136
ROBINA B. SCHOLES AND J. M. SHEWAN
simulate as closely as possible the natural environment from which the
sample was taken. This applies not only to the media used and in
respect of such factors as pH and salt content, but also to all steps in
the investigation process in the laboratory. Thus, some psychrophilic
bacteria are known to be extremely sensitive to temperatures much
above 10°C (see page 151) and if any of these are present, as they might
well be in samples from the colder parts of the World's oceans, then
exposure of a sample for even a few hours in a warm laboratory on
board ship would kill them off. Moreover, as Zobell and Conn (1940)
and Gunkel et al. (1961) point out, many marine bacteria are so heat
sensitive that the normal plating technique, using melted agar with a
temperature of solidification a t 46"C, is lethal to them. The converse of
this can also cause trouble ; thus some bacteria from warm tropical
waters are very sensitive to cold, and cannot survive 24 h at a temperature of 0 to 3°C. Consequently, storage of such waters in t i refrigerator for subsequent examination has to be avoided. Similar kinds of
precautions may well be necessary in the examination of materials
from great depths where they are subjected to considerable hydrostatic pressures. Exposure on deck and in the laboratory to ordinary
atmospheric pressures could conceivably kill some of the more sensitive
obligate barophiles, although technically this would be a most difficult
thing to avoid.
B. 2lrlenbbrane Jilters
Provided the above preliminary precautions are taken, the question
now is how to further investigate thc sample. I n many waters the
numbers of bacteria are so few (Ill00 ml) that normal procedures for
examination and isolation cannot satisfactorily be used. The introduction of the membrane filter has undoubtedly been one of the major
advances in this field (Oppenheimer, 1952; Jannasch and Jones, 1959;
Kriss, 1963). The latter author, in particular, used it extensively in his
work and obtained more constant results than with the Koch plate
procedure. The method is in fact a convenient way of concentrating
the water volume under investigation. Large volumes, up to several
litres, can usiially be used, although clogging of the pores by suspended
material can be a disadvantage. Moreover, the method can be adopted
for direct counts (Kriss, 1963) and by iising a variety of media (see
below) both in the liquid or solid form the presence and numbers of
various physiological groups of bacteria can readily be obtained
(Oppenheimer, 1952 ; Tennant and Erdman, 1955 ; Henderson, 1959;
Kriss, 1963). The less numerous organisms which would be missed by
other techniques can thus be picked up.
ROBINA B. SCHOLES AND J. M. SHEWAN
simulate as closely as possible the natural environment from which the
sample was taken. This applies not only to the media used and in
respect of such factors as pH and salt content, but also to all steps in
the investigation process in the laboratory. Thus, some psychrophilic
bacteria are known to be extremely sensitive to temperatures much
above 10°C (see page 151) and if any of these are present, as they might
well be in samples from the colder parts of the World's oceans, then
exposure of a sample for even a few hours in a warm laboratory on
board ship would kill them off. Moreover, as Zobell and Conn (1940)
and Gunkel et al. (1961) point out, many marine bacteria are so heat
sensitive that the normal plating technique, using melted agar with a
temperature of solidification a t 46"C, is lethal to them. The converse of
this can also cause trouble ; thus some bacteria from warm tropical
waters are very sensitive to cold, and cannot survive 24 h at a temperature of 0 to 3°C. Consequently, storage of such waters in t i refrigerator for subsequent examination has to be avoided. Similar kinds of
precautions may well be necessary in the examination of materials
from great depths where they are subjected to considerable hydrostatic pressures. Exposure on deck and in the laboratory to ordinary
atmospheric pressures could conceivably kill some of the more sensitive
obligate barophiles, although technically this would be a most difficult
thing to avoid.
B. 2lrlenbbrane Jilters
Provided the above preliminary precautions are taken, the question
now is how to further investigate thc sample. I n many waters the
numbers of bacteria are so few (Ill00 ml) that normal procedures for
examination and isolation cannot satisfactorily be used. The introduction of the membrane filter has undoubtedly been one of the major
advances in this field (Oppenheimer, 1952; Jannasch and Jones, 1959;
Kriss, 1963). The latter author, in particular, used it extensively in his
work and obtained more constant results than with the Koch plate
procedure. The method is in fact a convenient way of concentrating
the water volume under investigation. Large volumes, up to several
litres, can usiially be used, although clogging of the pores by suspended
material can be a disadvantage. Moreover, the method can be adopted
for direct counts (Kriss, 1963) and by iising a variety of media (see
below) both in the liquid or solid form the presence and numbers of
various physiological groups of bacteria can readily be obtained
(Oppenheimer, 1952 ; Tennant and Erdman, 1955 ; Henderson, 1959;
Kriss, 1963). The less numerous organisms which would be missed by
other techniques can thus be picked up.