After bacterial culture studies were conducted on term and preterm HM, the results revealed three major findings. First, high-ranked bacteria were common to both groups. Staphylococcus epidermidis, a commensal skin bacterium, was the most frequent bacterial pathogen in both groups, being detected in approximately 80% of the batches. CoNS, Staphylococcus aureus, and Pseudomonas fluorescens were contaminants found at high rates. These results are in line with previous studies [6–9]. These bacteria were reaffirmed to be common commensal bacteria with a high risk of causing contamination, regardless of the donor's living environment.
Second, the total bacterial count was significantly higher in preterm than term HM, and the bacterial species count was greater in preterm HM.
Third, there were differences in bacterial profiles between term and preterm HM. The characteristics of four bacteria species in the term HM are as follows. Serratia liquefaciens and Pseudomonas putida are often isolated from water and soil environments [12, 13]; Pseudomonas putida, especially, is rarely isolated from clinical specimens [13]. Pantoea agglomerans is a Gram-negative bacterium commonly present in fecal material and soil, but it is an uncommon cause of infection in children[14]. The spore-forming bacterium Bacillus cereus was detected in three batches. Preterm HM had more Enterococcus faecalis and Enterobacter aerogenes contamination. These bacteria are classified as enterococci that reside in the human gastrointestinal tract, and they are frequently reported to cause nosocomial infections [15, 16]. There have been cases of outbreaks in the NICU due to contamination of tap water [17]. Pseudomonas fluorescens is widely found in water supplies and can also be isolated from medical devices [18]. Staphylococcus lugdunensis, the CoNS species, is sometimes clinically treated the same as Staphylococcus aureus [19]. It is a commensal skin bacterium [20] and is reportedly a common cause of community-acquired and nosocomial infections [19, 21]. Stenotrophomonas maltophilia is commonly isolated from water, soil, and fecal material and detected in hospitals, especially in the water supply [17, 22].
According to a report by Urrea et al. [23, 24], Enterococcus species, Staphylococcus aureus, and CoNS such as Staphylococcus epidermidis are the leading Gram-positive bacteria, while Escherichia coli, Enterobacter species, Pseudomonas species, and Klebsiella species have been reported to be the organisms most frequently responsible for nosocomial infections in NICU. Of the five species present at significantly different rates in the preterm HM, four species were relevant. The result suggests that preterm HM tends to be contaminated with bacteria that can cause nosocomial infections in NICUs.
We made three main conclusions based on these results. The first is that preterm donors visit the NICU to meet their infants, and bacteria prevalent in the NICU environment may adhere to their clothing, resulting in them bringing the bacteria home. Therefore, there is a contamination risk by such bacteria, not only when HM is expressed in the NICU but also when expressed at home. Although data related to the expression environment (location, methods) were not collected in this study, it is likely most donors expressed at home because the study period overlapped with the coronavirus outbreak, and many NICUs had restricted visiting times.
Second, several preterm samples included HM that had been expressed before donor registration. First of all, as a basic premise, HMBs always provide hygiene instructions, such as pre-breast expression wiping and disinfection of the breast pump, at donor registration, regardless of whether the donor gave birth prematurely or not. Term mothers may voluntarily register as a donor if they delivered a term baby, are currently breastfeeding, and have excessive breast milk supply. After registration, they donate milk to the HMB. However, preterm mothers may provide HMB with milk, which is kept in stock for infants admitted to the NICU. Therefore, there is a higher risk that preterm HM is contaminated with bacteria.
Our third point is that the unique circumstances of preterm donors should also be considered. Several factors contribute to a stressful expression environment for preterm donors, including physical separation from their infants, the provision of a structured feeding schedule, the lack of privacy (when expressing in a hospital), the exhaustion and anxiety associated with an infant’s hospitalization, and long expression periods. These factors can also affect HM production [25–28]. Considering the situation, it is understandably more difficult to pay attention to hygiene precautions compared with when expressing at home, and HM may be contaminated by more bacteria.
These results tell us that hygiene education is more important for preterm donors. However, their physical and psychological circumstances need to be taken into consideration. HMBs need to provide less burdensome and more hygienic expression instructions for these mothers. It may also be necessary to communicate more frequently with preterm donors and to follow up the instructions by observing how they are expressing and storing milk at home. In addition, it may be necessary to survey the NICU situation at each institution and discuss hygiene instructions for preterm donors with the NICU staff.
Although the results of this study did not show a significant difference in the pass/fail score according to the bacterial culture test criteria established by the HMB (Table 1), in the future, better hygiene instructions will not only reduce the breast milk transmission risk but also contribute to reducing the wastage of valuable donated HM, a pertinent issue for HMBs.
Limitations
We did not investigate the location or method of expression, so the influence of these factors on the culture results is unknown. Additional environmental factors, such as living arrangements and sibling status, may also affect the bacterial profile of HM and will be investigated in the future.