How Does 70% Alcohol-based Hand Sanitizers Kill Microbes ?

Hand sanitizers often contain alcohol such as ethyl alcohol as an active ingredient other ingredients may include water and glycerin. Hand sanitizer has reduced the levels of microorganisms by killing them, just like other disinfectants do. 

  The effectiveness of hand sanitizers depend on quantity used duration of exposure and frequency of use. Usually 70% alcohol-based hand sanitizers, if rubbed thoroughly over finger and hand surfaces for 30 seconds followed by complete air drying can effectively reduce populations of bacteria, fungi and some viruses.

How alcohol-based hand sanitizers kill microorganisms ?

  • Alcohols disturb the normal arrangement of lipids structure of bacterial plasma membrane. The plasma membrane of the bacteria is formed by several phospholipid molecules.
  • Phospholipid is composed of a phosphate group and a lipid. The phosphate group is hydrophilic and the lipid is hydrophobic.
  • The phospholipid bilayer of plasma membrane which is formed by two layers of phospholipids is arranged in such a way that the hydrophilic parts are directly exposed to the internal and external environments of the cell. 

Plasma membrane of microorganisms

  • The reason being is that the internal and external environments of the cell are occupied with water.
  • In most cases the hydrophilic ends are always in contact with water. The hydrophilic nature provides the membrane more stability in the aqueous solutions.
  • When phospholipids come in contact with alcohol, the lipid molecules move out of place and break the arrangement as the hydrophilic ends chase for water. Thus it increases the membrane fluidity.
  • This alcohol stressed condition changes the shape of proteins that are present on the membrane.
  • When the proteins are not in normal shape they cannot perform their routine job. Therefore the microorganisms cannot survive.

Hand sanitizers are commonly used in pharmaceutical companies, hospitals and clean rooms.
Particularly alcohol-based hand sanitizers will be used in combination of water.

Why Alcohol solution with 70% alcohol and 30% of water effectively kills bacteria ?

  • Alcohol in combination of water evaporates slowly, therefore increases the contact time of the sanitizer and thus works effectively.
  • Water acts as a catalyst and plays an important role in denaturing the proteins.
  • The 100% alcohol coagulates the proteins rapidly and forms a protein layer. This will prevent coagulation of other protein molecules. This is the reason why the 70% alcohol is found to be more effective than 100%.

Prokaryotic Ribosome: structural subunits and function

 Ribosomes present in all living cells. Ribosomes are made up of several RNAs and proteins. They act as site of protein synthesis where mRNA translation takes place. Ribosomes link each amino acid in the order specified by the codons on mRNA to form a polypeptide chain.

Bacterial ribosome structure

  • Several ribosomes come together to form polysome during the protein synthesis. The rate of protein synthesis depends on the number of ribosomes.
  • Bacterial cell contains about     10000 ribosomes and they contribute 30% of total dry weight of the cell.
  • In Bacterial cell ribosomes are present in the cytoplasm.
  • More number of ribosomes provide granular appearance to the cytoplasm.
  • Ribosomes are of two types 70s and 80s.
  • The 80s ribosomes are present in eukaryotic cells.
  • Bacterial cell contains 70s ribosomes
  • Here the "s" denotes svedberg units.
  • Svedberg units indicate the rate of sedimentation during ultracentrifugation.
  • The sedimentation rate depends on size, shape and weight of the molecule, for example svedberg value is more for the heavier molecules than the lighter molecules.
  • These units are named after Theodor Svedberg from sweden who discovered the principle of ultracentrifugation. He was awarded the nobel prize in 1926.

Structure of 70s Ribosome

  • The 70s ribosome is made of two subunits a small and a large subunit.
  • Smaller subunit is of 30s and the larger one is a 50s.
  • Similarly the 80s ribosomes in eukaryotic cell consist of a 40s and 60s subunits.

30s Subunit of Ribosome

  • The smaller subunit in 70s ribosome(30s) has 16 sRNA and comprise of 1540 nucleotides bound to 21 proteins.

50s Subunit of Ribosome

  • The larger subunit has 5 sRNA and 23 sRNA.
  • The 5sRNA comprise of 120 nucleotides bound to 31 proteins.
  • The 23sRNA comprise of 2900 nucleotides bound to the 31 proteins.

Protein synthesis by Ribosome

  • During the protein synthesis the mRNA binds to the smaller subunit(30s).
  • The larger subunit(50s) links amino acids by providing the peptide bonds to the polypeptide chain.
  • The two subunits are combined together. The strength of the attachment depends on the concentration of Mg++ ions.

Difference between Laminar air flow and Biosafety cabinets

 Laminar airflow cabinet(hood) and biosafety cabinet(hood) appear to be one and the same but there are many differences between these two cabinets. Of course both of these cabinets are intended to give protection to the user and the environment at different levels.
Let's understand the main function differences and types of these two cabinets.

Laminar Airflow Cabinet

A laminar airflow cabinet is an enclosed unit that creates a microbial free environment.
This microbial free environment is created by high efficiency particulate air filters which are popularly known as hepa filters these HEPA filters.

  These HEPA filters capture all the airborne particles that enter the cabinet. The pore size of HEPA filter is usually 0.3 microns which can retain bacteria, fungal spores and other particles.

  HEPA filters are so constructed that the air reaching out to the filter is passed through two more filters in order to remove the particles that are larger than 0.3 microns.

There are two types of laminar airflow cabinets based on the direction of the airflow

  1. Vertical laminar airflow cabinet
  2. Horizontal laminar airflow cabinet.

  • In the vertical laminar airflow air moves from the top of the cabinet towards the workbench.
  • Similarly in the horizontal laminar airflow the air flows from the behind towards the operator.

Working principle of laminar air flow cabinet

The air from the surrounding environment is pulled by the blower and released into the working area through the HEPA filters. From the workbench most of the air is moved towards the face of the operator and then back to the environment.

  • The laminar airflow cabinets provide a unidirectional airflow with fixed velocity and pressure.
  • The pressure inside the cabinet does not allow the external air entering the cabinet.
  • Some laminar airflow units equipped with UV light to sterilize the interior surfaces and components before the operation.
  • The laminar airflow cabinets are used for culturing of non-infectious organisms as they do not provide protection to the operator.

Biosafety cabinet

Unlike the laminar airflow cabinets the biosafety cabinets not only give protection to the sample but they also protect the operator and the surrounding environment.
Based on the level of protection biosafety cabinets are classified into three classes -
  Class 1
  Class 2
  Class 3

Class 1 Biosafety cabinets

  • Class 1 biosafety cabinets provide only the operator and environment safety. They do not provide product safety that is being handled.
  • The class 1 biosafety cabinets pull the surrounding air from the operator side. The air inside takes the particles away from the operator and then pass through the HEPA filter before it is discharged into the environment.
  • In this case the surrounding air is coming in direct contact with the product on the work surface and therefore increasing the risk of product contamination.
  • In this way the class 1 biosafety cabinets protect the operator and the environment but not the product.

Class 2 Biosafety cabinets

  • In The class 2 biosafety cabinets the air enters into the cabinet through the front grille.
  • In the laminar airflow the airflow is towards the operator whereas in this case the air is pulled from the front grille. Which means the dirty air is pulled from the operator side and thus providing safety to the operator.
  • The contaminated air is then pushed below the workbench and taken upwards to the plenum. From the plenum the air is passed through the HEPA filter.
  • Usually 30% of the air will be sent back to the environment through the HEPA filter the remaining 70% of the air passes through another hepa filter and enters the working surface of the cabinet.

The class 2 biosafety cabinets provide the following
  • a front opening with continuous inward airflow
  • hepa filtered vertical and unidirectional airflow on the working surface
  • HEPA filtered air to the surrounding area

Depending on the exhaust system the airflow velocity and airflow rate the class 2 biosafety cabinets are further classified as 
type A1 A2 type B1 and B2.

  • The most widely used cabinets are belong to the type A2 of class 2 Biosafety cabinets.

Class 3 Biosafety cabinets

  • The class 3 biosafety cabinets are totally enclosed containers and provided with two gloves attached to the system to perform operations.
  • Hence these cabinets are also called as glove boxes the inflow and outflow of air occurs through HEPA filters.
  • These cabinets are also equipped with a transfer cabinet that allows sterilization of material before they leave the glove box.

Class 3 Biosafety cabinet 

  • The exhaust air is either treated with double HEPA filters or single HEPA filter followed by incineration.
  • The class 3 biosafety cabinets provide complete protection to the operator environment and the product.
  • Hence the class 3 is suitable for all biosafety level organisms.

Behaviour of bacteria towards stimull - Taxis or Tactic response

 The motile prokaryotes enjoy a selective advantage over their non-motile counter parts under certain environmental conditions. When they encounter gradient of physical and chemical agents in nature, the flagellar machinery functions to develop a response towards the stimulus.

  • In a positive response, bacteria move towards the stimulus e.g. nutrients, light. temperature, etc.
  • In a negative response the bacteria move away from the stimulus.

The movement of prokaryotes towards or away from the stimulus or the environment is called taxis or tactic response. There are several types of tactic responses.

1]. Chemotaxis

  • Movement of an organism towards the chemical attractant or away from the repellants is called chemo taxis.
  • Prokaryotic cell surface possesses special proteins, called chemo receptors.
  • Chemo receptors may be located either in the periplasmic space or the plasma membrane.
  • The tactic responses develop due to gradients of concentration of various chemicals.

2]. Phototaxis

  • The tactic responses exhibited by phototrophic (photosynthetic) prokaryotes to a gradient of light Intensity is called phototaxis.
  • They accumulate towards intense allowing most efficient photosynthesis.
  • Like the chemo receptors, bacteria possess photoreceptor.

3]. Magnetotaxis

  • The movement of bacteria towards the Earth's magnetic field or to local magnetie fields (magnets placed near the bacterial culture) is called magnetotaxis.
  • It has been observed in Aquaspirillum magnetotacticum.
  • It is due to the presence of a chain of magnetite (Fe3O4) inclusions (or magnetosomes) within the cell, which facilitates the positioning of bacteria as a magnetic dipole.
  • Magnetotactic bacteria move toward the oxygen-deficient sediments in the aquatic environment.

4]. Osmotaxis 

  • Movement of prokaryotes in response to the osmotic gradient is called osmotaxis.
  • It is related to the concentration gradient of disolved solutes.