The media undergoes colour changes depending upon the organism present. The unreacted tubes are a translucent red and purple colour.
Interpretation of results
The Overnighter is a screening test which can be used for rapid determination of the presence of a Staphylococcus aureus or Streptococcus mastitis infection.
A small sample of milk (2-3 drops) is placed into a chamber within vials of selective growth media. It is important that the vial is flicked sharply after the milk sample has been loaded to push the milk sample down into the base of the chamber. Any excess milk should then be tapped off, as excess milk in the top of the vial may result in non-specific fermentation. Incubation can be done on top of a household hot water cylinder, on the heat exchange unit of the vat chiller, or even in a pocket on an inner layer of clothing.
Colour changes on Incubation
See details below for the expected colour changes on incubation. Further pictures are also available in the
The red tube becomes pineapple yellow. This colour change will start to be evident within about 12 hours, with the yellow colouration appearing from the bottom of the tube. The colour change is usually fully developed after 24 hours of incubation. Some Staph aureus organisms do seem to have a long growth lag however, especially from samples from high somatic cell count cows. Where colour change has not occurred, you should continue to incubate the tubes for up to 48 hours.
Streptococcus
The purple tube becomes grey/black or a pale yellow/grey, and there is no change in the red tube. These colour changes generally happen quickly, after about 12 hours of incubation.
Contaminated Tubes
Many of the potential contaminating organisms are blocked by the selective media, but occasionally, if teat cleaning has not been done well, one may grow. This turns both tubes black/brown.
