MacConkey

MacConkey Agar Nº 2                                                                          Cat. 1035

For the identification of enterococci in the presence of coliforms and non-fermenting organisms in water, foods and clinical samples

Practical information                                                                            

Aplications                                                                    Categories

Detection                                                                      Coliforms

Industry: Water / Clinical / Food

Principles and uses                                                                                                                                           

MacConkey Agar Nº 2 is a slightly selective and differential medium especially useful for the recognition of enterococci in the presence of coliforms and non-lactose fermenters from water, sewage, food and clinical samples.

Bacteriological  peptone  provides  nitrogen,  vitamins,  minerals  and  amino  acids  essential  for  growth.  Lactose  is  the  fermentable  carbohydrate  energy source, causing a pH drop and a color change in the pH indicator (Neutral red), and bile precipitation. Bile salts Nº 2 and crystal violet are the selective agents,  inhibiting  Gram-positive,  bile-tolerant  microorganisms  such  as  staphylococci  and  non-fecal  streptococci.  Sodium  chloride  supplies  essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent.

The presence of intestinal enterococci, is an indicator for faecal contamination, especially when the contamination occurred a long before and the less resistant coliform bacteria, including Escherichia coli, may already be dead when the analysis is carried out.

Formula in g/L                                                                                                                                            

Bacteriological agar                                                                               13,5    Bacteriological peptone                                                                               20

Bile salts                                                                                                  1,5    Crystal violet                                                                                           0,001

Lactose                                                                                                    10    Neutral red                                                                                               0,05

Sodium chloride                                                                                         5

Preparation                                                                                                                                            

Suspend 50 grams of medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 ºC for 15 minutes. Cool to 45-50 ºC, mix well and dispense into plates.

Instructions for use                                                                                                                                            

- Inoculate and incubate at 35±2 ºC for 18-24 hours.

- Enterococci grow as intensely red, small colonies surrounded by a zone of pale red precipitate.

- Non-lactose fermenting bacteria form colorless colonies.

Quality control                                                                                                                                            

Solubility               Appareance                     Color of the dehydrated medium                   Color of the prepared medium                 Final pH (25ºC)

w/o rests               Fine powder                    Beige-pink                                                     Red-violet                                                7,2±0,2

Microbiological test                                                                                                                                            

Incubation conditions: (35±2 ºC / 18-24 h).

Microrganisms                                                                        Specification                               Characteristic reaction

Salmonella enteritidis ATCC 13076                                        Good growth                               Colony color: Colorless

Escherichia coli ATCC 25922                                                 Good growth                               Colony color: Pink-red (bile precipitate)

Staphylococcus aureus ATCC 25923                                     Inhibited growth

Enterococcus faecalis ATCC 29212                                       Good growth                               Colony color: Red

Storage                                                                                                                                            

Temp. Min.:2 ºC Temp. Max.:25 ºC

Bibliography                                                                                                                                            

Mac Geachie J. and Kennedy A.C. J. Clin. Path. 16. 32-38, 1963