Growth of Selected Bacteria in Processed Human Milks ¹

Survival and/or growth of Pseudomonas fluorescens , Micrococcus luteus , Micrococcus sp. 102 and Staphylococcus aureus was studied in raw, frozen, pasteurized or heat-sterilized pooled human skim milks. Growth response of P. fluorescens at 25°C was essentially the same in the untreated and treated milks with an increase in count/ml of 4 log cycles during a 24-h incubation period. Counts (per ml) of S. aureus in raw, pasteurized and frozen milks increased approximately 3 log cycles. Growth of S. aureus in heat-sterilized milk was somewhat less. Micrococcus sp. 102 could not be recovered from raw and frozen milk after incubation at 37°C for 14 and 24 h, respectively. The count (per ml) of this organism in pasteurized milk decreased approximately 2 log cycles during the 24-h incubation period, whereas counts in sterilized milk increased approximately 2 log cycles. In no instance did M. luteus survive in raw, pasteurized or frozen milks at 37°C even for short periods after inoculation. The organism survived in heat-sterilized milk for 14 h at 37°C, but then decreased in numbers until the organism could be recovered from only 1 of 5 pools after 48 h of incubation.