Bulk Milk Bacteriology

Bulk milk bacteriology is a useful tool for investigations of mastitis (clinical and subclinical) and high bulk milk bacterial counts (“Bactoscan” or “TBC/TVC”).

How to take a bulk milk sample

Bulk milk samples should be taken from a well agitated tank (a few minutes at least) and into a sterile pot. Depending on the type of bulk milk tank on the farm, the sample may be taken either through the lid at the top of the tank or via the milk outlet at the bottom where the tanker usually collects the milk. If the latter is done, since this is a part of the tank regularly accessed, it is particularly prone to the build up on of bacteria near the outlet. Therefore, it is important that milk is allowed to run through first to clear any bacteria present. Clean gloves should always be worn.

Compared to taking an individual milk sample from a cow with mastitis, bulk milk samples are much more prone to bacterial overgrowth which can significantly affect the interpretation of the results. It is crucial that samples are kept cool immediately after being taken. This should involve the use of ice packs and an insulated box, especially if the sample has to be posted to the laboratory.

An alternative to taking a sample yourself is to ask National Milk Laboratories (NML) to do it for you – a test known as a “Bacto-Breakdown”- if the farmer is provided with this service. This can be requested when the laboratory takes their weekly sample for milk fat, protein, SCC and Bactoscan tests. In fact, they can be asked to do it only if the Bactoscan is raised to a certain level in order to maximise the chance of getting a satisfactory result. Alternatively samples can be frozen on the farm when a Bactoscan reading is due. If the Bactoscan comes back high, then a bulk milk bacteriology can be performed. Some bacteria will die off due to the freezing but if the Bactoscan is suitably high, this is unlikely to significantly affect the interpretation of the result.

Interpretation

A variety of tests are performed as part of a standard bulk milk bacteriology assessment. Targets depend on the individual laboratory and will be provided with any results given.

TBC (Total Bacterial Count) or TVC (Total Viable Count)

The TBC is the total number of colony forming units from 1 ml of milk measured using a standard microbiological laboratory technique. Bacteria have to be living to be detected and so this measure may also be called the Total Viable Count (TVC). The Bactoscan is superseding this method of counting bacteria in milk, which is a more rapid, and accurate technique that will also count bacteria that tend to multiply at lower temperatures (Psychrotrophs – see below).

Thermoduric count or Laboratory Pasteurisation Count (LPC)

Thermoduric bacteria can withstand high temperatures and high levels are associated with a problem with the parlour washing.

Psychrotrophs count

Psychrotrophs are a type of bacteria that come from the environment and grow under bulk tank refrigeration temperatures. They will inevitably grow to high levels if held at cold temperatures for prolonged periods, but they will tend to be raised when either there is a poor milk cooling or a dirty environment.

Pseudomonads count

Pseudomonads come from the environment but are not of non-enteric origin.

Coliform count

The coliform count is reflective of the degree of faecal contamination of the teat and udder and therefore of parlour hygiene. High levels will be seen when the environment is poorly maintained in combination with minimal pre-milking trat preparation.

Detection of significant pathogens

Targets are available for most of the commonly found causes of subclinical mastitis (S. aureus, S. uberis). However any S. agalactiae detected is a cause for concern and should prompt further investigation.

Limitations

Despite its usefulness, there are several limitations to bulk milk bacteriology that must be considered. The main consideration, which should form part of any interpretation, is that a bulk milk sample is only a snapshot in time. The sample taken today may be completely different to the one taken yesterday. This can be very frustrating! It may so happen that the day you take the sample, cows are particularly clean or dirty which can give a particularly low or high level of certain bacteria that may not be consistent with other parts of your investigation. Alternatively, cows with subclinical mastitis due to S. aureus or S. uberis, which are often intermittently shed, may not be present contrary to your suspicions. This can be particularly awkward when you have just persuaded the farmer the benefits of doing the test and they have just paid in excess of £100.

Therefore the optimal way to analyse bulk milk bacteriology is to not over-interpret one result or rely on the result for your diagnosis. It should only be seen as an adjunct to a more thorough investigation of the presenting problem. Therefore repeated samples may need to be taken and the farmer should be warned of this possibility.

References and further reading

Biggs, A., 2009. Mastitis in Cattle, 1st Edition. The Crowood Press Ltd.

Blowey, R., Edmondson, P., 2010. Mastitis Control in Dairy Herds, 2nd Edition. CABI International.