MRS Agar (ISO) is a medium for the enumeration of m
esophilic lactic acid bacteria according to
This medium was originally developed in 1960 by de
Man, Rogosa & Sharpe for the cultivation
and enumeration of
spp. from various sources and is intended as a subs
Tomato Juice Agar. This original formulation has be
en adapted and adjusted to pH 5.7
according to ISO 15214:1998.
Nutrition is provided by enzymatic digest of casein
, glucose, meat & yeast extracts whilst
polyoxyethylenesorbitan monooleate, magnesium and m
anganese sulphates act as growth
stimulants. Selectivity against streptococci & moul
ds is provided by ammonium citrate and
sodium acetate. Used at low pH, ammonium citrate al
lows growth of lactobacilli whilst inhibiting
a number of other organism groups.
Although MRS Agar (ISO) is optimised to be
selective for Lactobacilli, some growth of
spp. and Pediococci may also occur.
Occasionally, sterilisation of this medium at 121
C for 15 minutes, in some autoclaves, may
cause the pH to fall outside of the specified pH li
mits 5.7 +/- 0.1. In these rare cases,
adjustment of the medium using acetic acid or sodiu
m hydroxide is recommended.
Powder: fine, slightly cohesive, light tan powder w
ith some lumps
Finished medium: clear, tan gel
5.7 ± 0.1
NR – Not regulated
Method for reconstitution
Disperse 70 grams of powder in 1 litre of deionised
water. Allow to soak for 10 minutes, swirl to
mix, then sterilise by autoclaving at 121
C for 15 minutes. Cool to 47°C and mix well before
dispensing into sterile Petri dishes. Dry the agar
surface before use.
Grams per litre
Dehydrated culture media: 10-25
7 days at 2-8
Using the pour plate method, inoculate 1ml of the t
est sample (serial dilutions should be used)
in to the Petri dish, before pouring over the molte
n agar. Mix carefully & allow to solidify.
Overlays may be used if required.
Surface inoculations may also be used.
Incubate microaerobically at 30°C for 72 hours + 3
Count all colonies.
spp. may form large, slimy colonies which may hinde
r the development of
other colonies, thus causing an underestimation of
the number of lactic acid bacteria.
Due to the possible development of microorganisms o
ther than lactic acid bacteria, it may be
necessary in some cases to confirm the colonies obt
ained using simple techniques such as Gram
stain or test for catalase.
Minimum Q.C. organisms
(according to ISO/TS 11133)
WDCM 00013 (inhibited)
de Man, J.C., Rogosa, M and Sharpe, M.E. (1960). A
medium for the cultivation of lactobacilli.
ISO 11133-2:2003/Amd.1:2011(E) Microbiology of food
and animal feeding stuffs – Guidelines
on preparation and production of culture media – Pa
rt 2: Practical guidelines on performance
testing of culture media – Amendment 1: test microo
rganisms for commonly used culture
ISO 15214:1998 Microbiology of food and animal feed
ing stuffs – Horizontal method for the
enumeration of mesophilic lactic acid bacteria – Co
lony count technique at 30