Interpretation of bacteriological culture

Bacteriological culture of milk samples

To monitor the microbiological quality of the farm's milk, one can take a bulk tank milk sample for bacteriological culture. In the case of contaminated samples or samples with no growth, the best option is to collect a new sample and start a new culture.The pathogens responsible for clinical and subclinical cases of mastitis that are most commonly observed are:



"Contagious" mastitis pathogens:

  • Staphylococcus aureus
  • Streptococcus agalactiae
  • Mycoplasma spp.

"Environmental" mastitis pathogens:

  • Non-agalactiae streptococci: Streptococcus uberis, Streptococcus dysgalactiae, Enterococcus faecalis and other streptococci
  • Coagulase-negative staphylococci
  • Coliform bacteria (Escherichia coli, Klebsiella pneumoniae, Enterobacter aerogenes)

Bacteriological of bulk tank milk

Bacteriological culture of bulk tank milk is useful to monitor the microbiological quality of the farm's milk. We should  look for all mastitis pathogens, with a specific focus on the contagious types, since it will give us an indication of the udder health on farm. Bulk tank milk cultures can never replace individual sample and cultures of mastitis cases. The result of bulk tank samples should also be interpreted carefully: a one-time bulk tank milk sample offers very little reliability.
An individual count should be carried out for each type of pathogen. The objectives to achieve are:

  • Streptococcus agalactiae: total absence (< 5 CFU/ml). Higher counts indicats the presence of infected animals in the herd.
  • Staphylococcus aureus: < 50 CFU/ml.
  • Coagulase-negative staphylococci (CoNS): < 300 CFU/ml.
  • Non- agalactiae streptococci: 500-700 CFU/ml. Special attention should be paid to the possible presence of S. dysgalactiae.
  • Coliform bacteria: < 100 CFU/ml.
Results (CFU/ml) of a bulk tank milk culture (Source: University of Minnesota, published in NMC)
Types of bacteria Normal values Moderate values High values Very high values
S. agalactiae < 5 5-50 50-100 >100
S. aureus < 50 50-200 200-500 >500
Non-agalactiae streptococci 500-700 700-1.200 1.200-2.000 <2.000
Coliform bacteria < 100 100-400 400-700 >700
CoNS < 300 300-500 500-750 >750

 Contaminated samples

A milk sample is considered to be contaminated when the growth of more than three different pathogens is observed at the time of reading. The most likely reason for contamination is a poor sample collection technique. The growth of S. agalactiae can never be the result of contamination, as the pathogen does not survive outside of the udder.

Samples with no growth

At the time of reading there is no bacterial growth on the culture plate. The possible causes of a lack of growth are:

  • presence of antibiotics or disinfectants in the sample,
  • presence of natural inhibitors in the milk (first days after calving, e.g. lactoferrin),
  • natural elimination of the microorganisms by the udder's immune system,
  • presence of pathogens that only grow in very selective culture media and special conditions (Mycoplasma).
  • insufficient number of microorganisms in the sample,
  • microorganisms that are difficult to isolate (S. aureus, irregular excretion, etc.).
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