It has been critically observed that the overall effect of
‘low temperature’
exclusively depends on the specific organism and the intensity of the application.
Example.
At temperatures ranging between 0–7°C (i.e., the ordinary refrigerator), the actual
rate of metabolism
of majority of microorganisms gets reduced substantially to such an extent that they
are rendered incapable of either synthesizing
toxins* or causing reproduction.**
Thus, one may conclude that
‘ordinary refrigeration’ exerts a distinct bacteriostatic effect
i.e.,
stops the multiplication vis-a-vis growth of microbes.
Psychotrophs***,
however, are found to grow appreciably but slowly particularly at the
refrigerator temperature conditions ; and may change the very
appearance and taste of food products
after a certain lapse of time.
Salient Features.
The various salient features of microbes in a ‘cold’ environment are as follows :
(1) A few microbes may even grow at
sub-freezing temperatures (i.e., below the freezing
temperature).
(2) Sudden exposure to
sub-freezing temperatures invariably render bacteria into the ‘dormantstate’;
however, they do not kill them (bactericidal effect) ultimately.
(3)
Gradual Freezing is observed to be quite harmful and detrimental to microorganisms, perhaps
due to the fact that the ice-crystals which eventually form and grow do disrupt the
cellularas well as the molecular structure of the microorganisms.
(4)
Life-Span of Frozen Vegetative Microbes—Usually remain active for a year upto 33% of
the
entire initial population, whereas other microbial species may afford relatively very
scanty survival rates.
Desiccation
In order to have both
normal growth and adequate multiplication the microorganisms do require
water. Desiccation
represents a typical state of microbes in the absence of water ; however, their
growth and reproduction remain restricted but could sustain viability for several years. Interestingly, as
soon as
‘water’ is duly made available to them the said organisms resume their usual growth and division
as well. This highly specific ability has been adequately employed in the laboratory manipulations
whereby the microbes are carefully
preserved by lyophilization.*
It has been duly observed that the ensuing resistance of the vegetative cells to undergo the
phenomenon of desiccation changes with the
specific species as well as the microorganism’s environment.
Example : Gonorrhea**
organism, Neisseria gonorrhoeae (Gonococcus), possess an ability to
withstand dryness only upto a duration 60 minutes hardly ; whereas,
Tuberculosis*** bacterium,
Mycobacterium tuberculosis
(Bacillus) may even remain completely viable for months together at a
stretch.
Important Points :
Following are certain important points which should always be borne in
mind :
(
a) An invariably susceptible microbe is found to be appreciably resistant when it gets duly
embedded in
pus cells, mucous secretions, and in faeces.
(
b) In contract to microbes the viruses are usually found to be quite resistant to the phenomenon
of
‘desiccation; however, they do not exhibit resistance comparable to the bacterial
endospores.
(
c) Importantly, in a typical hospital environment (setting) the presence and subsequent ability
of some particular
dried bacteria and endospores do remain absolutely viable, such as :
beddings, clothings, dust particulate matters, and above all the disposable (used) dressings
from patients may contain infectious organisms strategically located in dried pus, faecal
matter, mucous secretions, and urine.
Osmotic Pressure
Osmotic pressure
refers to–‘the pressure which develops when two solutions of different
concentrations are duly separated by a semipermeable membrane’.
In actual age-old practice, the preservation of food products
viz., pickles, fruits, are duly accomplished
by the use of high-concentrations of
salts and sugars which eventually exert their effects onaccount of the osmotic pressure. The most logical and probable underlying mechanism being the creation
of an extremely hypertonic environment due to the presence of these substances (salts and sugars) at
high concentrations that enables water to leave the
microbial cell precisely. In fact, the preservation
afforded by the
osmotic pressure very much resembles to that caused by desiccation (see Sectionbesides, the glaring fact that both processes evidently deny the microbial cell of the requisite
quantum of moisture essentially required for its normal growth. Dehydration of the
microbial cell actually
renders the
plasma membrane to shrink away from the respective cell-wall (i.e.,plasmolysis),
whereby the consequent cell stops growth (and hence reproduction), and it may not cause an instant
death. In a broader perspective, the fundamental principle of
osmotic pressure is largely exploited in
the prolonged preservation of food products.
Examples :
(a) Concentrated Salt Solutions (Brine Solution) may be used profusely in the
preservation and cure of meats, fish, vegetables, pickles etc.
(
b) Concentrated Sugar Solutions (Sugar Syrup) may be employed, extensively in the preservation
of lime juice, fruits etc.
Radiation
Radiation
refers to — ‘any form of radiant energy emission or divergence, as of energy in
all directions from luminous bodies, radiographical tubes, particle accelerators, radioactive elements,
and fluorescent substances’.
It has been established beyond any reasonable doubt that
radiation exerts its various effects on
the cells, depending upon its wavelength, intensity, and duration as well. Generally, one may come
across
two kinds of radiation which would cause a bactericidal effects on microbes, or usually referred
to as the
‘sterilizing radiation’, namely :
(
a) Ionizing Radiation, and
(
b) Nonionizing Radiation.
Each of the aforesaid types of radiation shall be treated individually in the sections that follows :
Ionizing Radiation
The
ionizing radiation normally possess a wavelength distinctly shorter in comparison to the
nonionizing radiation (size < 1 nm)
e.g., γ-rays, X-rays, or high-energy electron beams.
vividly depicts that the said
ionization radiation invariably carries a significant quantum
of energy ranging between 10
–5 nm (γ-rays) to 10–3 nm (X-rays).
γ
-Rays : These are emitted by radioactive cobalt (Co),
X-Rays :
These are produced by X-ray machines, and
Electron Beams :
These are generated by accelerating electrons to high energies in specialmachines.
Visible light plus other forms of radiant energy invariably radiate
via space as waves of
various lengths.
Ionizing radiation
viz., γ-rays and X-rays possess a wavelength shorter than 1 nm.
Nonionizing radiation
viz., UV-light has a wavelength ranging between 1–380 nm, where
the visible spectrum commences.
Salient Features.
The various salient features of the Ionizing Radiation are as stated under :
(1) The
γ-rays usually penetrate deeply but would essentially require reasonably longer duration,
extended to several hours, for the sterilization of relatively
large masses.
(2)
High-energy electron beams do possess appreciably lower penetrating power ; however,
need only a few seconds of exposure to cause sterilization.
(3) Major causative effect of
ionizing radiation being its distinct ability to the ionization of
water, which in turn gives rise to
highly reactive hydroxyl radicals [OH•]*. Interestingly, these radicals
critically interact with the cellular organic components, especially the DNA, and thereby kill the
cell ultimately.
(4)
High-energy electron beams (ionizing radiation) has recently gained an enormous worldwide
acceptance, recognition, and utilities for the exclusive sterilization of such substances as :
pharmaceuticals, disposable dental materials,
and disposable medical supplies. A few typical
examples are :
plastic syringes, catheters, surgical gloves, suturing materials.
Note. Radiation has virtually replaced ‘gases’ for the ultimate sterilization of these items.
Nonionizing Radiation
Predominantly the
nonionizing radiation possesses a distinct wavelength much longer than thatof the corresponding ionizing radiation, invariably greater than about 1 nm.
Example : UV-light :
The most befitting example of the nonionizing radiation is the UV-light,
which is able to cause
permanent damage to the DNA of exposed cells by virtue of creation of newer
additional bonds between the
‘adjacent thymines’ strategically present in the DNA-chains, as illustrated
in Figure : 7.6. The said figure evidently shows the formation of a
thymine dimer after being
exposed duly to the
UV-light whereby the adjacent thymines may be rendered into a cross-linked
entity.
Importantly, in the absence of the visible light, this particular mechanism is usually employed bya cell to afford the repair of the prevailing damage caused.
In reality, these
‘thymine dimers’ are found to cause effective inhibition in correcting replication
of the DNA in the course of division (reproduction) of the cell. It has been duly established that the
UV-wavelengths at nearly 260 nm
are most effective and useful for killing microbes due to the fact
that these are exhaustively absorbed by the cellular DNA.
Advantages of UV Light :
are as given under :
(1) It controls and maintains the miroorganisms in the air.
(2) A
‘UV-Radiation Lamp’ or a ‘Germicidal Lamp’ is abundantly and profusely employed
in a variety of such sensitive areas as :
operation theaters, hospital rooms, nurseries, and
cafeterias.
(3)
UV Light or Radiation is invariably employed to sterilize a plethora of highly sensitive
biological products
commonly used in the therapeutic armamentarium, such as : serum,
toxins,
and a variety of vaccines.
(4)
UV Light is also employed to sterilize the drinking water in homes, hospitals, and public
places.
(5)
UV Radiation is also used for the sterilization of the ultimate treated ‘municipal-waste
waters’
for agriculture and horticulture purposes.
Disadvantages of UV Light :
These are as stated under :
(1)
UV Radiation is found to be not very penetrating in nature ; and, therefore, the microorganisms
intended to be killed should be exposed almost directly to the UV-rays.
(2) Besides, such microbes that are adequately shielded (protected) by means of
textiles, coloured,
glass,
and paper (i.e., textured cellulose materials) are observed to be least affected
by the
UV radiation.
(3)
Serious Problem. In fact, UV light poses a serious problem in causing permanent damage to
human eyes on direct exposure, besides, prolonged exposure may even cause
sun burns as
well as
skin cancers.
Note : (1) Antimicrobial effect of UV sunlight is on account of the exclusive formation of the
‘singlet oxygen in the cytoplasm’.
(2) Microwaves (in the microwave oven) do not exhibit any direct effect on the microbes,
but kill them indirectly by heating the food stuff.