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Methods of Microbial Control in Healthcare
Principles to Practices
Dr. Rajarshi Gupta
Part 1 - Topics to be covered
1. Role / Need of/ for Microbial Control Methods in healthcare2. Methods of microbial control-an overview3. Choice of methods 4. Factors affecting methods of microbial control 5. Methods of Sterilization - an overview6. Physical methods of sterilization
Need for microbial control in healthcare settings
SOMETHING WAS WRONG
And then came Semmelweiss:
Though ostracized by medical fraternity at that time, his work on the role of handwashing in reducing mortality due to 'Childbed fever' was monumental in infection control.
Endoscopy associated infection outbreaks1974-2004
Seoane-Vazquez E, et al. Endoscopy 2007;39:742-778
Pathogen Estimated surface survival Acinetobacter spp. 3 days - 5 months
C.difficile spores 5months
E.coli 1.5 hours - 16 months
Enterococci(including VRE) 5 days - 4 months
Klebsiella spp. 2 hours - 30 months
M. tuberculosis 1 day - 4 months
P.aeruginosa 6 hours - 16 months
Staphylococci(including MRSA) 7 days - 7 months
HSV Minutes
Influenza Hours
Hepatitis A Days
HIV Days
Hepatitis B Months
With so many bugs remaining viable for such a long time,lack of microbial control will invariably lead to transfer of these to susceptible hosts.
Microbial Control
Source Control
Transmission prevention
Sterilisation
Disinfection
Antisepsis
Anti-microbial therapy
Inanimate objects primarily
Mainly in animate
Definitions:• Sterilisation: Elimination of all microbial forms ( vegetative cells + spores)
• Disinfection: Elimination of atleast pathogenic of microbial forms applied to inanimate objects
• Antisepsis : Same principle as disinfection but applied to animate surfaces
• Cleaning: Elimination of visible dirt / other organic matter from animate or inanimate surfaces mainly using surface tension lowering agents ( necessary for dislodging biofilms with entangled bacteria where disinfectant or antiseptic can’t penetrate)
• Decontamination: Rendering an area / surface / instrument safe for handling• Asepsis: It is a practice for prevention of contact with microorganisms
Classification of methods of microbial control
Physical Methods Chemical Methods
Heat
Filtration
Radiation
Age old methods
Osmotic pressure(eg. high salt in pickles)
Refrigeration
( These only suppress bacterial multiplication but dont kill them)
Better and quality assured methods for killing microbes
RATIONALE TO DISINFECTION AND STERILIZATION
•Spaulding scheme: More than 30 years ago, Earle H. Spaulding
•Clear and logical classification scheme
•Retained, refined, and successfully used by infection control professionals and others when planning methods for disinfection or sterilization
RATIONALE TO DISINFECTION AND STERILIZATION(Spaulding scheme from more than 30 years ago, by Earle H. Spaulding)
•Critical items - Enter sterile tissue or vascular system (STERILISE ALWAYS)
•Semi-critical items - Contact mucous membranes or non-intact skin ( HIGH LEVEL DISINFECTION)
•Non-critical items – Contact with intact skin only (LOW LEVEL DISINFECTION)
Changes in concepts since 1981 and pitfalls of the Spaulding scheme•Oversimplification of categories
(Unresolved in Spaulding scheme – What to do if semi-critical items are used along with critical items?)
Eg. Bronchoscope + Punch biopsy forceps
•Increasing complexity in medical devices
1. Complex ventilator circuits
2. Advanced dialysis machines
3. Fibre-optic bronchoscopes
Complicating factors here are mainly •DIFFERENTIAL SENSITIVITY OF MACHINE COMPONENTS TO HEAT•NETWORK OF SMALL LUMENS REDUCE STERILISER PENETRATION
Sterilisation and disinfection for Prion diseases
Prions are resistant to common sterilisation/disinfection protocols
They are difficult to remove by cleaning as they stick to surfaces
Protocol : 1N sodium hydroxide (NaOH) and heat in an autoclave at 121°C for 30 minutes
• Understanding the importance of Biofilms in this context
•Bronchoscopes and endoscopy devices often get coated with biofilms containing organisms embedded in matrix.
Sterilization or disinfection of such scopes is effective only if they are ‘Cleaned’ first because the sterilizer or disinfectant will not penetrate the biofilm
In general prior ' CLEANING' is essential for any sterilisation/disinfection process to be effective
“I thot I saw a tweetybird”
“I thot I taw a pussy cat”
“ But the biofilmwill protect us!”
•Intrinsic resistance, emergence of multi-drug resistant (MDR) bugs and their resistance to disinfectants
Intrinsic resistance – Pseudomonas spp. and Burkholderia spp. To quarternary ammonium compounds.
Conflicting results from studies and emergent hypotheses propose cross resistance between disinfectants and antibiotics. But this isn't a problem if the disinfectant is used at recommended concentrations
1.Gentamicin resistance might encode reduced susceptibility to propamidine, quaternary ammonium compounds, and ethidium bromide.
2.MRSA strains might be less susceptible than methicillin-sensitive S. aureus
(MSSA) strains to chlorhexidine, propamidine,
and the quaternary ammonium compound cetrimide.
General factors affecting sterilization or disinfection
1. Contact time: Very important, always allow enough time for proper penetration. Longer the contact time more effective the process.
2. Temperature: Generally higher the temperature lesser the contact time.3. Pressure: Important in autoclaves as changes can alter phase boundary of
steam and thus prevent enough latent heat release.4. pH: Increase in pH improves the antimicrobial activity of some disinfectants
(e.g., glutaraldehyde, quaternary ammonium compounds) but decreases the antimicrobial activity of others (e.g., phenols, hypochlorites, and iodine.
5. Relative humidity is the single most important factor influencing the activity of gaseous disinfectants/sterilants, such as EtO, moisture also increases effectiveness of heat
General factors affecting sterilization or disinfection contd…6. Water hardness (i.e., high concentration of divalent cations) reduces the rate of kill of certain disinfectants because divalent cations
7. Organic matter: Neutralise disinfectant directly or form a physical barrier around microbes. Carbohydrates reduce sensitivity to heat
8. Concentration and potency: Always use the disinfectant at the recommended ‘cidal’ concentration.
9. Biofilms
10. Intrinsic resistance
Cleaning( Rationale behind…)
Removal of visible dirt from surfaces
Inorganic and organic materials that remain on the surfaces of instruments interfere with the effectiveness of sterilization and disinfection
If soiled materials dry or bake onto the instruments, the removal process becomes more difficult and the disinfection or sterilization process less effective or ineffective.
Cleaning(methods)
Manual cleaning Friction
Fluidics
Scrubbing clean with detergent( ie. Surface tension lowering agent)
Fluids under pressure after scrubbing
Fluids under pressure in narrow lumen tubes where scrubbing brushes cannot reach
Ultrasonic Cleaners
• Disrupt by cavitations and implosion(waves of acoustic
energy are propagated in aqueous solutions) the bonds
that hold particulate matter to surfaces using ultrasonic
energy (20 – 400kHz) the bonds that hold particulate matter to
surfaces
• Ultrasound alone does not significantly inactivate bacteria, sonication can act synergistically to increase the cidal efficacy of a disinfectant.
• Special concern – Ultrasound energy can break down cells and release endotoxins onto surgical instruments, leading to inflammatory responses in patients.
• Usually used for delicate objects
Ultrasonic Cleaners
Sterilisation( Physical methods)
1. Heat
2. Filtration
3. Radiation
A quick glance at the bacterial growth curve
and the death curve
Probability of killing bacteria depends on1. The concentration of bacteria2. The temperature3. Time of sterilisation
Any sterilization or disinfection procedure causes 90% reduction in bacterialpopulation in a given time. Total eleimination of bacteria is a theoritical concept and cannot be attained practically.
How long to sterilise?
Answer: The D-value
It is used to determine the time of contact for any sterilisation process.
It is the time required to reduce the organism number 10 times or by one log.
Thermal death time( concept)
Similar to D-value but it is the time taken to kill all organisms.
ie. it is a sum of successive D-values
Z-value : Measure of thermal resistance of spores.
It is the change in temp( ) to produce ℃10 fold change in thermal death time of spores
Types of heat sterilisation
Dry heat - Kills by oxidising cellular components--- SLOW
Moist Heat - Kills by coagulating cellular proteins using combination of heat and moisture ---- RAPID
IN GENERAL MOIST HEAT IS MORE EFFECTIVE THAN DRY HEAT.
BUT DRY HEAT IS MORE CONVENIENT
Dry heat sterilisation( namely the hot air oven)Mainly for glassware that can tolerate high temperaturesFor oils, greases that are waterproofMetallic objects and dry powders that can be damaged by moisture.
Recommended temp. and duration
180 degree celsius - 30 mins170 degree celsius - 1 hr160 degree celsius - 2 hrs
AUTOCLAVING - moist heat sterilisationPrinciple:
Saturated steam ( no admixture of air)
at more than 100 degrees celsius( usually 121 degees celsius)
at elevated pressure( usually 15lb/inch2)
Importance of the principle:
Recommended temperature at recommended pressure ensures steam to stay at PHASE BOUNDARY
ie. Steam condenses on contact with items to release latent heat
Above recommended temperature(superheated steam) or low pressure causes steam to fall out of phase boundary and does not condense immediately on contact
• Steam at phase boundary condenses into water and contracts to create a low pressure to cause ingress of more steam
-- Steam out of phase boundary does not condense to draw fresh steam and hence temperature in autoclave falls
-- Admixture with air causes causes less ingress of fresh steam
• Air in the autoclave occupies small spaces in porous loads and reduces steam penetration
Some relevant definitions:
Sterilisation holding time: Time for which the load is exposed to recommended tempertaure and recommended pressure
Heat penetration time: Time required by the load to reach the recommended temperature and pressure and for the steaam to penetrate the porous spaces.
Types of autoclaves
Basically 2 types of autoclaves
Gravity type:Steam being lighter displaces air through an exhaust below and slowly fills up the chamber
•Complete air removal is impossible
•Less steam penetration of porous loads
•Does not allow active drying of load
Vacuum assisted autoclaves:
•Air pumped out before steam is charged
•Better penetration of steam into loads
•Allows drying after autoclave cycle by inducing partial vacuum
Uses of autoclave types:
GRAVITY TYPES - For non porous loads that do not have difficult to access areas(eg. wide mouthed containers, objects with flat non porous surfaces)
VACUUM ASSISTED TYPES - For porous loads where steam penetration into the smallest spaces is desired
(eg. soiled cotton,linen, narrow tubes, lumened instruments)
LIQUID CULTURE MEDIA CAN'T BE STERILIZED IN A VACUUM ASSISTED AUTOCLAVEAS THE INITIAL VACUUM THAT IS GENERATED MIGHT VAPORISE THE MEDIA
Quality assurance of autoclaves
BOWIE-DICK HEAT SENSITIVE STRIPS - Changes colour( blue to black) at 121℃
- Checks air admixture. If air is present, steam does not penetrate the centre of the strip and the centre stays blue
- Does not monitor time of contact. Just monitors temperature
- To be placed in the part of the autoclave least accessible to steam.
Geobacillus stearothermophilus ATCC 7953 spore strips
- These spores are most resistant to autoclaving
- Test strip is exposed with the load for the same cycle conditions
- Autoclaved spore strips are incubated in broth at 55 to check for complete ℃destruction of spores( no growth)
- Keep positive control with an unsterilized spore strip from same lot.
Physical methods like pressure recording devices and temperature recording thermocouples can be placed in the least accessible part of the load to check whether recommended sterilization conditions are achieved there or not
Low-temperature steam formaldehyde(LTSF)Steam at low temperature (73 degrees celsius) and sub atmospheric
pressure( 263 mm of Hg) is ineffective for bacterial spores.
But mixed with Formaldehyde it becomes strongly sporicidal due to
synergy with steam.
Hence, allows an autoclave to perform at lower temeprature for heat sensitive items.
Low temperature plasma sterilisation
• Occurs at low temperatures (37 -44 )℃
• Utilises gas ionised by strong electric field( PLASMA)
• Sporicidal
• Commonly ionised hydrogen peroxide gas is used to cause free radical damage
Moist heat Disinfection
Boiling at 100 degrees celsius : Good for vegetative bacteria and HBV but unsatisfactory for spores. Not
recommended for porous loads where penetration might be an issue
Free steaming at 100 degrees celsius: Saturated steam at normal atmospheric pressure. Mainly for heat sensitive media such as XLD, TCBS, DCA.
Pasteurization( a form of heat disinfection)Process of decontamination of milk
Kills only vegetative forms and not spores
Low temperature holding method(LTH) - 145°F or 62.8 for 30 mins℃
High temperature short time method(HTST) - 161°F or 71.7 for 15 secs℃
THE PRODUCT MUST THEN BE KEPT REFRIGERATED TO PREVENT SPORES FROM GERMINATING
Pasteurization( concepts behind.....)
• Every particle of milk must be heated to the
recommended temperature for the recommended period.
• Why a temperature of 145°F for 30 minutes for LTH ?
Milk transmitted pathogen C.burnetti can
survive upto 143°F for 30 mins.
Radiation Sterilisation
Ultraviolet light - Most effective at around 265nm wavelength( ie. UV-C) Forms pyrimidine dimers in bacterial DNA Very poor penetration, hence used only for surface
sterilisation ( decontamination of surfaces in food industry, decontamination of air in biosafety cabinets)
Gamma rays - Ionising radiation damages vital cellular moleculesImmense penetration powerCan be used to sterilise objects of considerable thickness( packaged food, medical devices, thick plastic syringes)
UV-A( 315-400nm) and UV-B(280-315nm) are not germicidal
Filtration
• Used for sterilisation gases(eg. air) or liquids( eg. serum)• Filtration depends upon pore size and
charge carried on both filter material and organism surface
Namely
The asbestos pad Seitz filterThe diatomaceous earth Berkefeld filterThe porcelain Chamberland-Pasteur filter The sintered glass filter
Nitrocellulose membrane filters with varying pore sizes
0.22 microns -- for Pseudomonas diminuta
0.45 microns -- for coliforms
0.80 microns -- for general airborne pathogens
High efficiency particulate air (HEPA) filters
Air filters for very small particles mainly in a laminar airflow
Consists of a dense meshwork of filter fibres
HEPA FILTER PRINCIPLE
Inertial Impaction:Inertia works on large, heavy particles suspended in the flow stream.These particles are heavier than the fluid surrounding them. As the fluid changes direction to enter the fibre space, the particle continues in astraight line and collides with the media fibres where it is trapped and held
HEPA FILTER PRINCIPLE
Interception:Direct interception works on particlesin the mid-range size that are not quite large enough to have inertia and notsmall enough to diffuse within the flow stream. These mid-sized particles follow the flow stream as it bends through the fibre spaces. Particles are intercepted or captured when they touch a fibre.
HEPA FILTER PRINCIPLE
Diffusion:Diffusion works on the smallestparticles. Small particles are notheld in place by the viscous fluid and exhibit random(Brownian motion) within the flow stream. Asthe particles traverse the flow stream, they collide with the fibre and are collected.
Thus, particle size of 0.3 microns are the least filtered in an HEPA filter
So, the effciciency of any HEPA filter is assessed by its capability to filter particles of around 0.3 microns
Transmission prevention
"However stringent source control might be, some notorious bug
at some point of time will surely find some route of entry
into some individual to cause some disease"
SUMMARY
• Microbial control involves both killing microbes and preventing thir transmission.
• Sterilization and disinfection differs by the fact that sterilisation kills spores while disinfection does not.
• Cleaning is essential before sterilization.• Physical methods of sterilization include primarily heat, filtration and
radiation.• Choice of sterilization method depends on the object to be sterilized
(whether solid, liquid or gas, heat susceptibility, susceptibility to moisture )
• Transmission prevention is equally important in microbial control as sterilization and disinfection.
THANK YOU
References:
1. Guideline for Disinfection and Sterilization in Healthcare Facilities, 2008, Centres for Disease Control and Prevention
2. Microbiology, 5th edition, Pelczar, Michael J.
3. Mackie and McCartney, Practical medical microbiology, 14th edition, Collee, J.G
4. Guidance for Filtration and Air-Cleaning System to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks, Department of Health and Human Services Centers for Disease Control and Prevention ,National Institute for Occupational Safety and Health (NIOSH)
