Microbiological analysis of donor human milk over seven years from the Hearts Milk Bank (United Kingdom)

When maternal milk is unavailable, donor human milk (DHM) from human milk banks (HMBs) is the optimal alternative, as recommended by the World Health Organisation. The microbiota of DHM could contain opportunistic pathogens, which means rigorous microbiological screening for DHM, prior to pasteurisation, is recommended to safeguard recipients. Here, an analysis of 6863 DHM samples from 1419 donors at the Hearts Milk Bank between 2017 and 2023 showed approximately 70.1% of samples exhibited a total viable count (TVC) between 10³-10⁵ CFU/mL, while 18.3% yielded no growth; 11.5% of samples exceeded the 10⁵ CFU/mL threshold. Staphylococcus was the most prevalent genus, with S. epidermidis found in 61.5% of samples. A significant (p < 0.05) negative co-occurrence was observed between S. epidermidis and Gram-negative opportunistic pathogens. Overall, 16.8% of DHM samples failed to meet UK microbiological screening criteria, with 68.3% of these failures due to exceeding TVC thresholds. S. epidermidis accounted for approximately 10.2% of the total failed samples. The majority of DHM samples met the current microbiological criteria specified in the National Institute for Health and Care Excellence (NICE) clinical guidance (CG93), "Donor milk banks: service operation". The core species in DHM reflects microorganisms typically found on the skin. These findings highlight that the current UK thresholds and criteria could potentially be modified to increase the available supply of DHM without increasing microbiological risk.