Bacteria counts pose a significant raw milk quality concern. Explanations and benchmarks for common tests are presented.
Standard plate counts should be less than 10,000/ml. Preliminary Incubation counts should be less than 50,000/ml; a goal of less than 20,000/ml is achievable.
Strep ag in bulk milk cultures means infected cows are present in the herd. Infected cows should be identified and treated or culled.
Environmental streps are generally due to milking wet, dirty cows. Environmental Strep should be less than 750/ml.
Coliforms in tank counts should be less than 100/ml. Higher counts indicate dirty cows or milking system cleaning problems.
Bacteria in raw milk may pose a human health hazard as well as milk quality problems. As a consequence, raw milk is subject to many quality regulations including limits on the allowable number of total bacteria. To encourage production of low bacteria count milk, dairymen typically are rewarded with bonuses for maintaining the total bacteria count below certain levels. Often, a problem is recognized only when the dairy farm receives a report indicating the bacteria count is too high. If this occurs, all aspects of farm milk production practices should be scrutinized.
Minimizing raw milk bacteria counts requires understanding the tests used. Knowledge of conditions and practices that cause elevated counts is particularly important. Finally, proactive procedures should be identified to avoid reoccurrence of problems.
Collection of Raw Milk Samples
Growth conditions and time are key factors in all bacteria tests. All bacteria tests determine how many bacteria were present in the sample at the time testing began. As a result, sample handling procedures are critical in providing usable information.
The milk sample drawn from the bulk tank must come from well-mixed milk. If not, the sample will not accurately represent the actual tank bacteria count. Sampling a poorly mixed tank may result in either an over- or underestimate of bulk tank bacteria count, depending upon the concentration of bacteria in the portion of milk that was sampled.
The sampling device must be thoroughly sanitized; otherwise it may contribute bacteria and increase the count for the sample. The collection vial must be sterile and the procedure for filling the sample vial must prevent bacteria contaminated material from entering. Manure, soil or water contaminants will produce a misleading result.
Bulk tank samples need to be placed on ice immediately and be kept ice cold until delivered to the lab. Cold temperatures stop growth of many bacteria species, especially those that normally live in the udder. At the lab, the sample must be tested promptly so any bacteria present in the sample cannot multiply prior to testing.
Conditions that result in large numbers of bacteria in the tank will cause high test numbers when counted after 48 hours on the growth media. The reverse also holds true; low numbers of bacteria in the bulk tank milk lead to low test numbers.
Understanding Bacterial Tests
Most bacteriology reports list colony-forming units. What’s a colony-forming unit? When a milk sample is placed on growth media and evenly spread on the surface, individual bacteria are theoretically evenly spread out as well. However, either an individual bacterium or a clump of bacteria that stick together may grow to produce a colony. As a result, what is seen on the growth media are referred to as colony forming units. No distinction can be made between those originating from a single bacterium or a clump of bacteria at the start of the test.
Producers’ goals should be to meet or exceed the processor standard for maximum bonus awards. The following table provides some suggested benchmarks.
|Suggested Bacterial Test Standards for Quality Premiums
||US Regulatory Standard
|Standard Plate Count
|Lab Pasteurized Count
|Preliminary Incubation Count
||< 3x to4x the SPC or < 50,000/ml
Source: S. Murphy, NMC Regional Meeting Proceedings, 1997. Pages 34-42.
Common Raw Milk Bacterial Tests
Standard Plate Count (SPC)
The most basic milk bacteria test is the Standard Plate Count (SPC). Other names for this test are the raw count or the total bacteria count (TBC). There are several approved ways of conducting the Standard Plate Count, but the time and temperature conditions used for this test are standardized.
The reason for running an SPC is to determine how many bacteria are present in the farm bulk tank. However, test results show bacteria in the sample when it was received at the lab. If the sample is contaminated or mishandled, bacterial numbers will increase between the bulk tank and the lab. The sample will then fail to accurately reflect the tank count.
A sample for SPC is placed on growth media and incubated at 90 degrees F (32.2° C) for 48 hours. Under these conditions, all bacteria rapidly grow in the presence of adequate food to form visible colonies. After 48 hours, visible bacteria colonies are counted. Based on the sample dilution, a final number is calculated and expressed as the number of colony-forming units per milliliter or CFU’s/ml.
Bacteria counts in raw milk should be compared to appropriate benchmark numbers. The legal maximum based on PMO (Pasteurized Milk Ordinance) (USA) guidelines is 100,000 cfu/ml. A realistic and achievable goal is less than 10,000 cfu/ml. Many producers consistently attain 5,000 cfu/ml.
Bacterial species that grow rapidly under warm conditions (90°F, 32.2°C) produce high SPC counts. This includes all of the typical mastitis pathogens including Strep agalactiae, Staph aureus, Strep non-ag species and most other mastitis causing bacteria. Species that are shed from infected quarters cause elevated bacterial counts in raw milk. Strep bacteria tend to be released in very large numbers and can create elevated bulk tank SPC counts. Staph aureus is not normally released in large quantities into raw milk so it is not likely to elevate the SPC. Coliforms normally are not shed in large numbers into raw milk, plus cows with a serious case of coliform mastitis are typically diverted from the tank.
Cold conditions significantly reduce the growth rate of most mastitis-causing bacteria. Therefore, keeping milk very cold at all times is the best way of minimizing growth. Any milk cooling problems will also increase the SPC.
Common environmental bacteria may also cause elevated bulk tank bacteria counts.
These bacteria can enter the milking system via dirt, contaminated water or manure. Fall-offs, liner slips or careless rinsing of the milking cluster can carry contamination into the system.
Dirty milking systems provide a place for any of these bacteria to lodge, grow and develop into large numbers. The bacterial buildup may be transferred to the tank as fresh milk passes over it and becomes inoculated. Dirty pipelines, unwashed zones in the milk handling system, long milking times (8-10 hrs), no sanitizing, etc., are all factors that can cause bacterial buildup in the system. When these occur, the tank SPC can rise.
Preliminary Incubation Count (PI Count)
Sometimes, in spite of a milking system that appears clean and few mastitis cases, there is still a milk quality problem. How can this happen?
Certain bacterial groups are capable of growing under cool or cold conditions. These bacteria are termed psychrotrophic bacteria. While they may not grow rapidly under these conditions, they are able to grow.
Psychrotrophic bacteria are typically from sources outside the cow such as dirt, manure and contaminated water. Because they are exposed to a wide variety of ambient conditions, they are able grow in both cool and warm conditions.
As these bacteria grow in stored (bulk tank) milk, they have only one food source available to them- cold raw MILK! They produce enzymes that break down milk components to provide their food. These enzymes remain in the milk and can survive pasteurization conditions that destroy the bacteria. Such milk may develop problems post pasteurization, such as reduced shelf life and off-flavors.
How do you find psychrotophic bacteria? Not easily because their numbers may be quite low initially. The most common method involves growth conditions that provide extra time and cool temperatures to try and nurture them along. If present, their numbers will increase and when finally counted, using the SPC procedure, they will produce an elevated bacteria count. This is the basis of the preliminary incubation count or PI count.
The milk sample from the farm tank is handled normally and brought to the lab. It is held in a cool environment, 55 °F (12.7°C), for 18 hours (pre-incubation period). This temperature and time period allows psychrotrophic bacteria to increase significantly in numbers but limits or stops growth of any other bacteria that require warmer temperatures. During this pre-incubation, the number of total bacteria in the sample may increase substantially.
After the 18 hours of pre-incubation, the sample is then tested with the SPC procedure to determine the total number of bacteria present. The count is then compared to the SPC where no pre-incubation was used. If the PI procedure increased the starting bacteria count, then the PI count will be higher than the SPC number.
The value of the PI test lies in the comparison with the corresponding SPC. PI counts generally are higher than the SPC. If both are low, all is well.
If the SPC is low and the PI is high, then psychrotrophic bacteria are at work. When the PI is significantly higher (3-4X) than the SPC, it is likely that soil-borne bacteria, which grow well in cool temperatures, have entered the milk. Water contaminated with Pseudomonas species is often the source. Common sources of contamination are stagnant water in the cows’ environment and contaminated hose water in the parlor.
If both the PI and SPC are high, then mastitis organisms are probably causing both the PI and SPC to be elevated. 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 all 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.
The PI count is not a mandatory procedure. Processors use it to evaluate raw milk supplies to determine if producers are shipping milk that may be contaminated with these cold tolerant bacteria. Many processors have also based their bacteria standards for bonus payments on the PI test.
The key to minimizing PI counts is to manage all aspects of milk production on the farm limiting the introduction of bacteria into milk. This includes overall cow cleanliness, teat cleanliness and sanitization, cleanliness of the total milk handling system, adequate bulk tank cooling capacity, and effective cleaning and sanitization of the milking system.
PI counts should be low and similar to the SPC. As a general guide, the PI counts should stay less than 50,000 and be no more than 3-4 times the SPC. A 10,000 SPC would have a 40,000 upper limit for PI if this guideline were used. Herds with good milking hygiene and mastitis control should find this an achievable goal.
Thermoduric Bacteria and the Lab Pasteurized Count (LPC)
On occasion, the LPC or Laboratory Pasteurized Test is run. The LPC involves taking the raw milk sample and pasteurizing it for 30 minutes at 160°F (71.1°C). The sample is then tested with the SPC procedure and after 48 hours the count is made.
Most bacteria in raw milk are killed by pasteurization, including all typical mastitis organisms, but certain species may survive in small numbers. These are the thermodurics and they are a concern in all milk products, including cheese, yogurt or fresh milk. Thermodurics have developed mechanisms to resist heat and other lethal agents such as sanitizers. Most of these bacteria have an ability to create a protective form called a spore that is very tough to kill.
The spores end up in finished products and begin growing and damaging the milk product. The most effective way to minimize the LPC count is to prevent contamination of the milk with thermoduric bacteria. This means clean cows and clean equipment. Thermoduric bacteria are common in soil and fermented feedstuffs. When cattle are exposed to contaminated material, thermoduric bacteria get on their teats. Poor udder sanitization will allow problems to develop. Milkstone buildups in the system may protect some of these bacteria and allow them to multiply in the raw milk. The LPC test is a good estimate of both cow and system cleanliness.
Bacteria Species Evaluation
An additional helpful, and often necessary, step in milk quality evaluation involves determining the actual species of bacteria present. Some milk processors automatically do this if the SPC of a sample is above a certain level. This evaluation determines the predominant bacteria species so corrective action is focused at the correct target(s).
This test is run by plating a milk sample on special growth media that selects for coliform species of bacteria. Coliforms are fecal bacteria but are also found commonly in the environment. Coliform bacteria can cause mastitis; however, mastitic cows are generally not the cause of elevated coliform counts in the bulk tank. Coliform counts >50/ml in the tank suggest that manure and soil on the teats was not cleaned off. Another possibility is that the claw was somehow soiled with manure during use. Counts that get significantly higher than this suggest dirty equipment and cleaning practices need to be evaluated.
Cows infected with Strep agalactia typically shed huge numbers of bacteria into raw milk, elevating the SPC significantly, especially during 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 herd expansions and purchased dairy cattle can lead to reinfection. When Strep ag 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. PI counts may also be elevated if the species involved flourish in cool conditions.
Strep non-ag species 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 buildups of these bacteria between milkings if cows lay in wet, contaminated areas. Such conditions often exist in summer under shades and shade trees.
A target or goal for Strep non-ag species counts in bulk milk should be less than 750/ml. Counts greater than this indicate a problem. Possible causes of elevated Strep non-ag counts in bulk milk include infected cows, the environment, cleanliness of cows at milking time, system cleanliness and performance of the cooling system.
When teat preparation fails to remove all of the teat-end soil, bacteria may end up in the raw milk. Some of these bacteria appear to survive and grow on milk films 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. For this reason, it is recommended that filters be changed after 3.5-4 hours of milking.
Very high Strep non-ag species counts suggest at least two possible reasons. Infected cows may be contributing lots of bacteria and causing the problem. Monitor early fresh cows, since a large percentage of environmental Strep infections show up during this time. Any dry-period infections persisting into early lactation can result in mastitis here, too.
Inadequate cooling may also be a factor. Milk, held at temperatures around 45°F (7.2°C) for a couple of hours, allows these bacteria to grow rapidly. Milk needs to be cooled to 38-40°F (3.3-4.4°C) as quickly as possible and held there. Higher storage temperatures allow more rapid bacterial reproduction. Keep it cold!
Inevitably an unacceptable bacteria count will occur. Finding the cause or the source is critical. It may be as simple as a cooling tank failure or a combination of factors that come and go sporadically. The latter situation is the most difficult to resolve. There are several major categories of potential problems. Mastitis cows shedding bacteria in large numbers during clinical episodes, cleaning or sanitizing deficiencies, bacteria from dirty cows or cooling problems. The following table provides possible reasons for problems when they occur.
Sources of Microbial Contamination as Detected by Bacteriological Procedures
|PIC High vs SPC
|SPC High/no Increase in PIC
||Not Likely But Possible
|Coliform Count High
||Not Likely But Possible
* A more likely source.
For any of the tests discussed, the numbers must be kept in the low range to minimize quality problems and maximize bonus payments. This is best accomplished by minimizing the number of bacteria from all sources entering the tank. Clean cows and clean systems are the start. Keeping the milk as cold as possible at all times minimizes the growth of most bacteria that may enter the bulk tank. Proper collection and handling of the sample from farm to lab will reduce erroneous results.
Murphy, S.C. Raw Milk Bacteria Tests: Standard Plate Count, Preliminary Incubation Count, Lab Pasteurized Count and Coliform Count- What Do They Mean For Your Farm? 1997. NMC Regional Meeting Proceedings, pages 34-42.
Using Bulk Tank Milk Cultures in a Dairy Practice
National Mastitis Council
Detailed discussion of the correct use of bulk tank milk (BTM) testing and the potential pitfalls of the method. Very clear guidelines on how to interpret BTM results.
Bulk Tank Cultures Can Help Diagnose Problems
NMC Newsletter "Udder Topics", February 1997
Brief description of the actual tests performed on BTM samples and what the results may indicate in a herd.
Testing Bulk Tank Milk Samples
G.M. Jones & Susan Sumner
Thorough treatment of bulk milk testing, including a description of the various tests used, causes of high results and conditions that may adversely affect test results.
BULK TANK MILK CULTURE - INTERPRETING THE RESULTS
Outline form coverage of the specific mastitis organisms found in BTM, their origins, treatment and prevention.