When Do Breast Milk Thermophilic Heat-Stable Pathogens Survive the Donor Milk Process?

donor milk heat-stable pathogens survive pasteurization

Breast milk is filled with incredible microbiota capable of enhancing infant gut colonization and maturity. But sometimes bacteria come along for the ride. Those bacteria are not always beneficial for the recipient. Examples include Streptococcus, Staphylococcus, Bacteroides, Acinetobacter, and Lactobacillaceae.

Human milk bacteria can be a big concern for NICU clinicians who feed donor milk to high-risk infants. Not all donor milk is processed the same. That can lead to issues with heat-stable pathogens surviving the pasteurization process and being passed on to vulnerable infants.

Those heat-stable pathogens are thermophiles. And they thrive in high pasteurization temperatures.

What are thermophiles and why are they in breast milk?

Thermophiles are heat-stable microbes able to thrive in high temperatures, usually more than 45 °C (that’s 113 °F). They can be present in high concentration in breast milk, possibly due to a mother’s infection, or the way breast pump kits are cleaned and maintained. Improper breast pump cleaning can lead to bacteria transference.

Thermophilic heat-stable pathogens are able to withstand high temperature Holder pasteurization (HoP) donor milk processing. HoP heats donor milk to 62.5 °C (144.5 °F), which unfortunately does not destroy them. Instead, those little pathogens live through it, resulting in the dangerous potential to reach the most at-risk infants in the NICU.

Even if a mother appears healthy, she may still have those pathogens in her living tissue and fluids. If she donates breast milk for donor milk, those pathogens are sent with it. And donor milk pasteurization processes won’t be enough to eliminate them before the milk is distributed to infants in need.

Bacterial impact in some cases may be minimal due to gestational age and overall health of an infant. It’s a very different story when compromised milk is delivered to premature babies with weakened immune systems in neonatal intensive care.

Which thermophiles can live in breast milk?

A few examples of thermophiles found in breast milk include:


Bacillus cereus (B. cereus) is an environmental pathogen. Bacillus pathogenic spores are able to withstand some food industry and hospital cleaning procedures.1 B. cereus can cause severe intestinal infections in immunocompromised infants.


Although Micrococcus is considered a contaminant, there are those in the scientific community who believe it should be regarded as a pathogen. Certain Micrococcus strains can cause various types of infections.


It is estimated up to 3.5% of mothers carry Group B Streptococcus (GBS) in their breast milk. GBS is the leading cause of neonatal infections in developed countries.2


Clostridium difficile (C. diff.) is known to cause antibiotic diarrhea and can lead to infection disease with symptoms spanning diarrhea, pseudomembranous colitis, and toxic megacolon.3

How can thermophilic pathogens be removed from donor milk?

How can thermophilic pathogens be completely removed from donor milk if traditional pasteurization methods are no guarantee?

Retort processing can eliminate heat resistant pathogens and is used throughout the food industry. It has long been the go-to method for processing infant formula because of its ability to remove all traces of pathogens. Retort processing has become more popular in donor human milk processing for its ability to completely destroy thermophiles, molds, and bacteria. Medolac is one provider of human milk-based nutrition using the Retort processing method.

Additional pre-testing can also be performed to reduce the possibility of pathogens surviving in a finished donor milk product. This can be done by pre-screening pooled donor milk for Bacillus, Clostridium, and others. The pathogen-positive pooled milk is then discarded. However, not all milk banks pre-screen pooled milk or discard as a common practice, which means hospitals should always ask if it is performed to better understand the potential risk to their NICU patients.

Pathogens in donor milk are a real concern for NICUs caring for at-risk infants. The best way to provide donor milk nutrition is to investigate your options, learn about the best safety practices available, and identify the right solution for your unit and your patients.

Medolac Benefit Human Milk products use an FDA-approved Retort sterilization food preparation method to eliminate all bacterial spores and viruses. Learn more about the Benefit human milk-based nutrition line for NICU infants, or request information for your hospital.


1. Decousser, JW et al (2013). Bacillus cereus and severe intestinal infections in preterm neonates: Putative role of pooled breast milk. Am J Infect Control 2013 Oct;41(10):918-21. doi: 10.1016/j.ajic.2013.01.043

2. Nicolini, G et al (2018). Group B streptococcus late-onset disease, contaminated breast milk and mothers persistently GBS negative: report of 3 cases. BMC Pediatrics volume 18, Article number: 214

3. Davis, M Y et al (2016). Rapid change of fecal microbiome and disappearance of Clostridium difficile in a colonized infant after transition from breast milk to cow milk. Microbiome. 2016; 4: 53