Thermal Destruction of Escherichia coli and Klebsiella pneumoniae in Human Milk

Abstract

A continuous flow high-temperature short-time pasteurization system was used to determine kinetic parameters (D-and z-values) for thermal destruction of the bacterial pathogens, Escher-ichia coli and Klebsiella pneumoniae, in mature human milk. D-and z-values of each bacterium were determined from data on survivors enumerated on both selective media, Violet Red Bile agar or MacConkey's, and on a non-selective medium, nutrient agar (NA). For£. coli, D-values were determined at 58,60,62 and 64°C. The predicted value of D at 60°C is 31.5 s. The z-value for E. coli is 3.2°C. D-values for A", pneumoniae were determined at 52,56 and 58°C. Based on these data the predicted value of D at 60°C is 1.3 s. The z-value for A", pneumoniae, is 2.8°C. For both E. coli and K. pneumoniae, counts on NA tend to be higher than on selective media. This is undoubtedly due to the inhibitory nature of the selective media. This also suggests that some degree of thermal injury may occur for each organism. It is generally accepted that the best nourishment for healthy mature newborn infants is their own mother's milk (2). The best form of nutrition for the low birth-weight, preterm and sick newborn is not well defined, but recent studies suggest that protein-fortified term human milk and the infant's own mother's milk are at least as good as special preterm milk formulas (12,16,17,18,23). For babies that require human milk but cannot obtain it from their mother, some system of human milk banking must be established, which depends upon adequate and reliable supplies of donated breast milk. One major concern involved with banking of human milk is the microbiological quality of the milk. Human milk is seldom sterile and if collected unhygienically may contain large numbers of bacteria. Bacteria present in breast milk include the potential pathogens, E. coli and K. pneumoniae (4,7,9,11,22,24). Several reports associate disease outbreaks with ingestion of human milk contaminated with pathogenic bacteria. A case of nosocomial colonization and infection due to E. coli 125:k70 was epidemiologically linked to con-'Present address: Food and Drug Administration, Cincinnati, OH. sumption of contaminated expressed breast milk (20). Milk contaminated with K. pneumoniae was responsible for development of primary bacteremia in five patients in a newborn intensive care unit (5). Studies on the effect of pasteurization on E. coli are limited and all of the available data are based on long time heat treatments (1,6,8,9,13). Baum (1) studied thermal destruction of E. coli in human milk exposed to a temperature of 62.5°C for up to 15 min. Effects of processing on specific bacteria in human milk were studied by Raptopoulou-Gigi et al. (13). Both pasteurization (62.5°C, 30 min) and sterilization destroyed staphylococci, streptococci and E. coli in all samples tested. Initial bacterial loads were not given. Eyres et al. (6) reported that processing at 62.5°C for 30 min destroyed E. coli when samples were inoculated with 3.0 x 10 6 CFU E. coli/ml before processing. Lucas and Roberts (9) also showed that pasteurization at 62°C for 30 min eliminated all pathogenic E. coli although initial levels were not specified. Jones et al. (8) reported that pasteurization at 62.5°C for 10 min destroyed E. coli when initial levels were approximately 10 6 /CFU ml. A 5-min treatment at the same temperature reduced levels 3 to 5 orders of magnitude. Studies on the thermal stability of K. pneumoniae in human milk are not available. Data on thermal destruction of E. coli and K. pneumoniae in human milk suitable for use in optimizing short-time pasteurization are not available either. This study was designed to determine the kinetic parameters on thermal destruction of E. coli and K. pneumoniae in human milk primarily using HTST pasteurization conditions .