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Chemical Agents


  1. Phenols and Cresols
    1. Cause cell membrane damage
    2. Resistant to inactivation by organic matter
    3. Active against Gram +, Gram -, moderately active against Mycobacteria
    4. Little activity against spores
    5. Use: Discarded cultures, pipettes, other infected material
    6. Phenol: bactericidal 3-5%
  2. Halogens
    1. Chlorine, hypochlorites, inorganic/organic chloramines
    2. Bactericidal, sporicidal, virucidal, little activity against M. tuberculosis
    3. Release of free chlorine → strong oxidizing agent
    4. Activity decreased by presence of organic matter Iodine:
    5. Alcoholic/aqueous solutions: skin antiseptic
    6. Also active against M. tuberculosis
    7. Iodophores: Mixture of iodine with surface active agents, best antiseptic, eg. Betadine
  3. Metallic salts
  1. Mercury: combines with -SH group of bacterial proteins e.g. merthiolate: preservation of sera
  2. 1% silver nitrate: prophylaxis for gonococcal ophthalmia

  1. Aldehydes
    1. Formaldehyde
      1. Irritant, water soluble gas; Lethal to bacteria, spores, fungi, viruses
      2. Cheap
      3. Sterilization of rooms, furniture, clothing blankets, mattresses
      4. Less effective in presence of organic matter
    2. Glutaraldehyde
      1. More effective, less irritant
      2. 2% solution(Cidex): sterilization of heat sensitive instruments cystoscopes, bronchoscopes (Fiber optic scopes) for at least 3 hrs at alkaline pH       
  2. Alcohols
    1. Kill Bacteria, no action on spores, viruses
    2. 60-70%; presence of water essential
    3. Isopropyl alcohol: Better fat solvent, more bactericidal, less volatile
  3. Vapour phase disinfectants:
    1. Ethylene oxide (ETO.)
      1. Mechanism of Action : Alkylation Reaction especially of nucleic acids
      2. Various types used
        Pure ETO.: Highly explosive and flammable used hence can he in self contained system only
        ETO. + Chlorofluoro Carbon: CFC causes depletion of ozone layer.  Banned Now
        1. High pressure mixture
        2. Change in composition with time leading to increase in E.O. conc.
        3. Acid formation - corrosion
      3. ​​ETO Sterilization depends upon:
        Concentration of E.O. : usually > 700mg/l
        Temperature    : 2 types of cycles
        1. Cool cycle : runs at 37°C
        2. Hot cycle : 47 — 66°C (Average 55)
      4. ​​Biological Control : Spores of Bacillus globigi

Side Effects :

  1. Acute Reactions
    1. Headache, Nausea, Vomiting, Dizziness
    2. Bums, Blisters
    3. Severe toxic shock in cardiopulmonary bypass surgery patients
    4. Hypersensitivity reactions in haemodialysis patients
  2. Chronic Reactions
    1. Mutagenic
    2. Peripheral neuropathy
  3. Precautions during use
    1. Installation : Away from general area
    2. Kept at Negative pressure
    3. Minimum of 10 air exchange / hour in area having ETO
    4. Sterilizer
  • Immediate exhaust on opening the door of sterilizer
  • Aeration of Load is must

v. PEL (Permissible Exposure Level of ETO) is - 1PPM / 8 hours


vi. STEL (Short time exposure limit) 5PPM     


  1. Hydrogen peroxide –
    1. Broad spectrum antimicrobial activity with confirmed virucidal, bactericidal, fungicidal, myco bactericidal, and sporicidal activity.
    2. In the gas phase, hydrogen peroxide (also known as VHP for Vaporized Hydrogen Peroxide) demonstrates significantly greater antimicrobial efficacy than in the liquid form.
    3. A type of low temperature sterilization.
    4. Some systems only use hydrogen peroxide gas under vacuum
    5. Some use hydrogen peroxide in the presence of plasma.
    6. A plasma is essentially excited H2O2 produced by adding energy (in the form of heat or an electromagnetic field).
    7. The systems have a rapid cycle time (<1 hour)
    8. Provide sterile instruments for immediate use or sterile storage (with no extended aeration requirements as traditionally needed with ethylene oxide and formaldehyde)
  1. Surface active disinfectants- MOA: Reduction of surface tension
    1. Possess both hydrophobic and hydrophilic groups
    2. Anionic, cationic, non-ionic, amphoteric   
    3. Cationic, quaternary ammonium compounds, most important Bacteriostatic, Weak detergent, Gram +> gram -ve
      Active against enveloped viruses
    4. Anionic, common soap
      Strong detergent
      Weak antimicrobial
    5. ​​​Amphoteric, tego compounds
Extra edge:

Low temp sterilization
a. ETO
b. H2O2
c. Low temp steam form-aldehyde


Testing the efficacy of disinfectants

  1. Minimum inhibitory concentration (MIC): Lowest concentration of the disinfectant that inhibits the   growth of Salmonella typhi
  2. Phenol coefficient test
  3. Similar quantities of organisms added to rising dilutions of phenol and the disinfectant to be tested

Rideal-Walker: No organic matter added

Chick Martin: organic matter added

  1. Dilution of disinfectant which kills the organisms divided by dilution of phenol
  2. Phenol coefficient 1.0 Equal <1.0 Less   >1.0 More
  3. Capacity test (Kelsey and Sykes test)
  4. In use testS

Label For Bio-Medical Waste Containers/Bags


Biohazard Symbol

Cytotoxic Hazard Symbol




Recent Advances

  1. SOP’s- ‘Standard operating procedures’ is a written set of instructions for every test process or procedure, to set minimum acceptable standard of a laboratory. Also called laboratory procedure manual or bench book.
  2. QA = QC + IQA + EQA (QA: quality assurance; QC: quality control; EQA: external quality assurance)
  3. QC is applied to analytical phase of testing process

Classification of pathogens into Hazard groups BIOSAFETY LEVELS

Hazard    Group


Containment level


Organism most unlikely to cause human disease



May cause human disease,

May be hazard to laboratory workers

Unlikely to spread in community,

Effective prophylaxis & treatment available



May cause severe human disease,

Serious Hazard to laboratory workers

Pose a risk to spread in community,

Effective prophylaxis & treatment available



Cause severe human disease,

Serious Hazard to laboratory workers

Pose a high risk to spread in community,

No Effective prophylaxis & treatment



Ebola, Marburg, Lassa, variola are pathogens of hazard group 4

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