Control of microbial growth

Question 1

Aseptic technique

Answer 1

Sterile solutions, plastics, glassware, environment

Removal/ destruction of contaminating micro-organisms

Protective clothing

Question 2

Why control microbial growth

Answer 2

In the lab
Health of worker, health of others
Prevent contamination of environment
Prevent contamination of sample

In industry :
Hospitals – prevent spread of infection
Food industry, prevent food spoilage, disease
Pharmaceutical industry, avoid contamination of medical supplies

Question 3

Risk assessment

Answer 3

Classification of microbes

Procedure, eg risk of aerosol

Question 4

Classification of microbes

Answer 4

Biological safety class 1 – not known to cause disease (e.g. lab strains, Rhizobium)

Biological safety class 2 – may cause disease, treatable and unlikely to spread to others (e.g. Salmonella)

Biological safety class 3 – causes disease risk to workers and others, but treatable (Mycobacterium tuberculosis, Yersinia pestis)

Biological safety class 4 – serious disease high risk to workers and public (some viruses)

Question 5

sterilization

Answer 5

destruction or removal of all viable organisms, include spores

Question 6

Antiseptics

Answer 6

chemical agents that kill or inhibit growth of microorganisms when applied to tissue

Question 7

antisepsis

Answer 7

prevention of infection of living tissue by microorganisms

Question 8

disinfectants

Answer 8

agents, usually chemical, used for disinfection

Question 9

disinfection

Answer 9

killing, inhibition of vegetative cells -pathogenic organisms, generally on inanimate objects

Question 10

Antimicrobial agents

Answer 10

agents that kill microorganisms or inhibit their growth

~ bacteriocidal agents kill vegetative cells, not necessarily endospores

~ bacteriostatic agents inhibit growth

Question 11

Antibiotics:

Answer 11

microbial products or their derivatives that kill or inhibit growth of bacteria

Question 12

Pattern of cell death

Answer 12

• microorganisms are not killed instantly
• population death usually occurs exponentially
• tail end of slightly more resistant cells
• Assay of death -Viable count – generally used

Question 13

Measuring heat-killing efficiency

Answer 13

Thermal death time (TDT)

Decimal reduction time (D or D value)

Question 14

Decimal reduction time (D or D value)

Answer 14

time required to kill 90% of microorganisms or spores in a sample at a specific temperature

Question 15

Thermal death time (TDT)

Answer 15

shortest time needed to kill all microorganisms in a suspension at a specific temperature and under defined conditions

Question 16

Monitoring bacterial death

Answer 16

1. (Vital stain – microscopy)
2. Viable colony countcolony forming units (cfu)/ml or gram material. Quantitate survival.
3. Growth on subculture in media. Effectively sterile?

Monitor death by inability to divide and grow (2 and 3)

Question 17

Conditions Influencing the Effectiveness of Antimicrobial Agent Activity

Answer 17

population size - larger popn longer required

population composition - microorganisms differ markedly in their sensitivity to antimicrobial agents, spores

concentration of antimicrobial agent - although relationship is not linear

duration of exposure

temperature - higher temperatures usually increase amount of killing

local environment (many factors) e.g.:
~ pH - microorganism more readily killed at low pH
~ viscosity & concentration of organic matter - can dramatically affect efficacy of antimicrobial agents (e.g increased organic material in water – additional chlorine)