Lecture BIOD 8: Control of Microorganisms

1
Lecture BIOD 8 Control of Microorganisms
- This chapter specifically addresses the control
destruction of microorganisms, a topic of
immense practical importance.
- Many microorganisms are beneficial and
necessary for human well-being.
- However, microbial activities may have
undesirable consequences, such as food spoilage
and disease.
- It is essential to be able to kill
microorganisms or inhibit their growth to
minimize their destructive effects.

• Terminology of Microbial Control

- Sterilization is the destruction or removal of
all viable organisms from an object or from a
particular environment.
- Commercial sterilization is heat treatment of
canned foods to destroy Clostridium botulinum
endospores.

- Disinfection is the killing, inhibition, or
removal of pathogenic microorganisms usually on
a nonliving surface..
- Antisepsis is the prevention of infection of
living tissue by microorganisms.
- Sanitization is the reduction of the microbial
population to a safe level as determined by
public health standards.
2
- -cide is a suffix indicating that the agent
will kill the kind of organism in question (e.g.,
viricide, fungicide).
- -static is a suffix indicating that the agent
will prevent the growth of the type of organism
in question (e.g., bacteriostatic, fungistatic).

• Pattern of Microbial Death

- A microorganism is considered dead when it is
unable to grow in conditions that would normally
support its growth.
- Microorganisms are not killed instantly when
exposed to a lethal agent.
- Bacterial populations subjected to heat or
antimicrobial chemicals usually die at a constant
rate.
- Such a death curve, when plotted
logarithmically, shows this constant death rate
as a straight line.
- Time to kill a microbial population depends on
many factors, such as the number of microbes,
the microbial species, the life cycle phases of
the species (i.e., endospores), etc.

• Conditions Influencing the Effectiveness of
  Antimicrobial Agent Activity

(1) Population size larger populations take
longer to kill than smaller populations.
3
(2) Population composition microorganisms differ
markedly in their sensitivity to various agents.
(3) Concentration or intensity of the
antimicrobial agent higher concentrations or
intensities are generally more efficient, but the
relationship is not linear.
(4) Duration of exposure the longer the
exposure, the greater the number of organisms
killed.
(5) Temperature a higher temperature will
usually (but not always) increase the
effectiveness of killing.
(6) Local environment environmental factors,
such as pH, viscosity, and concentration of
organic matter can profoundly influence the
effectiveness of a particular antimicrobial agent.

• Use of Physical Methods in Control

• Heat

(1) Moist Heat
- Boiling water is effective against vegetative
cells and eucaryotic spores.
- Autoclaving (steam under pressure) is effective
against vegetative cells and most bacterial
endospores.
4
- Pasteurization a process involving brief
exposure to temperatures below the boiling point
of water.
- It reduces the total microbial population and
thereby increases the shelf life of the treated
material.
- It is often used for heat-sensitive materials
that cannot withstand prolonged exposure to high
temperatures there are 3 types

• Low-temperature long-term (LTLT)
  pasteurization-63C for 30 min
• High-temperature short-term (HTST) flash
  pasteurization-72C for 15 sec
• Ultrahigh temperature (UHT) pasteurization-140 to
  150C for 1 to 3 sec

(2) Dry Heat
- Can be used to sterilize moisture-sensitive
materials such as powders, oils, and similar
items.
- It is less efficient than moist heat because it
usually requires higher temperatures (160 to
170C) and longer exposure times (2 to 3 hrs).

• Heat-killing Efficiency

- Heat-killing efficiency in expressed in TDT, D
value, Z value, and F value.
- Thermal death time (TDT) is the shortest time
necessary to kill all microorganisms in a
suspension at a specific temperature and under
defined conditions.
5
- Decimal reduction time (D, or D value) is the
time required to kill 90 of the microorganisms
or spores in a sample at a specific temperature.
- Z value is the increase in temperature required
to reduce D to 1/10 of its previous value.
- F value is the time in minutes at a specific
temperature (usually 121.1C) necessary to kill a
population of cells or spores.

• Filtration

- Filtration is the passage of a liquid or gas
through a filter with pores small enough to
retain microbes.
- Filtration sterilizes heat-sensitive liquids
and gases by removing microorganisms rather than
destroying them.
- Microbes can be removed from air by
high-efficiency particulate air filters.
- Membrane filters composed of nitrocellulose or
cellulose acetate are commonly used to filter out
bacteria, viruses, and even large proteins.
6

• Radiation

- Ultraviolet (UV) radiation is effective, but
its use is limited to surface sterilization
because UV radiation does not penetrate glass,
dirt films, water, and other substances.
- Ionizing radiation (X rays, gamma rays, etc.)
is effective and penetrates the material.
However, it is not widely used because of cost
and concerns about the effects of the radiation
on food.

• Use of Chemical Agents in Control

- Phenolics used in labs and hospitals as
disinfectants act by denaturing proteins.
- Alcohols widely used disinfectants and
antiseptics will not kill endospores act by
denaturing proteins and possibly by dissolving
membrane lipids
- Halogens widely used antiseptics and
disinfectants iodine acts by oxidizing cell
constituents and iodinating cell proteins
chlorine acts primarily by oxidizing cell
constituents.
- Heavy metals effective but usually toxic act
by combining with proteins and inactivating them.
- Quaternary ammonium compounds cationic
detergents used as disinfectants for food
utensils and small instruments, and because of
low toxicity, as antiseptics for skin act by
disrupting biological membranes and possibly by
denaturing proteins.
7
- Aldehydes reactive molecules that can be used
as chemical sterilants may irritate the skin
act by combining with proteins and inactivating
them.
- Sterilizing gases (e.g., ethylene oxide,
betapropiolactone)-can be used to sterilize
heat-sensitive materials such as plastic petri
dishes and disposable syringes act by combining
with proteins and inactivating them.
- Vapor-phase hydrogen peroxide has been used to
decontaminate biological safety cabinets.