temperature
pH
Aeration
Lack of nutrients
Presence of toxic substances
survival structures- spores in bacteria and fungi, cysts in bacteria.
bacterial endospores:
Most unicellular bacteria do not produce any spores for survival
two genera of Gram +ve bacteria, Bacillus and
Clostridium, can form endospores (inside vegetative cells).
N.B. 1 spore/cell
Vegetative cells = growing cells
bacterial endospores:
Endospores are the most resistant forms of life known.
They can withstand temperatures well above the boiling point of water (10,000x more resistant to heat than vegetative cells) and conditions of extreme desiccation.
Endospores can remain viable in a metabolically inactive state for at least 300 years (viable spores recovered from
soil around roots of herbarium specimens)
and then germinate i.e. produce new vegetative cells (1 cell from each spore), in a few minutes when conditions become favourable for vegetative growth
bacterial endospores:
endospores are quite unlike vegetative cells. they are smaller and highly refractile (using phase contrast light microscopy they are phase bright).
They also differ physically and chemically, having a much lower water content (15% water compared with 70% water in vegetative cells)
and containing large amounts of a compound, calcium dipicolinate. Vegetative cells contain small quantities of this compound as an intermediate in the synthesis of the amino acid, lysine.
bacterial cysts:
modified vegetative cell. rod-shaped vegetative cell, for example in Azotobacter, becomes rounded and develops a thick, chemically and physically resistant cell wall.
Azotobacter cysts are resistant to desiccation, mechanical disintegration and UV & ionising radiation.
In contrast to endospores, however, cysts are not especially heat resistant
