Microbial Control

Outline

•  History •  Definition of terms •  Pattern of microbial death •  Antimicrobial agents

– Physical – Chemical

History

Ring- a - ring of rosies

Pocketful of posies

Atichoo ! Atichoo !

We all fall down

Bubonic Plague or the Black Death

•  Epidemic swept thru Europe in the Middle Ages (13th and 14th centuries)

•  40 million people were killed – About 1/3 of the population of the continent

•  Etiological agent: – Yersinia pestis Gram (-) rod

•  2 Vectors – Rat – Flea

Yersinia pestis - Gram (-) bacillus

Vectors - Rat and Flea

Bubonic Plague Infection

1. Flea bite with Yersinia pestis 2. Bacteria multiply in the bloodstream

Bacteremia 3. Bacteria localize in lymph nodes, especially axillary and groin areas

4. Hemorrhaging occurs in lymph nodes, resulting in reddish rash that will eventually become “black and blue” swellings or Buboes (hence the name Bubonic Plague or Black Death)

Bubonic Plague Infection

5. If untreated, about 50 % Mortality Rate 6. If bacteria spread to the lungs, it becomes Pneumonic Plague and is now highly contagious (Almost a 99 % Mortality Rate)

History

A pouch of sweet smelling herbs or posies were carried due to the belief that the disease was transmitted by bad smells

Ring- a - ring of rosies

Pocketful of posies

Atichoo ! Atichoo !

We all fall down

Humans vs. Microbes

•  Most of History, microbes have been winning the battle

•  In the last 100 yrs or so the battle has swung in our favor – Why?

•  Because of our increasing knowledge of how to Control Microbial Growth

Smallpox

Variola virus

Eradicated in 1977 (Somalia)

Terms :

Sterilization vs. Disinfection •  Sterilization

– destroying all forms of life •  Disinfection

– destroying pathogens or unwanted organisms

Disinfectant vs. Antiseptic •  Disinfectant

– antimicrobial agent used on inanimate objects •  Antiseptic

– antimicrobial agent used on living tissue

cidal vs. static

•  Bactericidal - kills bacteria •  Bacteristatic - inhibits bacterial growth

•  Fungicidal •  Fungistatic •  Algacidal •  Algastatic

�  Commercial Sterilization: Killing C. botulinum endospores

Microbial death

“a microbe is defined DEAD if it does not grow when inoculated into culture medium that would normally support its growth”

Death is defined as the inability of the

organisms to form a visible colony –James Jay (Modern Food Microbiology 6th ed)

Bacterial populations die at a constant logarithmic rate

“It takes more time to kill a large population of bacteria than it does to kill a small population, because only a fraction of organisms die during a given time interval.

Conditions Influencing Effectiveness of Antimicrobial Agent Activity

1.  Population size –  Larger population requires a longer time to die

2.  Population composition –  Microorganisms vary markedly on susceptibility –  Vegetative versus Spores –  Young versus Mature cells

5.  Concentration or Intensity of an Antimicrobial Agent –  The more concentrated an agent the more rapidly

microbes can be destroyed –  BUT sometimes an agent may be more effective at lower

concentrations (e.g. 70% alcohol)

Conditions Influencing Effectiveness of Antimicrobial Agent Activity

3.  Duration of Exposure –  The longer the exposure to an agent the more they will be

killed

6.  Temperature –  An increase in temperature at which a chemical acts often

enhances it activity –  Example: acids used in high T = more effective

7.  Local environment –  pH, organic matter, etc –  Controls or Protects the pathogen

Targets of Antimicrobial Agents

1. Cell membrane 2. Enzymes & Proteins 3. DNA & RNA

Methods of microbial control

•  Physical – Heat – Filtration – Radiation

•  Chemical

HEAT •  Flaming •  Incineration •  Hot-air sterilization •  Moist-air sterilization

Hot-air Autoclave

Equivalent treatments 170˚C, 2 hr 121˚C, 15 min

HOW DOES HEAT KILL MICROBES

•  MOIST HEAT – Kill effectively by degrading nucleic acids and by

denaturing enzymes and other essential proteins – May also disrupt cell membranes

•  DRY HEAT – Microbial death results from the oxidation of cell

constituents and denaturation of proteins

MOIST HEAT STERILIZATION

•  Thermal Death Point (TDP) – The lowest temperature at which a microbial

suspension in killed in 10 minutes

•  Thermal Death Time (TDT) – The shortest time needed to kill all organisms

in a microbial suspension at a specific temperature and under defined conditions

However, such a destruction is logarithmic and it is theoretically NOT POSSIBLE to “completely destroy” microbes in a sample

•  Decimal Reduction Time or D value – Time required to kill 90% of the

microorganisms or spores in a sample at a specified temperature

– Time required for the line to drop by one log cycle or tenfold

– Used to estimate the relative resistance of a microbe to different temperatures

The z Value and

The F Value

•  z Value – The increase in temperature required to reduce D

to 1/10 its value or to reduce it by one log cycle

•  F value – Time in minutes at a specific temperature needed

to kill a population of cells or spores – Usually 121°C

APPLICATION: After a food have been canned, it must be heated to

eliminate the risk of botulism arising from the presence of Clostridium spores

Example: IF the D value = 0.204 minutes It would take 12D or 2.5 minutes to reduce the spore number by heating at the specified temperature

APPLICATION:

•  If the z value for Clostridium spores is 10°C •  It takes a 10°C change in temperature to alter

the D value tenfold

•  THUS: if the cans are to be processed at 111°C rather than 121°C, the D value would increase by tenfold t 2.04 minutes

•  The 12D value = 24.5 minutes

THE USE OF PHYSICAL METHODS: FILTRATION •  Applicable for heat-sensitive materials that needs sterilization

•  2 types of filters –  Depth filters: consist of fibrous or granular materials that have been

bonded into a thick layer filled with twisting channels of small diameter –  Membrane filters: porous membranes; 0.2 µm pore sizes

•  Laminar flow hood versus biological safety cabinets (HEPA filters) –  High Efficiency Particulate Air –  Remove 99.97% particles –  for sterilizing AIR

THE USE OF PHYSICAL METHODS: RADIATION

•  IONIZING RADIATION •  X rays, gamma rays, electron beams •  Excellent as a sterilizing agent and penetrates deep into objects

•  NON-IONIZING RADIATION –  UV (about 260nm) –  Quite lethal but does not penetrate glass, dirt films, water and

other substances very effectively

•  Microwaves: kill by heat not usually antimicrobial