Microbial growth
Microbial Growth - refers to the # of cells, not the
size of the cells
Generation Time (Doubling Time) •
time required for a cell to divide •
most about 1 Hr. To 3 Hrs. •
E. coli - 20 minutes •
Mycobacterium tuberculosis - 24 Hrs. •
Requirements for growth
Physical •
Chemical •
Physical Requirements
Temperature w
psychrophiles (cold loving microbes ) •
range 0 C- 2O C •
mesophiles (moderate temp. loving microbes) •
range
20 C - 40 C •
thermophiles
range
(heat loving microbes) •
40 C - 100 C •
pH
Most bacteria grow between pH 6.5 - pH 7.5 w
Very few can grow at below pH 4.0 w
many foods, such as sauerkraut, pickles, and cheeses are
preserved from spoilage by acids produced during
fermentation
•
Osmotic Pressure
Microbes obtain almost all their nutrients in solution from w
surrounding water
Tonicity w
isotonic •
hypertonic •
hypotonic •
Chemical Requirements
Macro & Micro Elements w
C H ,O ,P ,K , I, N, S,
Ca, Fe Mg Na, Cl w
Oxygen
Bacteria can be classified base on their oxygen requirements w
Aerobic and anaerobic bacteria can be identified by growing them in
liquid culture:
1: Obligate aerobic bacteria gather at top of test tube to absorb maximal
amount of oxygen.
2: Obligate anaerobic bacteria gather at bottom to avoid oxygen.
3: Facultative bacteria gather mostly at the top, since aerobic respiration
is most beneficial; but as lack of oxygen does not hurt them, they can be
found all along the test tube.
4: Microaerophiles gather at upper part of test tube, not at top. Require
O2, but at low concentration.
5: Aerotolerant bacteria are not affected by oxygen, and they are evenly
spread along the test tube
Oxygen is lethal to some organisms
All organisms produce superoxide ( O2-) w
Superoxide is toxic to cells (steals electrons) w
Superoxide must be neutralized w
Superoxide dismutase
O2- + O2- + 2 H+ -------> H2O2 + O2 w
Hydrogen peroxide is also toxic to cells and it must be neutralized w
Catalase
2 H2O2 --------> 2 H2O + O2 w
Obligate Anaerobes lack: w
Superoxide dismutase ( SOD ) •
Catalase
•
Culture Media
1. Chemically Defined w
the exact chemical composition is known •
used to grow fastidious organisms •
2. Complex Media w
exact chemical composition is not known •
most bacteria and fungi are grown with this •
Special Culture Techniques
1. Anaerobic Bacteria w
a. Reducing Media •
b. Anaerobic Container •
c. Agar Stab •
d. Agar Shake •
2. Microaerophilic Bacteria w
grow best under reduced O2 levels and increased CO2 levels •
Normal Atmosphere
21 % O2
•
.3 to .03 % CO2
•
Selective Media
Inhibits the growth of some bacteria while selecting for the growth w
of others
Example: w
Brilliant Green Agar •
dyes inhibit the growth of Gram (+) bacteria •
selects for Gram (-) bacteria •
Most G.I. Tract infections are caused by Gram (-)
bacteria
EMB (Eosin Methylene Blue) w
dyes inhibit Gram (+) bacteria •
selects for Gram (-) bacteria •
G.I. Tract infections caused by Gram (-) bacteria •
•
Differential Media
Differentiates between different organisms growing on the same w
plate
Example: w
Blood Agar Plates (TSA with 5% sheep blood) •
used to differentiate different types of Streptococci
Selective and Differential Media
Mannitol Salt Agar w
used to identify Staphylococcus aureus
•
Mannitol Salt Agar w
High salt conc. (7.5%) inhibits most bacteria •
sugar Mannitol
•
pH Indicator (Turns Yellow when acid) •
Limiting factors in the environment
Lack of food, water or nutrients w
space w
accumulation of metabolic wastes w
lack of oxygen w
changes in pH w
temperature w
•
Phases of
Growth
4 Phases w
1. Lag Phase w
2. Log Phase w
3. Stationary Phase w
4. Death Phase
1. Lag Phase
Bacteria are first introduced into an environment or media w
Bacteria are “checking out” their surroundings w
cells are very active metabolically w
# of cells changes very little w
1 hour to several days w
2. Log Phase
Rapid cell growth (exponential growth) w
population doubles every generation w
microbes are sensitive to adverse conditions w
antibiotics •
anti-microbial agents •
3. Stationary Phase
Death rate = rate of reproduction w
cells begin to encounter environmental stress w
lack of nutrients •
lack of water •
not enough space •
metabolic wastes •
oxygen •
pH •
Endospores would form now w
4. Death Phase
Death rate > rate of reproduction w
Due to limiting factors in the environment w
Enumeration of Bacteria
Turbid culture - 10 million bacterial cells per ml w
Serial Dilution w