The method of preparation of the microbial suspensions for preparing the inoculum for the
assay of various antibiotics is clearly stated . In an event when the suspensions are duly
prepared by these methods, one may accomplish and observe that the growth characteristic features are
fairly uniform in order that the inoculum could be determined by carrying out the following trials.
For Method A. After the suspension is prepared, as given under
different volumes of it to each of several different flasks containing 100 ml of the medium specified in
(the volume of suspension suggested may be used as a guide). Using these
inocula, prepare inoculated plates as described for the specific antibiotic assay. While conducting cylinder-
plate assays, double layer plates may be prepared by pouring a seed layer (inoculated with the
desired micro-organism) over a solidified uninoculated base layer. For each Petri dish, 21 ml of the base
layer and 4 ml of the seed layer may be generally suitable. Fill each cylinder with the median concentration
of the antibiotic and then incubate the plates. After incubation, examine and measure
the zones of inhibition. The volume of suspension that produces the optimum zones of inhibition with
respect to both clarity and diameter determines the inoculum to be used for the assay.
For Method B. Proceed as descirbed for Method A and, using the several inocula,
carry out the procedure as described for the specific antibiotic assay running only the high and low concentrations
of the standard response curve. After incubation, read the absorbances of the appropriate
tubes. Determine which inoculum produces the best response between the low and high antibiotic concentrations
and use this inoculum for the assay.
Apparatus. All equipment is to be thoroughly cleaned before and after each use. Glassware for
holding and transferring test organisms is sterilised by dry heat or by steam.
Thermostatic control is required in several stages of a microbial assay, when culturing a microorganisms
and preparing its inoculum and during incubation in a plate assay. Closer control of the
temperature is imperative during incubation in a tube assay which may be achieved by either circulated
air or water, the greater heat capacity of water lending it some advantage over circulating air
Measuring transmittance within a fairly narrow frequency band requiers a suitable
spectrophotometer in which the wavelength of the light source can be varied or restricted by the use of
a 580 nm filter for preparing inocula of the required density, or with a 530 nm filter for reading the
absorbance in a tube assay. For the latter purpose, the instrument may be arranged to accept the tube in
which incubation takes place, to accept a modified cell fitted with a drain that facilitates rapid change of
contents, or preferably fixed with a flow-through cell for a continuous flow-through analysis. Set the
instrument at zero absorbance with clear, uninoculated broth prepared as specified for the particular
antibiotic, including the same amount of test solution and formaldehyde as found in each sample.
Cylinder-Plate Assay Receptacles
Use rectangular glass trays or glass or plastic Petri dishes (approximately 20 × 100 mm) having
covers of suitable material and assay cylinders made of glass, porcelain, aluminium or stainless steel
with outside diameter 8 mm ± 0.1 mm, inside diameter 6 mm ± 0.1 mm and length 10 mm ± 0.1 mm.
Instead of cylinders, holes 5 to 8 mm in diameter may be bored in the medium with a sterile borer, or
paper discs of suitable quality paper may be used. Carefully clean the cylinders to remove all residues.
An occasional acid-bath, e.g., with about 2M nitric acid or with chromic acid solution is needed.
Turbidimetric Assay Receptacles
For assay tubes, use glass or plastic test-tubes, e.g., 16 mm × 125 mm or 18 mm × 150 mm that
are relatively uniform in length, diameter, and thickness and substantially free form surface blemishes
and scratches. Cleanse thoroughly to remove all antibiotic residues and traces of cleaning solution and
sterilise tubes that have been used previously before subsequent use.
Microbial assays gain markedly in precision by the segregation of relatively large sources of
potential error and bias through suitable experimental designs. In a cylinder-plate assay, the essential
comparisons are restricted to relationships between zone diameter measurements within plates, exclusive
of the variation between plates in their preparation and subsequent handling. To conduct a
turbidimetric assay so that the difference in observed turbidity will reflect the differences in the antibiotic
concentration requires both greater uniformity in the environment created for the tubes through
closer thermostatic control of the incubator and the avoidance of systematic bias by a random placement
of replicate tubes in separate tube racks, each rack containing one complete set of treatments. The
essential comparisons are then restricted to relationships between the observed turbidities within racks.
Within these restrictions, two alternative designs are recommended; i.e., a 3-level (or 2-level)
factorial assay, or a 1-level assay with a standard curve. For a factorial assay, prepare solutions of 3 or 2
corresponding test dilutions for both the standard and the unknowns on the day of the assay, as described
under Preparation of the Standard and Preparation of the Sample. For a 1-level assay with a standard
curve, prepare instead solutions of five test dilutions of the standard and a solution of a single median
test level of the unknown as described in the same sections. Consider an assay as preliminary if its
computed potency with either design is less than 60% or more than 150% of that assumed in preparing
the stock solution of the unknown. In such a case, adjust its assumed potency accordingly and repeat the
Microbial determinations of potency are subject to inter-assay variables as well as intra-assay
variables, so that two or more independent assays are required for a reliable estimate of the potency of a
given assay preparation or unknown. Starting with separately prepared stock solutions and test dilutions
of both the standard and the unknown, repeat the assay of a given unknown on a different day. If the
estimated potency of the second assay differs significantly, as indicated by the calculated standard error,
from that of the first, conduct one or more additional assays. The combined result of a series of smaller,
independent assays spread over a number of days is a more reliable estimate of potency than that from a
single large assay with the same total number of plates or tubes.
Methods. The microbiological assay of antibiotics may be carried out by Method
A or Method B.
[A] Cylinder-Plate or Cup-Plate Method
Inoculate a previously liquefied medium appropriate to the assay (Tables 10.1 and 10.3) with the
requisite quantity of suspension of the micro-organisms, add the suspension to the medium at a temperature
between 40° and 50° and immediately pour the inoculated medium into Petri dishes or large rectangular
plates to give a depth of 3 to 4 mm (1 to 2 mm for nystatin). Ensure that the layers of medium are
uniform in thickness, by placing the dishes or plates on a level surface.
The prepared dishes or plates must be stored in a manner so as to ensure that no significant
growth or death of the test organism occurs before the dishes or plates are used and that the surface of
the agar layer is dry at the time of use.
Using the appropriate buffer solutions indicated prepare solutions of
known concentration of the Standard Preparation and solutions of the corresponding assumed concentrations
of the antibiotic to be examined. Where directions have been given in the individual monograph
for preparing the solutions, these should be followed and further dilutions made with buffer solution as
indicated Apply the solutions to the surface of the solid medium in sterile cylinders or in
cavities prepared in the agar. The volume of soluiton added to each cylinder or cavity must be uniform
and sufficient almost to fill the holes when these are used. When paper discs are used these should be
sterilised by exposure of both sides under a sterilising lamp and then impregnated with the standard
solutions or the test solutions and placed on the surface of the medium. When Petri dishes are used,
arrange the solutions of the Standard Preparation and the antibiotic to be examined on each dish so that
they alternate around the dish and so that the highest concentrations of standard and test preparations are
not adjacent. When plates are used, place the solutions in a Latin square design, if the plate is a square,
or if it is not, in a randomised block design. The same random design should not be used repeatedly.
Leave the dishes or plates standing for 1 to 4 hours at room temperature or at 4°, as appropriate,
as a period of pre-incubation diffusion to minimise the effects of variation in time between the application
of the different solutions. Incubate them for about 18 hours at the temperature indicated
Accurately measure the diameters or areas of the circular inhibition zones and calculate the results.
Selection of the assay design should be based on the requirements stated in the individual monograph.
Some of the usual assay designs are as follows.
One-Level Assay with Standard Curve
Standard solution. Dissolve an accurately weighted quantity of the Standard Preparation of the
antibiotic, previously dried where necessary, in the solvent specified and then dilute to the
required concentration, as indicated, to give the stock solution. Store in a refrigerator and use within the
period indicated. On the day of the assay, prepare from the stock solutions, 5 dilutions (solutions S1 to
S5) representing five test levels of the standard and increasing stepwise in the ratio of 4 : 5. Use the
dilution specified in Table 10.3 and a sequence such that the middle or median has the concentration
given in the table.
Sample solution. From the information available for the antibiotic preparation which is being
examined (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 with the same solvent as used for the
standard. Prepare from this stock solution a dilution to a concentration equal to the median level of the
standard to give the sample solution.
Method. For preparing the standard curve, use a total of 12 Petri dishes or plates to accommodate
72 cylinders or cavities. A set of three plates (18 cylinders or cavities) is used for each dilution. On
each of the three plates of a set fill alternate cylinders or cavities with solution S3 (representing the
median concentration of the standard solution) and each of the remaining 9 cylinders or cavities with
one of the other 4 dilutions of the standard solution. Repeat the process for the other 3 dilutions of the
standard solutions. For each unknown preparation use a set of three plates (18 cylinders or cavities) and
fill alternate cylinders or cavities with the sample solution and each of the remaining 9 cylinders of
cavities with solution S3.
Incubate the plates for about 18 hours at the specified temperature and measure the diameters or
the zones of inhibition.
Estimation of potency. Average the readings of solution S3 and the readings of the concentration
tested on each set of three plates, and average also all 36 readings of solution S3. The average of the
36 readings of soluiton S3 is the correction point for the curve. Correct the average value obtained for
each concentration (S1, S2, S4 and S5) to the figure it would be if the readings for solution S3 for that set
of three plates were the same as the correction point. Thus, in correcting the value obtained with any
concentration, say S1, if the average of 36 readings of S3 is, for example, 18.0 mm and the average of the
S3 concentrations on one set of three plates is 17.8 mm, the correction is + 0.2 mm. If the average
reading of S1 is 16.0 mm, the corrected reading of S1 is 16.2 mm. Plot these corrected values including
the average of the 36 readings for solutions S3 on two-cycle semilog paper, using the concentrations in
Units or μg per ml (as the ordinate logarithmic scale) and the diameter of the zones of inhibition as the
abscissa. Draw the straight response line either through these points by inspection or through the points
plotted for highest and lowest zone diameters obtained by means of the following expressions :
a + b + c − e
; H =
e + d + c − a
where L = the calculated zone diameter for the lowest concentration of the standard curve response line.
H = the calculated zone diameter for the highest concentration of the standard curve response
c = average zone diameter of 36 readings of the reference point standard solution.
a, b, d, e = corrected average values for the other standard solutions, lowest to highest
Average the zone diameters for the sample solution and for solutions S3 on the plates used for the
sample soluiton. If the sample gives a large average zone size than the average of the standard (solution
S3), add the difference between them to the zone size of solution S3 of the standard response line. If the
average sample zone size is smaller than the standard values, subtract the difference between them from
the zone size of solution S3 of the standard response line. From the response line read the concentration
corresponding to these corrected values of zone sizes. From the dilution factors the potency of the
sample may be calculated.
[A.2] Two-Level Factorial Assay
Prepare parallel dilutions containing 2 levels of both the standard (S1 and S2) and the unkown
(U1 and U2). On each of four or more plates, fill each of its four cylinders or cavities with a different test
dilution, alternating standard and unknown. Keep the plates at room temperature and measure the diameters
of the zones of inhibition.
Estimation of potency. Sum the diameters of the zones of each dilution and calculate the %
potency of the sample (in terms of the standard) from the following equation :
% potency = Antilog (2.0 + a log I)
wherein a may have a positive or negative value and should be used algebracially and
where a = 1 2 1 2
1 2 1 2
(U + U ) – (S + S )
(U + U ) + (S – S )
U1 and U2 are the sums of the zone diameters with solutions of the unknown of high and low
S1 and S2 are the sums of the zone diameters with solutions of the standard of high and low
I = ratio of dilutions.
If the potency of the sample is lower than 60% or greater than 150% of the standard, the assay is
invalid and should be repeated using higher or lower dilutions of the same solutions. The potency of the
sample may be calculated from the expression.
% potency × assumed potency of the sample
[A.3] Other Designs
(1) Factorial assay containing parallel dilution of three test levels of standard and the unknown.
(2) Factorial assay using two test levels of standard and two test levels of two different unknowns.
[B] Turbidimetric or Tube Assay Method
The method has the advantage of a shorter incubation period for the growth of the test organism
(usually 3 to 4 hours) but the presence of solvent residues or other inhibitory substances affects this
assay more than the cylinder-plate assay and care should be taken to ensure freedom from such substances
in the final test solutions. This method is not recommended for cloudy or turbid preparations.
Prepare five different concentrations of the standard solution for preparing the standard curve by
diluting the stock solution of the Standard Preparation of the antibiotic (Table 10.3) and increasing
stepwise in the ratio 4 : 5. Select the median concentration (Table 10.3) and dilute the solution of the
substance being examined (unknown) to obtain approximately this concentration. Place 1 mL of each
concentration of the standard solution and of the sample solution in each of the tubes in duplicate. To
each tube add 9 ml of nutrient medium (Table 10.3) previously seeded with the appropriate test organism
At the same time prepare three control tubes, one containing the inoculated culture medium (culture
control), another identical with it but treated immediately with 0.5 mL of dilute formaldehyde solution
(blank) and a third containing uninoculated culture medium.
Place all the tubes, randomly distributed or in a randomized block arrangement, in an incubator or
a water-bath and maintain them at the specified temperature (Table 10.3) for 3 to 4 hours. After incubation
add 0.5 mL of dilute formaldehyde solution to each tube. Measure the growth of the test organism by
determining the absorbance at about 530 nm of each of the solutions in the tubes against the blank.
Estimation of potency. Plot the average absorbances for each concentration of the standard on
semi-logarithmic paper with the absorbances on the arithmetic scale and concentrations on the logarithmic
scale. Construct the best straight response line through the points either by inspection or by means
of the following expressions :
a + b + c − e
; H =
e + d + c − a
where L = the calculated absorbance for the lowest concentration of the standard response line.
H = the calculated absorbance for the highest concentration of the standard response line.
a, b, c, d, e = average absorbance values for each concentration of the standard response line
lowest to highest respectively.
Plot the values obtained for L and H and connect the points. Average the absorbances for the
sample and read the antibiotic concentration from the standard response line. Multiply the concentration
by the appropriate dilution factors to obtain the antibiotic content of the sample.
Precision of Microbiological Assays
The fiducial limits of error of the estimated potency should be not less than 95% and not more
than 105% of the estimated potency unless otherwise stated in the individual monograph. This degree of
precision is the minimum acceptable for determining that the final product complies with the official
requirements and may be inadequate for those deciding, for example, the potency which should be
stated on the label or used as the basis for calculating the quantity of an antibiotic to be incorporated in
a preparation. In such circumstances, assays of greater precision may be desirable with, for instance,
fiducial limits of error of the order of 98% to 102%. With this degree of precision, the lower fiducial
limit lies close to the estimated potency. By using this limit, instead of the estimated potency, to assign a
potency to the antibiotic either for labelling or for calculating the quantity to be included in a prepara
tion, there is less likelihood of the final preparation subsequently failing to comply with the official
requirements for potency.*