516
Bacterial Cell Culture Methods
show growth. In a 10
12
dilution, all 5
tubes of medium would probably show no
growth as there would be only 1 organism
per 200 mL.
Transfer 3 (4 or 5 if possible) replicate
1 mL samples of each suitable dilution
to tubes of liquid or plates of solid
medium and incubate. A greater range
of dilutions (usually 5 or 6) is required
in this method, unless the result can be
predicted accurately.
After incubation, record the presence
or absence of growth in the recov-
ery cultures.
Statistical tables, for example, McGrady
tables have been compiled, enabling
the most probable number of organ-
isms per milliliter of the suspension
to be determined. The following ex-
ample of a McGrady table has been
computed
for
a
dilution
factor
of
10, and for 5 replicate tubes inoc-
ulated from each dilution of culture
(Table 2).
Theentriesinthetablearethenumberof
turbid tubes observed after inoculation of
thesamevolumefromanythreesuccessive
tenfold dilutions. The stated MPN is the
estimated mean viable count per inoculum
taken from the most concentrated of the
three dilutions being considered.
5
Characterization of Bacteria Using
Biochemical and Cultural Tests
5.1
Introduction
Because bacteria are small, morphological
characters have never proved satisfactory
Tab. 2
Values of the Most Probable Number (MPN) for 5 tubes inoculated from each of 3
successive 10-fold dilutions.
Signifcant number Numbers oF
replicate tubes showing turbidity
at 3 successive dilutions
MPN
Signifcant number Numbers oF
replicate tubes showing turbidity
at 3 successive dilutions
MPN
0
1
0
0.18
5
0
0
2.3
10
0
0
.
2
0
50
1
3
.
1
11
0
0
.
4
0
51
0
3
.
3
20
0
0
.
4
5
51
1
4
.
6
20
1
0
.
6
8
52
0
4
.
9
21
0
0
.
6
8
52
1
7
.
0
22
0
0
.
9
3
52
2
9
.
5
3
0
0
0.78
5
3
0
11.0
30
1
1
.
1
53
1
1
1
.
0
31
0
1
.
1
53
2
1
4
.
0
32
0
1
.
4
54
0
1
3
.
0
40
0
1
.
3
54
1
1
7
.
0
40
1
1
.
7
54
2
2
2
.
0
41
0
1
.
7
54
3
2
8
.
0
41
1
2
.
1
55
0
2
4
.
0
42
0
2
.
2
55
1
3
5
.
0
42
1
2
.
6
55
2
5
4
.
0
43
0
2
.
7
55
3
9
2
.
0
––
5
5
4
1
6
0
.
0
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