chap 2 Flashcards
Bacteria reproduce by
binary fission which involves the separation of a single cell into two more or less identical daughter cells, each containing, among other things, at least one copy of the parental DNA or by budding
The lag phase: phase 1
It is characterized by no increase in cell number, but the cells are actively metabolizing in preparation for cell division
It can last from one hour to several days
Enzymes and intermediates are formed and accumulate until they are present in concentrations that permit growth to resume
This phase depends on a wide variety of factors including:
-The time required for synthesis of division factors like enzymes & coenzymes
-The growth medium so that’s why it can be either short or very long
Exponential (log) phase: 2nd phase
It’s a pattern of balanced growth wherein all the cells are dividing by binary fission and are growing by geometric progression
The cells divide at a constant rate depending on the composition of the growth medium, the organism itself, and the condition of incubation (Tº, pH, etc..)
The rate of exponential growth of bacterial culture is expressed as generation timealso the doubling timeof the bacterial population
The fastest growing bacteria have generation times of 10-20 minutes under optimum growth conditions, others have of hours or even days.
c-Stationary Phase third phase
The growth rates slow the number of death cells balances the number of new cells & the population stabilizes
The metabolism slows down; the living cells maintain a slow metabolic activity
What are the factors that let the cells to enter this phase?
Environmental changes like:
-high cell density,
-exhaustion of nutrients,
-accumulation of waste products, pH changes, …
d-Death phase (Final Phase)
Physiological point at which cell deaths exceeds cell births, viable count declines
The cells quickly lose the ability to divide even if they are placed in fresh medium
The death rate increases
A small number of survivors may persist for months or even years
Like the log phase the death phase is so fast and exponential
The death phaseas all phases, can be slowed by lowering the temperature.
So to maintain maximum cell viability it’s best to grow bacterial cultures only to early stationary phase & then chill the culture
Bacterial growth over time can be graphed as cell number versus time : Growth Curve
The curve has 4 different phases:
lag phase-exponential (log) phase, stationary phase and death or decline phase.
requirements for microbial growth can be divided into two main categories:
a-Physical:
Temperature, pH, Osmotic Pressure & water activity
requirements for microbial growth can be divided into two main categories: b-Chemical:
Sources of carbon, nitrogen, Sulfur, phosphorus, trace elements, oxygen & organic growth factors
MO are classified into 3 primary groups according to the basis of their preferred range of T:
a-Psycrophiles:
b-Mesophiles:
c-Thermophiles:
a-Psycrophiles:
cold loving microbes
- MO can grow slowly @ low temperatures (0-15°C)
but they have their optimum growth T between 15°C-20°Ci
b-Mesophiles:
moderate temperature loving
c-Thermophiles:
heat loving microbes
Ci-Obligate psychrophiles:
- Can grow at 0°C
- Optimum growth 15°C
- Found mostly in deep oceans or in certain polar regions
- Rarely cause problems in food preservation
ii-Facultative psychrophiles
- Can grow at 0°C
- Optimum growth T 20°C-30°C
- Responsible for food spoilage
Psychrotrophs=
moderate psychrophiles
MesophilesModerate-temperature loving microbes
They grow @ ambient temperature
Optimum growth @ 25ºC-40ºC
Include most common type of microorganisms
Include most of the common spoilage and disease organisms (the optimum T for many pathogenic bacteria is about 37°C)
Thermophiles
Heat loving or high temperature adapted microbes
Capable of growth at high temperatures
Optimum growth temperature 50ºC-60ºC
Hyperthermophiles:
- Optimum growth temperature > 80◦C
Hot springs associated with volcanic activity
The pH of an environment affects bacterial growth
Most bacteria grow best between
6-8
In general, bacteria survive alkaline pH better than the acid pH;this is why number of foods like pickles and many cheeses are preserved from spoilage by acids produced by bacterial fermentation
Acidophiles
Optimum pH = 1-5.5
Include many fungi and some bacteria
Acids produced by bacterial fermentation used to preserve food: pickles, cheese
*Neutrophiles:
*prefer a pH in the range of 5.5 –8(most organisms fall in this category)
Alkalophiles:
pH= 8-8.5 to 11
Found in highly basic soda lakes and high carbonate soils
Alkalinity is rarely used to preserve food
Plasmolysis:
Interfere with growth
Highly osmotic environments prevent bacterial growth
Addition of sugars, salts can be used to preserve food: jellies, jams, salting of meat
In high solute environments, the water is bound to solutes and cannot interact with the enzymes so
this is a way to inhibit or to decrease bacterial growth.
-Fungi (molds and yeasts) are more resistant than bacteria to high or low osmotic pressure
Halophile:
organisms adapted to high salts concentration
Obligate halophiles:
require High salt concentration for their growth (30%)
Ex: Dead Sea
Facultative halophile
More common
2% salt concentration
Few species of facultative halophiles can tolerate 15%
Autotroph:
CO2 is the carbon source
- Capnophiles:
require high concentrations of CO2
- Heterotroph:
Carbon Source = Organic compounds
Nitrogen:
Makes up about 14 % of dry weight of bacterial cell.
-Primarily used to form the amino acids, nucleic acids, and coenzymes
-Sources of nitrogen:
NH4+ (ammonium ion), Nitrates NO3-& N2from amino acids and proteins
nitrogen fixation;
-Some important bacteria use gaseous Nitrogen N2 directly from the atmosphere -Some organisms that use the nitrogen fixation are free living mostly in the soil but others live cooperatively in Symbiosis with the rots of legumes like alfalfa, beans soybeans,…
Sulfur
Considered as a micronutrient
Obtained from organic sources: sulfur containing compounds or from inorganic sources
It’s the constituent of many amino acids and several coenzymes
phosphorus
It is essential for the synthesis of nucleic acids & phospholipids of cell membranes
It’s obtained from inorganic sources (inorganic PO4—)
Required cofactors for enzymes by microorganisms
Potassium, Magnesium, Calcium
a-Obligate aerobes:
Organisms that require oxygen to live
b-Obligate anaerobes:
*Bacteria or organisms that are unable to grow or even survive in the presence of oxygen
c-Microaerophiles:
•Organisms that grow best when low concentration of oxygen are present
d-Facultative anaerobes:
They can live with & without O2 Tend to grow better/faster when O2 is present Can generate ATP by either organic or inorganic means and have the means to detoxify O2
Ex: E.coliin the human intestinal tract
e-Aerotolerant anaerobes:
Organisms that are unable to utilize molecular oxygen in their ATP generation but otherwise they tolerate it fairly well
Essential organic compounds an organism is unable to synthesize
Vitamins: essentially organic equivalents of trace elemnets
Function as coenzymes
Amino acids, purines, pyrimidines
Trace elements
Elements required in relatively small amounts: iron, copper, zinc
Employed as enzyme cofactors
They are sometimes added to a laboratory medium
Usually present in tap water and other components of media
