Effects of Nutrient Concentration on Microbial Growth
A. To grow, microbes need a variety of substances called NUTRIENTS that they obtain from their environment and use for the production of energy and biosynthesis of cellular macromolecules. Each nutrient plays an important role in overall growth of the cell. Although at this point in the semester we are concerned with ways microbes satisfy their own nutritional needs, we will see in the last unit that in doing so they also recycle elements in the environment and so drive all biogeochemical cycles on the planet. Without microbes we’d all be dead!
Equation for Cell Growth
MONOMERS + INFORMATION + ENERGY ( POLYMERS ( MACROMOLECULES ( NEW CELL
1.source of monomers: from medium (pre-formed or reconfigured from whatever molecules are available)
2.source of information = DNA (unique to each organism) ( RNA ( proteins
3.source of energy: needed for both synthesis and maintenance. Comes from biochemical reactions. Bacteria and Archaea have many special tricks to get energy, far more than animals.
B. Nutritional categories - Microbes can be classified by their sources of energy and carbon (the nutrient that is needed in largest quantity for cell growth).
1.Energy source (E-source)
a. if organic molecules are used for energy (e.g. sugars, amino acids) then the microbe is classified as an
ORGANOTROPH (we are organotrophs)b. if inorganic molecules are used for energy (e.g. H2S, NH3) = LITHOTROPH
c. if light is used for energy = PHOTOTROPH
2.Macronutrients: Cell requires ~ 30 elements. Needs 6 in large amounts (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous, Sulfur - remember as
CHNOPS, pronounce it like the liqueur and you won’t forget it), some only in trace amts., some in between.a. Carbon source (C-source)
a. can be CO2 , in which case the microbe is called an AUTOTROPH
b. can be organic molecules = HETEROTROPH (we are heterotrophs)
Note: many bacteria can form all organic molecules from any of a wide variety of C-sources. E.g., Pseudomonas strains (soil organisms) can grow on hydrocarbons, a variety of sugars, fats, and polysaccharides, etc. The full description of microbe typically refers to both its E- and C-sources; lithotrophic autotroph, organotrophic heterotroph (this is what we are), etc. We will see many examples of these.
b. Nitrogen source
The typical bacterium is 15% nitrogen (found in proteins, nucleic acids, cell wall). Most bacteria get their nitrogen from either amino groups (-NH2), or ammonia (NH3), or nitrate (NO32-). Some bacteria can fix atmospheric nitrogen (N2) to form ammonia (NH3).
c. Phosporous source
Generally available as phosphate ion (PO43-) or organic molecules (e.g. RNA). Often limiting for growth.
d. Sulfur source
Required for 2 amino acids and for some vitamins. Available as sulfate (SO42- ) or sulfide (S2- ).
3.Micronutrients
Potassium (K), Magnesium (Mg), Calcium (Ca), and Iron (Fe) are required in small but significant amts. They act as cofactors for many enzymes and are important in cell structures (e.g., Mg2+ required by ribosomes for protein synthesis). Sodium (Na+) is not required by many microbes (unlike animals, where nerve system requires Na+ and K+). Marine microbes do require sodium (remember true halophiles require high NaCl concentrations compared to halotolerant microbes that can put up with it but don’t need the NaCl).
Trace elements also needed: Cobalt (Co), Zinc(Zn), Molybdenum(Mo), Copper(Cu), Manganese(Mn), Nickel (Ni), Selenium (Se) -- usually required by a small number of enzymes.
C. Adaptations to limited nutrients. On the first day of class we talked about the fact that with their fast generation times microbes would take over the planet, except that they are 1) preyed upon by other microbes and that 2) food (nutrients) is (are) usually not abundant. Just as they can adapt to any other constraint on growth, some microbes are also adapted to environments of limited nutrients (=OLIGOTROPHIC; microbes that grow optimally under conditions of limited nutrients are OLIGOTROPHS, if microbes require high nutrient levels they are COPIOTROPHS, remember by thinking "copious").
Because periods of starvation are probably experienced by most free-living microbes, starvation survival is very important.
Microbes have adapted to periods of low nutrient availability in several ways:
Some synthesize increased amts. of carrier proteins for uptake of nutrients across the plasma membrane. This allows microbes to obtain and use a larger proportion of the few nutrient molecules available. Remember too that microbes can move nutrients against the concentration gradient as a way of packing in necessary nutrients (what types of transport mechanisms allow microbes to move nutrients against concentration gradients? - don’t forget this stuff, the final is comprehensive!)
Some microbes can synthesize enzymes needed to use a different nutrient. For example, if the sugar glucose (the C- and E-source) is in short supply, some microbes can make enzymes to take up and use the sugar lactose instead, if lactose is more abundant.
Most important mechanism - Many microbes adjust the rates at which they metabolize nutrients and the rates at which they synthesize the new macromolecules required for growth (like RNA and proteins) to fit the availability of the least plentiful nutrient. Both metabolism and growth are slowed, but no energy is wasted synthesizing products that can’t be used and growth is as rapid as the conditions will allow. For example, under starvation conditions the bacteria, Vibrio (V. cholera ( disease cholera), will form very small cells. These very small cells are not spores but these cells have very low rates of metabolic activity and are fairly resistant to environmental stresses. When conditions become favorable again the cells will swell in volume and will increase metabolic activity.