Lecture 12: Microbial nutrition, growth and survival Flashcards
in order to grow all organisms require -
> Chemical elements for synthesis of cell constituents
Energy-generating system
N.B. Bacteria are nutritionally the most versatile
Autotrophs
light or oxidation of inorganic compounds
Heterotrophs
organic compounds
Major nutritional types- heterotrophs
> energy source- Oxidation of organic compounds
C-source- Organic compounds
Examples- All animals, fungi, protozoa, and most bacteria
Major nutritional types- autotrophs
> Energy source- Oxidation of inorganic compounds i.e NH3, H2S
C-source- CO2
Examples- Certain bacteria e.g. nitrifiers, sulphur oxiders
Major nutritional types- phototrops-autotrops
> Energy type- Light
C-Source- CO2
Examples- plants, algae, and photosynthetic bacteria
Nutritional requirements: most important elements
> Carbon
Hydrogen
Oxygen
Nitrogen
Sulphur
Phosphorous
Nutritional requirements: Other elements in small amounts
> Potassium
Sulphur
Magnesium
Calcium
Iron
And some specific elements in very low concentrations
And vitamins
Bacterial growth
> A typical bacterium reproduces by binary fission
Growth defined as orderly increase in all major constituents of an organism
Involves 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.
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)
Factors effecting microbial growth
> Nutrient availability
Temperature
pH
Aeration – oxygen/carbon dioxide
Moisture content
Presence of inhibitory > substances
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
Notably two genera of Gram +ve bacteria, Bacillus and Clostridium, can form endospores (inside vegetative cells). N.B. 1 spore/cell
Vegetative cells = growing cells
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 are 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
Gruinard island - Britain’s ‘anthrax island’
> Bacillus anthracis endospores were released from bombs dropped on the tiny Scottish island to wipe out a flock of sheep.
1942 test was carried out because of fears that Germany might attack the UK with biological weapons.
Anthrax can be contracted by skin contact, ingestion or inhalation, but it is through inhalation that it is at its most deadly and proves to be fatal in about 95% of cases, even with medical treatment.
Experts on biological weapons have suggested that 100kg of anthrax sprayed on a major city could kill more than 3m people.
The island was quarantined for 48 years
Decontaminated with formaldehyde .
Biological weapons- microorganisms
> Soon after the terrorist attacks of 9/11, on October 4, 2001, Bob Stevens, an employee of American Media, was diagnosed with systemic anthrax.
This confirmed diagnosis raised the concern that his disease was the result of an act of bioterrorism.
The last case of inhalational anthrax in the United States was 25 years earlier in 1976. The subsequent discovery of Bacillus anthracis spores at Bob Stevens’ place of work further supported the initial view that the incident resulted from the illegitimate use of the pathogen.
Amerithrax
> Five Americans were killed and 17 became ill in what became the worst biological attacks in U.S. history.
The ensuing investigation by the FBI and its partners, code-named ‘Amerithrax’, was one of the largest and most complex in the history of law enforcement.
Letters containing spores were subsequently discovered.
The use of the U.S. postal system as a method for weapon dissemination set off national panic and the FBI began an investigation.
In August 2008, the Justice Department and FBI officials announced a breakthrough in the case and released documents and information showing that charges were about to be brought against Dr. Bruce Ivins, who was an American microbiologist and senior biodefence researcher at the United States Army Medical Research Institute of Infectious Diseases (USAMRIID).
On July 29, 2008 he died of an overdose of tylenol in an apparent suicide after learning that criminal charges were likely to be filed against him by the FBI for an alleged criminal
connection to the anthrax attacks.
No formal charges were ever actually filed against him for the crime and no direct evidence of his involvement has been uncovered.
Biological weapons- microorganisms
> As a result of the 2001 anthrax attacks, the developing field of ‘microbial forensics’ was thrust into the spotlight.
Its occurrence created an urgent need for robust forensic interpretation of microbial evidence from acts of bioterrorism or biocrimes.
A major component of a microbial forensic investigation is the precise determination of the cause and the source.
DNA fingerprinting of B. anthracis
> The anthrax letter attacks clearly demonstrated the importance of being able to identify the exact strain of Bacillus anthracis, which was involved.
This was an essential requirement in the investigation.
Forensic science has increasingly become dependent on DNA evidence for the identification and elimination of criminal suspects. Likewise, microbial forensics has built
upon this success, with some modifications, to make DNA fingerprinting fundamental to the identification of specific micro-organisms used in biocrimes.
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.
