The growth profile, thermotolerance and biofilm formation of Enterobacter sakazakii grown in infant formula milk

Gamma radiation sensitivity of Enterobacter sakazakii in dehydrated powdered infant formula

The observed Enterobacter sakazakii D10-values for tryptic soy broth and dehydrated powdered infant formula were 0.27 +/- 0.05 and 0.76 +/- 0.08 kGy, respectively. A decrease of approximately 3 log in the dehydrated powdered infant formula was obtained by irradiation with 3.0 kGy or rehydration with hot water at 80 degrees C. No recoverable bacteria were found in the powdered infant formula irradiated at 5.0 kGy and stored, either before or after rehydration. A radiation dose of up to 5.0 kGy had no marked effect on the sensory properties of the dehydrated powdered infant formula after rehydration and heating. Gamma radiation could potentially be used to inactivate E. sakazakii in dehydrated powdered infant formula; however, nutritional studies need to be conducted before the use of radiation can be recommended.

Effects of preculturing conditions on lag time and specific growth rate of Enterobacter sakazakii in reconstituted powdered infant formula

Enterobacter sakazakii can be present, although in low levels, in dry powdered infant formulae, and it has been linked to cases of meningitis in neonates, especially those born prematurely. In order to prevent illness, product contamination at manufacture and during preparation, as well as growth after reconstitution, must be minimized by appropriate control measures. In this publication, several determinants of the growth of E. sakazakii in reconstituted infant formula are reported. The following key growth parameters were determined: lag time, specific growth rate, and maximum population density. Cells were harvested at different phases of growth and spiked into powdered infant formula. After reconstitution in sterile water, E. sakazakii was able to grow at temperatures between 8 and 47 degrees C. The estimated optimal growth temperature was 39.4 degrees C, whereas the optimal specific growth rate was 2.31 h(-1). The effect of temperature on the specific growth rate was described with two secondary growth models. The resulting minimum and maximum temperatures estimated with the secondary Rosso equation were 3.6 degrees C and 47.6 degrees C, respectively. The estimated lag time varied from 83.3 +/- 18.7 h at 10 degrees C to 1.73 +/- 0.43 h at 37 degrees C and could be described with the hyperbolic model and reciprocal square root relation. Cells harvested at different phases of growth did not exhibit significant differences in either specific growth rate or lag time. Strains did not have different lag times, and lag times were short given that the cells had spent several (3 to 10) days in dry powdered infant formula. The growth rates and lag times at various temperatures obtained in this study may help in calculations of the period for which reconstituted infant formula can be stored at a specific temperature without detrimental impact on health.

Enterobacter sakazakii: an emerging pathogen in powdered infant formula

Enterobacter sakazakii represents a significant risk to the health of neonates. This bacterium is an emerging opportunistic pathogen that is associated with rare but life-threatening cases of meningitis, necrotizing enterocolitis, and sepsis in premature and full-term infants. Infants aged <28 days are considered to be most at risk. Feeding with powdered infant formula (PIF) has been epidemiologically implicated in several clinical cases. Infants should be exclusively breast-fed for the first 6 months of life, and those who are not should be provided with a suitable breast-milk substitute. PIF is not a sterile product; to reduce the risk of infection, the reconstitution of powdered formula should be undertaken by caregivers using good hygienic measures and in accordance with the product manufacturer's food safety guidelines.

A chromogenic plating medium for the isolation and identification of Enterobacter sakazakii from foods, food ingredients, and environmental sources

A chromogenic agar, R&F Enterobacter sakazakii chromogenic plating medium (ESPM), was developed for isolating presumptive colonies of E. sakazakii from foods and environmental sources. ESPM contains two chromogenic substrates (5-bromo-4-chloro-3-indoxyl-alpha-D-glucopyranoside and 5-bromo-4-chloro-3-indoxyl-beta-D-cellobioside), three sugars (sorbitol, D-arabitol, and adonitol), a pH indicator, and inhibitors (bile salts, vancomycin, and cefsulodin), which all contribute to its selectivity and differential properties. On ESPM, 79 pure culture strains of E. sakazakii (10 clinical isolates and others from food and environmental sources) yielded blue-black (three strains were blue-gray) raised colonies, 1 to 2 mm in diameter with and without halos after 24 h at 35 degrees C. Other enteric organisms plus Pseudomonas aeruginosa yielded white, yellow, green, or clear colonies with and without clear halos. Of these genera, only Shigella sonnei and one Pantoea strain produced blue-black to blue-gray colonies. ESPM was used to isolate E. sakazakii from a variety of foods: corn, wheat, and rice flours; powdered infant formula; dairy products (dried milk, whey, and caseinates); cereals; and environmental sources. Most false-positive results on ESPM were eliminated by observing acid production on either sucrose or melibiose after 6 h at 35 degrees C on a R&F E. sakazakii screening medium (ESSM) biplate. In an analysis of 240 samples, the number of samples positive for E. sakazakii by the ESPM-ESSM method and the U.S. Food and Drug Administration protocols (violet red bile glucose agar and tryptic soy agar) were 27 and 16, respectively, with sensitivity and specificity values of 100.0 and 96.9% versus 59.3 and 43.7%, respectively. These data support the fact that E. sakazakii confirmation should be based on more than one confirmation system. Both the API 20E and Biolog Microlog3 4.20 systems should be used for confirmation of E. sakazakii isolates.

Performance of media for recovering stressed cells of Enterobacter sakazakii as determined using spiral plating and ecometric techniques

A study was done to determine the performance of differential, selective media for supporting resuscitation and colony development by stressed cells of Enterobacter sakazakii. Cells of four strains of E. sakazakii isolated from powdered infant formula were exposed to five stress conditions: heat (55 degrees C for 5 min), freezing (-20 degrees C for 24 h, thawed, frozen again at -20 degrees C for 2 h, thawed), acidic pH (3.54), alkaline pH (11.25), and desiccation in powdered infant formula (water activity, 0.25; 21 degrees C for 31 days). Control and stressed cells were spiral plated on tryptic soy agar supplemented with 0.1% pyruvate (TSAP, a nonselective control medium); Leuschner, Baird, Donald, and Cox (LBDC) agar (a differential, nonselective medium); Oh and Kang agar (OK); fecal coliform agar (FCA); Druggan-Forsythe-Iversen (DFI) medium; violet red bile glucose (VRBG) agar; and Enterobacteriaceae enrichment (EE) agar. With the exception of desiccation-stressed cells, suspensions of stressed cells were also plated on these media and on R&F Enterobacter sakazakii chromogenic plating (RF) medium using the ecometric technique. The order of performance of media for recovering control and heat-, freeze-, acid-, and alkaline-stressed cells by spiral plating was TSAP > LBDC > FCA > OK, VRBG > DFI > EE; the general order for recovering desiccated cells was TSAP, LBDC, FCA, OK > DFI, VRBG, EE. Using the ecometric technique, the general order of growth indices of stressed cells was TSAP, LBDC > FCA > RF, VRBG, OK > DFI, EE. The results indicate that differential, selective media vary greatly in their abilities to support resuscitation and colony formation by stressed cells of E. sakazakii. The orders of performance of media for recovering stressed cells were similar using spiral plating and ecometric techniques, but results from spiral plating should be considered more conclusive.

Enterobacter sakazakii: a coliform of increased concern to infant health

The first cases of neonatal meningitis believed to have been caused by Enterobacter sakazakii were reported in 1961. Prompted by several subsequent outbreaks of E. sakazakii infections in neonates and an increasing number of neonates in intensive care units being fed rehydrated powdered infant formula, considered to be a source of the pathogen, public health authorities and researchers are exploring ways to eliminate the bacterium or control its growth in dry infant formula, processing environments and formula preparation areas in hospitals. Reviewed here are advances in taxonomy and classification of E. sakazakii, methods of detecting, isolating and typing the bacterium, antibiotic resistance, clinical etiology and pathogenicity. Outbreaks of E. sakazakii infections in neonates and adults are summarized. Reports on the presence of E. sakazakii in clinical settings, the environment and foods and food processing facilities are reviewed. Tolerance of the pathogen to environmental stresses, its behavior in powdered and rehydrated infant formulae and hazard analysis and risk management are discussed. Research needs are presented.

Biofilm formation, extracellular polysaccharide production, and cell-to-cell signaling in various Enterobacter sakazakii strains: aspects promoting environmental persistence

Enterobacter sakazakii is considered an opportunistic pathogen and has been implicated in food-associated cases of meningitis or enteritis, especially in neonates and infants. The organism has been detected in various types of food and in food production units, but so far only powdered infant formula has been linked to outbreaks of disease. Survival and persistence in such environments requires the ability to adapt to high osmotic potentials and/or dry conditions. Fifty-six E. sakazakii strains were evaluated for several features important for persistence and survival: (i) biofilm formation and the putative production of cellulose as one of the components of the extracellular matrix, (ii) adherence to hydrophilic and hydrophobic surfaces, (iii) the production of extracellular polysaccharides, and (iv) the ability of E. sakazakii to produce cell-to-cell signaling molecules. Pellicle and flock formation was observed in 21 of the strains grown in Luria-Bertani broth and in 44 of the strains grown in brain heart infusion broth. Calcofluor-stained fibrils, observed microscopically in every (fragile or rigid) pellicle, suggested the presence of cellulose as an extracellular compound in this type of biofilm. Twelve isolates did not form any pellicle or flocks under either condition. Twenty-three of the isolates exhibited the potential to adhere to glass surfaces in shaken cultures, and 33 strains showed biofilm formation at the air-solid interface of polyvinyl chloride microtiter wells. Sixteen isolates adhered to both surfaces. Twenty-four of the isolates tested produced a milky, viscous mass, considered as extracellular polysaccharide. High-performance liquid chromatography analysis of the polysaccharide revealed the presence of glucose, galactose, fucose, and glucuronic acid. Thin-layer chromatography analyses performed on ethyl acetate extracts of cell-free supernatants of the 56 strains indicated the presence of two different types of acylated homoserine lactones (3-oxo-C6-HSL and 3-oxo-C8-HSL). These findings illustrate the ability of E. sakazakii to produce cell-to-cell signaling molecules.

A top-down proteomics approach for differentiating thermal resistant strains ofEnterobacter sakazakii

Thermal tolerance has been identified as an important factor relevant to the pathogenicity of Enterobacter sakazakii in human neonates. To identify a biomarker specific for this phenotypic trait, intact protein expression profiles of 12 strains of E. sakazakii were obtained using liquid chromatography mass spectrometry. Proteins were extracted from the bacterial cells, separated by reversed-phase liquid chromatography and mass analyzed. At the end of the chromatography run, the uncharged masses of the multiply charged proteins were determined via automated software routines. The resulting data provided an accurate mass expression profile of the proteins found in the individual strains. From the individual expression profiles, it was possible to identify unique proteins corresponding to strains with thermal resistance. One protein found only in the thermal tolerant strains was sequenced and identified as homologous to a hypothetical protein found in the thermal tolerant bacteria, Methylobacillus flagellatus KT. The protein sequence of this protein was then used to reverse-engineer PCR primers for the gene sequence associated with the protein. In all cases, only thermal tolerant strains of E. sakazakii produced amplified PCR products, demonstrating the specificity of this biomarker.

Fortifying Fresh Human Milk with Commercial Powdered Human Milk Fortifiers Does Not Affect Bacterial Growth During 6 Hours at Room Temperature

OBJECTIVE

To evaluate the growth of resident aerobic mesophilic flora and added Enterobacter sakazakii in fresh, unfortified human milk; fresh human milk fortified with two commercial powdered fortifiers differing in iron content; and infant formula prepared from powder.

SUBJECTS

Eight mothers provided preterm breast milk samples.

METHODS

Breast milk samples were divided into three aliquots: unfortified, fortified with fortifier containing 1.44 mg iron/14 kcal, and fortified with fortifier containing 0.4 mg iron/14 kcal. Aliquots of formula were prepared. Breast milk and formula aliquots were divided into two test samples. Half were inoculated with low amounts of E sakazakii; half were not. All test samples were maintained at room temperature (22 degrees C), serially diluted, and plated onto agars after 0, 2, 4, and 6 hours. Plates were incubated at 35 degrees C and enumerated.

STATISTICAL ANALYSES

Data were analyzed using repeated measures analysis of variance. P<.05 was considered significant.

RESULTS

There were no differences in colony counts of aerobic bacteria among uninoculated or among inoculated human milk samples at any time; counts did not increase significantly over 6 hours. There were no differences in colony counts of E sakazakii among inoculated human milk samples at any time; counts did not increase significantly over 6 hours. Aerobic bacteria and E sakazakii colony counts from infant formula did not increase significantly over 6 hours.

CONCLUSIONS

During 6 hours at 22 degrees C, fresh human milk and formula had negligible bacterial growth; fortifying human milk with powdered fortifiers did not affect bacterial growth.

Survival and growth of Enterobacter sakazakii in infant rice cereal reconstituted with water, milk, liquid infant formula, or apple juice

AIMS

To determine survival and growth characteristics of Enterobacter sakazakii in infant rice cereal as affected by type of liquid used for reconstitution and storage temperature after reconstitution.

METHODS AND RESULTS

A commercially manufactured dry infant rice cereal was reconstituted with water, apple juice, milk, or liquid infant formula, inoculated with a 10-strain mixture of E. sakazakii at populations of 0.27, 0.93, and 9.3 CFU ml(-1), and incubated at 4, 12, 21 or 30 degrees C for up to 72 h. Growth did not occur in cereal reconstituted with apple juice, regardless of storage temperature, or in cereal reconstituted with water, milk, or formula and stored at 4 degrees C. The lag time for growth in cereal reconstituted with water, milk, or formula was decreased as the incubation temperature (12, 21 and 30 degrees C) was increased. Upon reaching maximum populations of 7-8 log10 CFU ml(-1), in some instances populations decreased to nondetectable levels during subsequent storage which was concurrent with decreases in pH.

CONCLUSIONS

Enterobacter sakazakii initially at very low populations can rapidly grow in infant rice cereal reconstituted with water, milk, or infant formula.

SIGNIFICANCE AND IMPACT OF THE STUDY

Reconstituted infant rice cereal can support luxuriant growth of E. sakazakii. Reconstituted cereal that is not immediately consumed should be discarded or stored at a temperature at which E. sakazakii and other food-borne pathogens cannot grow.

Microbiological, epidemiological, and food safety aspects of Enterobacter sakazakii

Enterobacter sakazakii is considered to be an opportunistic pathogen and has been implicated in foodborne diseases causing meningitis or enteritis, especially in neonates and infants. The U.S FoodNet 2002 survey rate of invasive infections with this organism in infants under 1 year of age was 1 per 100,000 infants. Severity of the disease is a matter of concern. In a recent study on the occurrence of E. sakazakii in production environments from food (milk powder, chocolate, cereal, potato, and pasta) factories and households, this organism was isolated with varying frequency from nearly all environments examined, strongly indicating that it is widespread. Stationary phase E. sakazakii cells are remarkably resistant to osmotic and drying stresses compared with other species of the Enterobacteriacae. In this article, we review the literature on this organism with special respect to the information relevant for food safety.