microbial disinfection Flashcards
in terms of microbial disinfection, what part of the bacterial growth curve do we aim to maximise?
- aim to maximise the death or decline phase
define sterilisation
- kills off all microorganisms (including spores)
- not an absolute state
define disinfection
- is to kill, inhibit or remove pathogens to levels that prevent infection risk on inanimate objects & surfaces
define antisepsis?
- is to kill, inhibit or remove pathogens to levels that prevent infection risk in skin, mucous membranes & tissues
define sanitation
- is reducing microbes to a “safe” or “acceptable” level
- imprecise, non-scientific term
define asepsis
- is the prevention of microbial contamination by excluding or removing microbes
define decontamination
- is cleaning & disinfection
define biocidal
- lethal to microbes by irreversible inhibition (bactericidal, virocidal, germicidal)
define biostatic
- is the reversible inhibition of reproduction
what are the 2 ways that viability can be determined?
- direct identification & enumeration (direct count)
2. indirect indicators - presence of metabolic by-products
what are the limitations of viability measures for infectious organisms?
- sensitivity, specificity & limits of detection for counts
- for molecular methods - theres no differentiation between viable microorganisms vs presence
- for serological methods - theres no differentiation between current infection vs a previous exposure
- microbe could be viable but in a non-culturable state
- death of a microbe can be difficult to define
when are microbes more susceptible to death by disinfectants?
- microbes in the growth phase
what factors influence a microbe’s susceptibility to death?
- stage of growth
- clumping of cells
- environmental conditions
- microbial communities (protection)
- specific strain variability
- intracellular parasites
what are bacterial endospores? and what basis do they form?
- is a survival mechanism used by some bacteria
- bacteria that form endospores can remain dormant for decades & when environmental conditions become favourable, they become activated & germinate
- endospores form the basis of sterilisation standards
how do viral envelopes affect susceptibility to disinfectants, heat & environmental degradation?
- enveloped viruses are more susceptible to disinfectants, heat & environmental degradation than non-enveloped viruses
what is sublethal injury?
- sublethal injury is a reversible state of injury where cell can recover in favourable conditions
- is a viable but non-culturable state
- increases susceptibility of death for these microbes
how are biofilms formed?
- biofilms are formed when 3D bacterial communities attach to a surface
- bacteria are embedded in an extracellular polymeric substance
what are biofilms protected from?
- physical removal
- disinfectants, antiseptics & antibiotics
- immune system
name the 4 physical methods of disinfection
- heat
- ionising radiation
- gases
- chemicals
when is dry heat used as a physical method of disinfection?
- 160*C at 60-90mins
- for porous solids (powders) & non-aqueous liquids
- for items that cannot be subjected to moist heat as it is non corrosive
when is moist heat used as a physical method of disinfection?
- 100*C at 3 mins
- more efficient compared to dry heat
- most effective method is pressurised steam (autoclave)
- autoclave kills bacteria, viruses & spores
- pressurised steam allows water to store more energy to kill microbes
define the decimal reduction time (D value)?
- is the time it takes in minutes for a tenfold reduction in specified bacterial population at a specified temperature
- 1 log reduction = 90% reduction of bacteria
- D value extends below the x-axis, meaning absolute sterility is not possible
define thermal death time (Z value)
- is the temperature change required to change the D value tenfold (lower Z = more heat susceptible)
- measures resistance to different levels of heat
how does ionising radiation work as a physical method is disinfection? what are its pros & cons?
- causes DNA damage & forms free radicals that are lethal to microbes
- cons: space, cost, safety
- pros: does not use heat or moisture, penetrates packaging which reduces risk of contamination after radiation treatment
what are the applications of ionising radiation?
- single use materials
- medical materials (sutures, syringes)
- pharmaceuticals
- lab equipment
what are the pros & cons of using alkalising gases as a physical method of disinfection?
pros
- highly effective against a wide range of organisms
- highly diffusible
- can be used for heat sensitive material & electronics
cons
- toxicity
- corrosion, flammable
- special equipment & facilities needed
how is filtration used as a physical method of disinfection?
- small pore sizes filter out different microbes
- 10nm needed for viral sterility
what are 3 considerations for chemical methods of disinfection
- material being disinfected
- contamination with organic material
- contact time required
when are acids & alkalis used as a chemical method of disinfection? what are the benefits of using them?
- used for large scale or industrial/commercial disinfection
- cheap, readily available
- resistant to organic matter
- has detergent/cleaning action
- effectively kills a range or microbes & biofilms
but is corrosive
when are alcohols used as a chemical method of disinfection?
- mainly for skin disinfection
- often used in conjunction with iodine or chlorhexidine
- works better with 70% alcohol mixed with water
- fast acting but short residual activity
when are aldehydes used as a chemical method of disinfection?
- used for heat-sensitive equipment & materials
- wide antimicrobial spectrum, highly effective but have some toxicity
what are the benefits & limitations of using quaternary ammonium compound as a chemical method of disinfection?
- is a general cationic surface-active disinfectant with cleaning action
- prolonged contact time is possible
- lower user & patient toxicity & irritability
- inactivated by organic matter, hard water & soaps
- low activity against gram negative bacteria
how is chlorhexidine used as a chemical disinfectant?
- 0.5% water solution is relatively non-irritant & can be used on skin & mucous membranes but not ears
- 0.5% in alcohol is used for skin gels & rubs
- good residual activity
- best if 5-10min contact time
what are the main benefits & limitations of chlorine?
- wide pathogen range & is highly effective, including spores
- cheap & accessible
- inactivated by organic material
- corrosive for stainless steel & fabrics
- irritants esp mucous membranes
- store & make fresh
what is iodine commonly used for during chemical disinfection?
- used for treat dips & anti fungal scrubs
- as it is effective against fungal infections, low irritant & effect against a wide range of organisms
what are oxidising agents used for during chemical disinfection?
- good for environmental application
- per oxygen biocides have agriculture & industrial uses (low toxicity & corrosiveness, wide range & generally good effectiveness)
why can disinfectants sometimes fail?
- microbes can become resistant to the disinfectant
- improper use (not fit for purpose, not made up correctly, insufficient contact time or excessive organic matter load)
- most disinfectant failure is b/c of poor contact between pathogen & disinfectant
what is disinfectant contamination and which disinfectants are susceptible to disinfectant contamination?
- some organisms can grow & reproduce in disinfectants
- QATS, chlorhexidine & synthetic phenolics
what is disinfectant contamination and which disinfectants are susceptible to disinfectant contamination?
- some organisms can grow & reproduce in disinfectants
- QATS, chlorhexidine & synthetic phenolics
how should carcasses be disposed of to prevent dead animals from being a source of decontamination?
- carcasses require composting, deep pit burial or incineration
