Growth and survival of spore forming Bacillus cereus in human milk exposed to different processing, and storage...

Human milk (HM) is the preferred nutrition for all infants, including preterm and low-birth-weight infants, but it may still pose infectious risks even after pasteurization. Data on the behavior of Bacillus cereus in HM are limited. We therefore investigated the growth and survival of B. cereus in defrosted HM under processing and storage conditions typical of neonatal intensive care unit practice, in vitro.

Frozen HM from five mothers was thawed, pooled, and screened for baseline contamination, then inoculated with a clinical B. cereus isolate (~1.5 × 10⁵ CFU/mL). Samples were allocated to three groups: (A) native HM warmed to 37°C for 3 h; (B) pasteurized HM subsequently warmed to 37°C; and (C) pasteurized HM stored frozen (-21°C for 1 week), thawed, and then warmed to 37°C. B. cereus growth and survival were assessed at 0.5, 1, 2, and 3 h.

Standard pasteurization did not eradicate B. cereus, achieving only a 1-log reduction in CFU/mL. Subsequent warming to 37°C permitted rapid regrowth, with counts returning to the initial inoculum (~1 × 10⁵ CFU/mL) after 3 h. Pasteurization followed by freezing did not further reduce bacterial counts but effectively inhibited proliferation, maintaining lower levels (~1 × 10⁴ CFU/mL) during subsequent warming.

HM should be fed as soon as it reaches feeding temperature and preferably within 1 h. When immediate feeding is not feasible, freezing HM after pasteurization is advisable to inhibit B. cereus growth.