Bulk Tank Bacteria Concerns
by Winston Ingalls, Ph.D.
West Agro, Inc., Kansas City, MO
A significant raw milk quality concern is the bacteria count. Bacteria in milk potentially represent a public health concern in addition to a milk quality concern. For dairymen, raw milk bacteria counts represent an economic concern since in many cases the quantity of bacteria allowed in raw milk is directly related to bonus payments. Pocket book issues quickly command attention.
Bacteria numbers in milk are determined by testing the bulk tank samples that are collected by milk haulers prior to transferring the farm bulk tank contents to the tank truck. Sampling is mandatory and the bacteria count is determined using a standard test format referred to as the Standard Plate Count (SPC), Plate Loop Count (PLC) or simply the raw count.
Test results provide an estimate of the total number of bacteria present in the sample. The number, expressed as bacteria per ml, does not identify species of bacteria present, just the estimated total number. A typical number for example may be 9,000 per ml.
This value represents the number of bacteria that have entered the tank from all possible sources. It includes bacteria in milk from infected cows, bacteria normally present on teat skin and bacteria found in dirt on the outside of the teats and udder. It also includes bacteria in dirt, water and manure that may have entered the cluster during fall-offs, liner slips or when components are rinsed off with water and contamination is carried into the system. Finally it may be from bacteria buildup on inadequately cleaned milk contact surfaces anywhere in the system. Milk cooling problems may also contribute to elevated bacteria counts because warm temperatures accelerate bacteria growth.
The objective is to keep raw milk bacteria counts as low as possible and when problems occur each of these issues has to be considered.
Additional Test Procedures
Two other routine bacteria tests are often run in addition to the SPC in an effort determine if the source of bacteria in the milk is infected cows, dirty cows or dirty systems.
Preliminary Incubation Count-PI
The Preliminary Incubation (PI) count attempts to determine the presence of bacteria that tend to grow in cool–cold conditions. Typically such bacteria originate from sources outside the udder. It may be possible for certain Strep non-ag bacteria to also elevate the PI when they are shed from infected quarters, but typically, elevated PI counts are linked to external sources, not infected cows.
PI counts generally are higher than the SPC. When significantly higher (3-4X), it suggests soil related bacteria, that grow well in cool temperatures, have entered the product. Water contaminated with Pseudomonas species is often the source. It may be stagnant water that cattle have access to or contaminated hose water used in the parlor.
Failure to cool milk adequately and quickly during and after milking provides favorable conditions for these bacteria to grow. Old, cracked rubber tubing, especially around milk inlet locations, is a place where these bacteria may collect and build up.
Between milkings, bacteria in soil films on equipment surfaces may continue to grow and cause PI problems. Sanitizing system components prior to milking will kill most bacteria while failure to sanitize will allow them to go directly into the bulk tank and create problems. Use of acid sanitizers as the last step of the cleanup procedure can help reduce problems. The sanitizer kills the bacteria while the acid condition limits bacterial growth for extended periods.
It may be possible for certain Strep non-ag bacteria to also elevate the PI when they are shed from infected quarters, but typically, elevated PI counts are linked to external sources, not infected cows.
Contagious bacteria reside primarily in the udders of infected cows and on the teat skin.
PI counts for raw milk should be 20,000/ml or less. In many cases the PI count is quite similar to the SPC when all things are done correctly and that should be the objective.
Laboratory Pasteurized Count (LPC)
The LPC test determines the presence of bacteria that can survive exposure to temperatures of 145 degrees F for 30 minutes. These conditions kill all typical mastitis causing bacteria from inside or outside the udder but certain environmental species may survive, grow and produce damaging enzymes that cause problems in fluid milk or cheese. Obviously, bacteria capable of surviving these conditions are tough customers and not something you want to find in milk.
Bacteria associated with LPC counts are found in dirt and feedstuffs like silage. They can survive in milk films or buildups in the system and develop very resistant forms called spores, which can survive exposure to high temperatures and sanitizers. Control requires minimizing the number entering the system and eliminating places for them to hide during cleaning. An LPC count should be less than 100/ml and if elevated above this level, look at cow cleanliness and locations in the system that fail to clean adequately.
Bacteria Species Evaluation
An additional helpful and often necessary milk quality evaluation involves determining the bacteria species making up the total count. Some milk processors automatically do this if the SPC of a sample is above a certain level. This evaluation determines the predominant bacteria species which allows corrective action to be focused at specific targets.
Cows infected with Strep agalactia typically shed huge numbers into raw milk and can elevate the SPC significantly, especially around clinical outbreaks. Since the interior of the udder is the only place this bacterium is found in any quantity, its presence in bulk milk at any level indicates infected cows. It is not coming from mud, manure or bedding because it needs the internal environment of the udder to survive.
Many dairy farms have completely eliminated Strep ag but the current dairy climate involves many herd expansions and lots of dairy cattle being relocated. When it appears in a herd previously free of the problem, the source is infected cows. Have cows been purchased without a background check? This is a common way for it to enter. Always, buyers beware!
Environmental streps (Strep non-ag species)
When differential counts indicate high numbers of Strep non-ag species it may represent several different issues. Cows infected with Strep non-ag species can shed large numbers of organisms into raw milk and cause a big increase in the SPC. In some instances PI counts may also be elevated if the species involved flourish in cool conditions.
These bacteria thrive in the environment of the cow. They can be found in bedding, manure and on various body sites. The teat and teat ends may develop buildup of these bacteria between milkings if cows lay in wet, contaminated areas and get dirty. Such conditions often exist in summer under shades and shade trees when cows seek relief from heat. These bacteria can also cause serious infections if they gain entry to the udder.
A target or goal for Strep non-ag species counts in bulk milk should be less than 750/ml. Counts greater than this may point to several issues.
Elevated Strep non-ag counts in bulk milk require a look at infected cows, the environment and cleanliness of cows at milking time, system cleanliness and performance of the cooling system. Each can be a factor.
When teat preparation fails to remove all of the teat-end soil, bacteria associated with the soil may end up in the raw milk. It appears possible for some of these bacteria to survive and grow on milk films which may be found in hard to clean areas such as gaskets and around milk nipples. They may also grow on milk filters during extended milking times inoculating the milk passing through the filter. It is recommended that filters be changed after 3.5-4 hours of milking.
Very high counts suggest at least two possible reasons. Infected cows may be contributing lots of bacteria and causing the problem. Monitor early fresh cows because there is a tendency during this period for a large percentage of environmental Strep infections to show up. They may be dry-period infections that are carried into early lactation.
Inadequate cooling may also be a factor. Milk, held at temperatures around 45 degrees F for a couple of hours, allows these bacteria to grow rapidly. This could include certain Strep non-ag species. It is always a factor to consider. Milk needs to be cooled to 38-40 degrees F as quickly as possible and held there. Higher storage temperatures allow more rapid bacterial reproduction. Keep it cold!
Coliforms are another group of bacteria that show up in bulk milk. Processors may have an established upper limit for coliform levels in raw milk and their presence is associated with contamination from manure and bedding materials. They occasionally cause serious mastitis problems in individual cows but generally infected cows are not the cause of elevated coliform counts in bulk tank milk.
The coliform count in raw milk should be less than 100/ml and the count typically is much less when things are done properly. Coliform counts in the hundreds/ml may indicate a problem with dirty cows being milked. When it rises into the thousands it often means there are dirty zones in the milking system where these bacteria are growing and sending large numbers into the raw milk.
Bacteria counts need to be tracked over time to determine if there are trends of any type. A sudden unexpected spike and then a return to normal may indicate a one-time issue or a short-term problem. Cooling problems, clinical mastitis or cleaning system failure may be the reason. A trend of increasing counts or long periods of high counts may indicate a different problem. All the reports and numbers can be very confusing so it helps to develop a few benchmarks or goals to serve as reference points.
The SPC should be less than 10,000/ml. When things are done properly it should be between 1000 and 10,000/ml.
Some have suggested the PI standard be less than 50,000/ml but a realistic goal may be less than 20,000/ml. Higher counts suggest problems with cleaning, sanitizing or milking wet cows. Maintaining low PI counts requires doing everything required for a low SPC plus a little extra.
Strep ag should not be present in bulk milk. Zero! Its presence means infected cows. Find them, treat them or cull them.
Environmental Streps should represent less than 750/ml. They typically are from exterior locations of the cow including dirty cows. One key is to milk clean, dry cows. The other is to identify infected cows and keep them out of the tank.
Coliforms. These are linked to manure. The counts in the tank should be less than 100/ml. Elevated counts points to dirty cows or cleaning and sanitizing problems.