It has been amply established that each and every type of microorganism possesses
specific
well as
definitive growth-requirements.
Salient Features.
The salient features of the important and vital cultural characteristics are as
stated under:
(1) A plethora of microbes may be grown either
on or in a cultural medium*.
(2) A few microorganisms could be cultivated (grown) in a medium comprising specifically
organic chemical entities**,
whereas some others require solely inorganic chemical entities.
(3) Certain microbes do require
complex natural materials*** only for their normal growth.
(4) Importantly, there are certain critical microbes that may be carefully and meticulously
propogated only in a
living host or living cells, and cannot be grown in an usual artificial
laboratory medium.
Example: Rickettsias****
prominently require a definitive host in which they may growconveniently and generously, for instance: (a) an arthroped*****; (b) a chick embryo (i.e., a fertilized
chicken egg)
; and (c) a culture of mammalian tissue cells. In reality, the host being employed as an
extremely complex specified and articulated ‘
medium’ essentially required for such nutritionally
demanding microorganisms
.
(5)
Specific physical parameters: Besides, certain highly critical and specific array of nutrients,
each type of microorganism predominantly needs certain particular physical parameters for
its natural and normal growth.
Examples :
(
a) Microbes growing at high temperatures (e.g., Thermophilic bacteria): Some organisms do
prefer to grow and thrive best at temperatures ranging between 40° and 70°C (104° and
158°F) ; and hence, fail to grow below 40°C
e.g., Thermoactinomyces vulgaris; Thermus
aquaticus;
and Streptococcus thermophilus.
(
b) Microbes growing at low temperatures : Certain microorganisms grow best in the cold environmental
conditions and simply cannot grow above 20°C
e.g., Vibrio marinus strain MP-1 ;
and
Vibrio psychoerythrus.
(
c) Pathogenic bacteria: A host of organisms that are solely responsible for causing diseases in
humans do essentially require a temperature very close to that of the human body (
i.e., 37°C
or 98.4°F)
e.g., Salmonella typhi; Vibrio cholerae; Mycobacterium tuberculosis; Clostridium
tetani; Shigella dysenteriae; Treponema pallidum; Bordetella pertussis; Rickettsia rickettsii
etc.
(
d) Gaseous environment: It is equally important to have requisite gaseous environment for the
substantial growth of the microorganisms.
Examples:
(1) Aerobic Microbes: These are of two kinds, namely :
(
i) Facultative Aerobes i.e., microbes that are able to live and grow preferably in an environment
devoid of oxygen, but has adapted so that it can live and grow in the presence of oxygen.
(
ii) Obligate Aerobes i.e., microorganisms that can live and grow only in the presence ofoxygen.
(2)
Anaerobic Microbes: These microorganisms can live and grow in the absence of oxgyen,
and are of
two types, namely :
(
i) Facultative Anaerobes i.e., microbes that can live and grow with or without oxgyen.
(
ii) Obligatory Anaerobes i.e., microorganisms that can live and grow only in the absence of
oxygen.
(
e) Light (i.e., UV-Light): UV-Light provides a source of energy necessary for the growth of
certain microbes
e.g., cyanobacteria (blue green algae). Interestingly, some organisms may be indifferent
to light or at times may even prove to be quite deleterious to their legitimate growth.
(
f) Liquid Culture Medium: It has been observed that each and every type of microorganism
invariably grows in an absolutely typical characteristic manner in various
liquid culture medium with
variant composition, such as:
(
i) Sparse or abundant growth—as could be seen in a liquid medium.
(
ii) Evenly distributed growth—as seen spread throughout the liquid medium.
(
iii) Sedimented growth—as may be observed as a sediment usually at the bottom.(iv) Thin-film growth—as could be seen on the surface of the liquid culture medium.
(
v) Pellicle growth—as may be observed as a scum at the top.
Example: Salivary pellicle
—The thin-layer of salivary proteins and glycoproteins that quickly
adhere to the tooth surface after the tooth has been cleaned; this amorphous, bacteria-free layer may
serve as an attachment medium for bacteria, which in turn form
plaque.
(
g) Solid Culture Medium: It has been amply demonstrated that microorganisms invariably
grow on
solid culture medium as colonies* which are markedly distinct, compact masses of cells
evidently visible with a naked eye (macroscopically). In fact, the ensuing
colonies are usually characterized
based upon, their particular shape, size, consistency, texture, colouration, compactness, and other
several vital characteristic features.