Chapter 6: Microbial Growth
How do bacteria grow?

Not in size

Increase in population size

One cell divides into 2 new cells – binary fission
Binary fission
Binary fission:

Attachment of chromosome to p.m.; replication of DNA;
new p.m. and cell wall laid down between the 2
chromosomes

This is the way that each new daughter cell gets one
chromosome
Number of
generations
Number of cells
Log of # of cells
10X = #
0
1
0
1
2
0.3
2
4
0.6
3
8
0.9
4
16
1.2
5
32
1.5
6
64
1.8
7
128
2.1
8
256
2.4
9
512
2.7
10
1024
3.0
11
2048
3.3
12
4096
3.6
How can we describe growth?
2n = no. of cells in n generation
Generation time
Nt = N0 x 2n
Growth problems
If Staphylococcus aureus has a doubling time (generation
time) of 30 minutes and 5 hours have passed, how many
generations have been produced?
a. How many 30 minute time chunks are in 5 hours?
= 10
ANSWER: 10 generations
Growth problems
If Staphylococcus aureus has a generation time of 30 minutes
and 5 hours have passed, how many bacteria will be
present at the end of the time period?
a.
b.
c.
We have already determined that 10 generations will
occur.
2n = # cells at n generation
210 = # cells at the 10th generation
ANSWER = 1024 cells
Growth problems
If Staphylococcus aureus has a generation time of 30 minutes
and 5 hours have passed, how many bacteria will be
present at the end of the time period if we start with
3,000 cells?
Nt = N0 x 2n
Nt = 3,000 x 210
= 3,072,000 cells at the end of 5 hours

Growth curve: lag, log, stationary, and death phases

What occurs in each?
How can population size be counted?
(Advantages and disadvantages of each method)
1. Direct methods
A. Microscopic count with hemacytometer/
Petroff Hauser counting chamber
Petroff Hauser counting chamber

B. Plate counts – dilution series and plates

C. Filtration

D. Coulter counter/flow cytometer/Fluorescence
activated cell sorter (FACS)
2. Indirect methods
A. Dry weight
B. Metabolic activity
C. Turbidity
Turbidity
Growth requirements of microbes
A. Temperature:
Thermophiles
Mesophiles
Psychrophiles
B. pH: acidophiles
C. Osmotic pressure (# of solutes in solution)
Halophiles
D. Oxygen:
Types:
Obligate aerobe
Facultative anaerobe
Obligate anaerobe
Aerotolerant anaerobe
Microaerophilic
Enzymes needed to survive in presence of
oxygen:
Catalase
Peroxidase
Super oxide dismutase (SOD)
E. Nutrients
C, N, P, S elements needed
Mg, Fe, etc. trace elements needed
Media:
Defined or complex
Selective vs. differential
Special
Mannitol salt agar – selective medium
Blood agar – differential medium
The End