Oxytetracycline Selectavial
SV204 For the isolation of yeasts and moulds
from food specimens.
Introduction
Yeasts and moulds play an important role in
spoilage of food. A wide range of fungi are
involved in spoilage processes including plant
pathogens that are able to disfigure and damage
crops before harvest, to moulds that cause
1
losses of quality in storage . In addition to
damaging the appearance of food products,
certain moulds are able to produce toxic
metabolites, mycotoxins, which pose a potential
2
health hazard to humans . Several factors
including a reduction in content of the traditional
preservatives sugar and salt in certain foods in
response to health concerns and changes in
consumer habits, such as less frequent
shopping, are leading to increased mould
1
spoilage of foodstuffs .
In the laboratory most yeasts and moulds can be
easily cultured on simple media. However,
cultures develop relatively slowly, up to 4-6
weeks in some cases, and are sometimes
overgrown by more rapidly multiplying bacteria
that are also present in the sample. To reduce
the possibility of such overgrowth a primary
requirement of media used for the enumeration
of yeasts and moulds is the ability to prevent
bacterial growth. As fungi are able to grow over
a wide pH range this inhibition was for many
years achieved by the use of low pH media such
3
as acidified potato dextrose agar .
4,5
Work by Mossel et al. demonstrated that the
use of acidic media could have an adverse
affect on the recovery of yeasts and moulds and
recommended instead a neutral medium,
Glucose Yeast Extract Agar. This could be
made selective for fungi with addition of a broad
spectrum antibiotic, Oxytetracycline, to eliminate
bacterial contamination to produce
Oxytetracycline Glucose Yeast Extract (OGYE)
4,5
medium .
For enumeration of yeast and mould colonies
colonies, surface plating methods are preferred
over pour plates as restriction of oxygen supply
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and the heat of molten agar can inhibit growth or
place additional stress on already damaged
6
cells .
Description
Each MAST Oxytetracycline Selectavial
contains an accurately assayed quantity of
antibiotic in a soluble, non-interfering carrier
substance.
SV204
Content
Oxytetracycline
™
Selectavial
Oxytetracycline
50mg
Concentration
in 1 litre medium
100mg/litre
Directions
1. Label Petri dishes using the self-adhesive
labels provided.
2. Prepare and sterilise OGYE basal medium
as directed. Cool to 55ºC and hold in a water
bath at this temperature.
3. Reconstitute the contents of one vial using
5ml of sterile deionised water. The best
method is to aseptically add the diluent using
a sterile needle and syringe. Draw the
diluent into the syringe and after removing
the plastic cap of the vial, inject through the
rubber stopper of the vial. The lyophilised
supplement will rapidly dissolve and may be
withdrawn into the syringe.
3. Add the antibiotic solution to 500ml of
medium and discard the needle into an
approved container.
DO NOT TRY TO RE-SHEATH AN
EXPOSED NEEDLE.
4. Mix gently but thoroughly to evenly distribute
the selective agents.
5. Pour culture plates and allow to set.
6. Prepared culture plates may be used
immediately or stored in plastic bags at 2-8°C
for up to one week before use.
NB 11/98 V0.1
In Use7
References
-1
Prepare a 10 food homogenate of specimen
using either a stomacher or blender by
homogenising 25g or 25ml of sample in 225ml of
prepared diluent.
-1
Using the 10 homogenate and a suitable range
of decimal dilutions inoculate the surface of an
agar plate by a suitable method for enumeration.
Incubate the plates at 25°C for three to five days.
Plates should be incubated for up to 14 days
before being discarded.
Count the colonies according to the inoculation
method used and calculate the number of colony
forming units (cfu) per gram or millilitre of original
test sample.
MAST is a Registered Trademark
1. James JM. Modern food microbiology. 4th
Edition New York: Van Nostrand Reinhold,
1991: 641-651
2. Williams AP. Methodological developments in
food mycology. J Appl Bacteriol. 1989; 67
(Supplement): 61S-67S
3. Sharf JM. (Editor). Recommended methods
for the microbiological examination of foods.
2nd Edition. New York: American Public
Health Association, 1966
4. Mossel DAA, Visser M, Mengerink WHJ. A
comparison of media for the enumeration of
moulds and yeasts in foods and beverages.
Lab Prac. 1962; 11: 109-112
5. Mossel DAA, Kleynen-Semmeling AMC
Vincentie HM, Beerens H, Catsaras M.
Oxytetracycline-glucose-yeast extract agar for
selective enumeration of moulds and yeasts in
foods and clinical material. J Appl Bacteriol.
1970; 33: 454-457
6. Pitt JI. Food mycology - an emerging
discipline. J Appl Bacteriol. 1989; 67
(Supplement): 1S-9S
7. Roberts D, Hooper W, Greenwood M (editors).
Practical food microbiology. 2nd Edition.
London: Public Health Laboratory Service,
1995:160-161
NB 11/98 V0.1