Microbiological
Assay
Introduction
• A microbiological assay defined as qualitative or quantitative determination of chemical
compound from a simple or even complex material with the use of microorganisms.
• Also called microbial assay
• Used to determine the potency of a drug in animals or man and monitoring and controlling anti-
microbial chemotherapy
• Many anti-microbial agents, which inhibit the growth of microorganisms (antibiotics) or are
essential for their growth (vitamins and amino acids).
• RELATIVE POTENCY: defined as the term used to express the biological activity of a
sample preparation compared to a standard preparation.
Advantages:
Simple
Specific
Minimum requirements of space, labour, materials and time
Inexpensive and
Convenient method
Disadvantages:
Required skilled
technician
Required proper
calibration
Less reproducible
Chances of greater
error
Not used if a good
Microbiological assay of an
antibiotic
• The microbiological assay of an antibiotic is based upon a comparison of the
inhibition of growth of micro-organisms by measured concentrations of the
antibiotics under examination with that produced by known concentrations of
a standard preparation of the antibiotic having a known activity.
• To be utilised for demonstrating the therapeutic efficacy of antibiotics.
• Assay: it is a physical or biological testing method to determine content or
quality of substances.
Two general methods are usually employed:
I) The cylinder-plate (or cup-plate) method (method A) and
II) the turbidimetric (or tube assay) method(method B)
• The cylinder-plate method depends upon diffusion of the antibiotic from a vertical
cylinder through a solidified agar layer in a Petri dish or plate to an extent such that
growth of the added micro-organism is prevented entirely in a zone around the
cylinder containing a solution of the antibiotic.
• The turbidimetric method depends upon the inhibition of growth of a microbial
culture in a uniform solution of the antibiotic in a fluid medium that is favourable to
its rapid growth in the absence of the antibiotic.
Culture media used for antibiotic assay
• Dissolved all the required ingredients
(table1) in sufficient water to produced 1000ml
• pH can be maintained by adding 1M solution of
HCl or NaOH.
• Followed by the sterilization of the media
• FTM, ATM and SCDM are used.
• Composition of FTM, ATM and SCDM?????
Buffer solution
• Prepare by dissolving the following quantities given in Table2.
• sufficient water to produce 1000 ml after sterilisation,
adjusting the pH with 8 M phosphoric acid or 10 M potassium
hydroxide.
Preparation of standard
• A Standard Preparation is an authentic sample of the
appropriate antibiotic for which the potency has been
precisely determined by reference to the appropriate
international standard.
• The Potency of the standard preparation may be
expressed in International Units or in μg per mg of the
pure antibiotic.
• Ex: Dissolve a quantity of the standard preparation of a
given antibiotics in the solvents(table3). Dilute the
preparation to get the required concentration as stated
and stored in a refrigerator.
• Usually prepared in the ratio of 1:1.5
Preparation of the test sample
• From the information available for the substance
under examination (the “unknown”), assign to it an
assumed potency per unit weight or volume, and on
this assumption prepare on the day of the assay a
stock solution and test dilution as specified for each
antibiotic in Table4 but with the same final diluents
as used for the Standard Preparation.
• The assay with 5 levels of the Standard requires only
one level of the unknown at a concentration
assumed equal to the median level of the standard.
Preparation of Test organism
• The test organism for each antibiotic is listed in Table, together with
its identification number in the American Type Culture Collection (ATCC).
Preparation of inoculums
• Inoculums is the mixture of microbes along
with the culture media in which it is growing.
Steps involved:
Maintain the test microbes on slant of medium A
and transfer to a fresh slant once a week.
Incubate the slant at the specified temperature
for 1day
Using 3ml of slant solution, wash the microbes
from agar slant on to a large surface of medium
A such as a Roux bottle containing 250ml of
agar media
Incubate for 1day at the required temperature
Wash the growth from the nutrient surface using
50ml of saline solution.
Store the test microbes under refrigerator
Methods of Microbiological Assay
• A. Cylinder plate or
Cup plate method
• B. Turbidimetric or Tube
Assay method
A. Cylinder plate or cup plate method
• A previously liquefied medium with the required
quantity of microbial suspension is inoculated
• The suspension is added to the medium at a temperature
between 40-50 degree and inoculated medium is
immediately poured
• The solution are applied to the surface of the solider
medium in sterile cylinder or in ager cavities
• They are incubated for about 18 hours at the temperature
indicated
• The quantities estimation of antibiotic is
accurately measuring the done by
circular inhibition zones . diameter or areas
of the
Simple image
Measurement of zone of inhibition
Standard curve of microbial assay of
antibiotic (sample)
B. Turbidimetric or Tube Assay method
• Advantage- shorter incubation period for the growth
of the test organism(usually 3 to 4 hrs)
• Disadvantage-The presence of solvent
residues inhibitory substances affects more.
• Not recommended for cloudy or turbid preparation.
• Five different concentration of the standard solution
are prepared for preparing the standard curve.
• 1mm of each concentration of the standard solution
of the sample solution are placed in each of the tubes
in duplicate at 9 ml of nutrients medium previously
seeded with the appropriate test organism at to each
other
• Five tubes containing the inoculated culture medium
with standard drug with a specific dose and test
organism.
• Five tubes containing culture medium with test
organism and the test sample with different dosages.
• Another one treated immediately with 0.5 ml of
dilute formaldehyde solution(blank)
• All the tubes are placed in an incubator and
maintain at the specified temperature- 37°C for 3
to 4 hour.
• The growth of the test organism is measured by
determining the absorbance at 530 nm of its
against the blank.
• The standard calibration card is prepared and the
absorbance obtained for the sample is plotted on it
to obtain the concentration of the test antibiotic
SPECTROMETER FOR ABSORBANCE
