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

Desiccation resistance and persistence of Cronobacter species in infant formula

Cronobacter is a newly described genus which includes opportunistic pathogens formerly known as 'Enterobacter sakazakii'. These organisms have been isolated from a wide variety of sources, including powdered infant formula (PIF). This review focuses on the desiccation survival of Cronobacter, and its relevance to vehicles of infection. Due to its probable natural habitat of plant material, the organism has an array of survival mechanisms which includes resistance to desiccation and osmotic stresses. The organism can survive for long periods of time (>2years) in the desiccated state, and can be recovered from a large number of powdered foods in addition to powdered infant formula. On reconstitution, the organism may rapidly multiply and present a risk to immunocompromised infants. It is expected that an improved understanding of the nature of Cronobacter persistence may aid in further improved control measures and eliminate the bacterium from the critical food production environments.

Detection of Enterobacter sakazakii and other pathogens associated with infant formula powder by use of a DNA microarray

Pathogen detection is critical to the process of generating and testing powdered infant formula (PIF). An obstacle associated with PIF microbial surveillance is that most current procedures are time-consuming and labor-intensive. We have developed a rapid, DNA microarray-based detection technique to identify 10 different pathogenic bacteria associated with PIF contamination based on the 16S-23S rRNA gene internal transcribed spacer (ITS) sequences and wzy (O antigen polymerase) gene. Using this procedure, Enterobacter sakazakii, Salmonella enterica, Klebsiella pneumoniae, Klebsiella oxytoca, Serratia marcescens, Acinetobacter baumannii, Bacillus cereus, Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli O157 were identified. One hundred eighty-five strains were used to validate the microarray assay (including 134 target pathogen strains and 51 closely related bacteria). Twenty-seven probes reproducibly detected multiple pathogens with high specificity and sensitivity (0.100 ng genomic DNA or 10(4) CFU/ml). Twenty-one real PIF samples were tested by the microarray with 100% accuracy. The data presented reveal that the designed oligonucleotide microarray is a promising method for basic microbiology, clinical diagnosis, food safety, and epidemiological surveillance.

[Effects of the reconstitution and storage conditions of powdered infant formula (PIF) on the survival and growth of Enterobacter sakazakii]

The effect of the reconstruction and storage conditions of powdered infant formula (PIF) on the survival and growth of three Enterobacter sakazakii strains, ATCC 29004, HT 022 and HT 028, was investigated. D values of E. sakazakii ATCC 29004 and HT 022 at 60 degrees C were 3.6 and 1.6 min, respectively, and that of HT 028 at 52 degrees C was 1.6 min. The effect of the temperature of the water used for the reconstruction of PIF on the inactivation of the three E. sakazakii strains was also investigated. One to 2 log order inactivation occurred at 70 degrees C, and above 5 log order inactivation at 80 degrees C. Storage tests at 5, 10 and 25 degrees C showed that none of the strains could grow at 5 degrees C, HT 028 grew slightly at 10 degrees C, and at 25 degrees C all three strains started growth after 4 hr incubation and reached up to 8 log CFU/mL after 16 hr incubation. From the above results, it is concluded that a suitable temperature of the hot water for reconstruction of PIF is above 70 degrees C, and the preferred storage temperature of reconstructed PIF, which is recommended to be consumed within 2 hr, is below 5 degrees C.

Cronobacter ('Enterobacter sakazakii'): current status and future prospects

The genus Cronobacter accommodates the 16 biogroups of the emerging opportunistic pathogen known formerly as Enterobacter sakazakii. Cronobacter spp. are occasional contaminants of milk powder and, consequently, powdered infant formula and represent a significant health risk to neonates. This review presents current knowledge of the food safety aspects of Cronobacter, particularly in infant formula milk powder. Sources of contamination, ecology, disease characteristics and risk management strategies are discussed. Future directions for research are indicated, with a particular focus on the management of this increasingly important bacterium in the production environment.

Resistance of Enterobacter sakazakii (Cronobacter spp.) to environmental stresses

AIM

To gain a better understanding of the survival and persistence of Enterobacter sakazakii in severe environments.

METHODS AND RESULTS

We evaluated the resistance of Ent. sakazakii to various environmental stresses, including heating, drying, water activity (a(w)), and pH. The resistance of Ent. sakazakii to heat varies widely among strains. Most tested strains of Ent. sakazakii exhibited unusual resistance to dry stress, which depends on drying media. Growth of most strains occurred within 24 h at 37 degrees C when the initial a(w) of the medium was adjusted to 0.94 with sucrose or sodium chloride. The minimum pH for growth within 24 h at 37 degrees C was 3.9 or 4.1 for most strains tested. Additionally, there did not appear to be any relationship between resistance to stresses and biofilm-forming ability in Ent. sakazakii planktonic cells.

CONCLUSIONS

These results indicate that Ent. sakazakii is much more resistant than other Enterobacteriaceae to environmental stresses. Moreover, it is likely that Ent. sakazakii has cross-resistance to dry and thermal stresses.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings of this study will contribute to an improved understanding of the survival and behaviour of Ent. sakazakii, which will lead to improved strategies for preventing outbreaks of Ent. sakazakii infection.

Development and application of a novel peptide nucleic acid probe for the specific detection of Cronobacter genomospecies (Enterobacter sakazakii) in powdered infant formula

Here, we report a fluorescence in situ hybridization (FISH) method for rapid detection of Cronobacter strains in powdered infant formula (PIF) using a novel peptide nucleic acid (PNA) probe. Laboratory tests with several Enterobacteriaceae species showed that the specificity and sensitivity of the method were 100%. FISH using PNA could detect as few as 1 CFU per 10 g of Cronobacter in PIF after an 8-h enrichment step, even in a mixed population containing bacterial contaminants.

The importance of hygiene in the domestic kitchen: Implications for preparation and storage of food and infant formula

Aims: Public concerns relating to food safety remain high with most attention focused on manufactured foods and those served in catering operations. However, previous data have suggested that the home may be the main location for cases of food-borne disease. The aim of this paper is to review the microbiological risks associated with hygiene in the domestic kitchen related to food and infant formula safety.

Methods: Compared to other food sectors, research on consumer food hygiene, domestic food-handling and preparation of infant formula is relatively understudied. Behavioural and microbiological studies of consumer hygiene and the domestic kitchen have been reviewed to incorporate research relating to the safety of infant formula.

Results: Incidence data identify the home as an important location for acquiring food-borne disease. The domestic kitchen can be used for a variety of purposes and is often contaminated with potentially harmful micro-organisms such as Campylobacter and Salmonella. Consumer hygiene habits have frequently been found to be inadequate and relate both to microbial growth, survival and cross-contamination. Due to the reduced immune response of infants, the activities associated with the preparation of infant formula and associated bottles and equipment are of particular concern.

Conclusions: Cumulatively, the data suggest that more effort should be made to educate the consumer in food hygiene, especially when the kitchen is used to reconstitute infant formula. This information needs to be provided in a form appropriate for use by consumers.

Characterization of Cronobacter recovered from dried milk and related products

BACKGROUND

Cronobacter is a recently proposed genus consisting of six genomospecies that encompass the organisms previously identified as Enterobacter sakazakii. Cronobacter are opportunistic pathogens and are known to cause serious infections in infants, particularly neonates. High case fatality rates have been associated with infections and acute sequelae can occur in survivors with severe ramifications on neurological development. Infant formula has been identified as one route of transmission for infection in infants. However, the primary reservoirs for subsequent contamination of foods with Cronobacter remain undefined due to the ubiquitous nature of these organisms. More recently, infections in adults have been reported, especially amongst the elderly and patients who are immunocompromised. To help prevent the transmission of infection, it is important to identify the main food sources for Cronobacter. The aim of this study was to identify and characterize Cronobacter isolated from dried-milk and related products available in an Egyptian food market.

RESULTS

In total sixteen Cronobacter strains were isolated from 152 dairy-based products. These were identified and characterized using pheno- and genotyping experiments. Real-time PCR confirmed the detection of Cronobacter. Following antibiotic susceptibility tests, 3 strains showed resistance to trimethoprim and/or neomycin. Phenotype profiles were generated based on key biochemical distinguishing tests. Pulsed-field gel electrophoresis (PFGE) identified 8 PFGE types amongst the collection of strains. Repetitive sequence based PCR (rep-PCR) analysis identified 3 rep-PCR types amongst the collection of strains. Sequencing of the recN gene was used to differentiate among the recently described species of Cronobacter.

CONCLUSION

This study identified the presence of Cronobacter in dried milk and related products sourced from the Nile-Delta region of Egypt. Although the majority of the strains were susceptible to the antibiotics tested, resistance was observed in three isolates, highlighting the risks associated with Cronobacter contamination in foods. Phenotype and genotype analysis should be applied to further characterize Cronobacter spp. and prevent its transmission into food products.

Evaluation of an antimicrobial ingredient prepared from a Lactobacillus acidophilus casein fermentate against Enterobacter sakazakii

Previously two antimicrobial peptides, IKHQGLPQE (caseicin A) and VLNENLLR (caseicin B), were identified following the fermentation of sodium caseinate with the proteolytic strain Lactobacillus acidophilus DPC 6026. This study evaluated the ability of these peptides to kill Enterobacter sakazakii ATCC 12868 spiked in reconstituted infant formula. The survival of E. sakazakii populations in reconstituted infant formula containing a sodium caseinate fermentate was compared with survival in formula containing positive (monocaprylin) and negative controls. The L. acidophilus DPC 6026 sodium caseinate fermentate reduced pathogen numbers by >4 log CFU/ml at 37 degrees C, comparing favorably with the activity of monocaprylin. Additionally, E. sakazakii NCTC 8155 was inoculated into pasteurized, reconstituted infant formula (6 log CFUlml) followed by the addition of increasing concentrations of the fermentate (0.21 to 6.7% [wt/vol]). At a concentration of 0.21% (wt/vol), pathogen viability was maintained over 4 h at 6.0 log CFU/ml. In contrast, pathogen numbers increased approximately 100-fold in the control formula in the same time frame. At higher final fermentate concentrations (-3.33% [wt/vol]), numbers were reduced to 0 log CFU/ml over 60 min. The spectrum of activity of the fermentate against other foodborne pathogens was also determined and shown to be effective against Escherichia coli O157:H7 and Listeria innocua. Results indicate the potential of this fermentate as a built-in protection mechanism against E. sakazakii strains in reconstituted infant formula.

Enterobacter sakazakii: an emerging pathogen in infants and neonates

BACKGROUND

Enterobacter sakazakii (ES) is an emerging pathogen associated with the ingestion of contaminated reconstituted formula that causes necrotizing enterocolitis, sepsis, and meningitis in low-birth-weight preterm neonatal infants. Necrotizing enterocolitis remains the most common gastrointestinal surgical emergency in these infants. In recent years, the International Commission on Microbiological Specifications for Foods has ranked ES a "severe hazard for restricted populations." Because of its resistance to certain antibiotics, better understanding of ES pathogenesis is needed to aid in the development of new preventive strategies.

METHODS

Review of pertinent English-language literature.

RESULTS

Neonatal and older infants appear to be at the highest risk, although adult ES infections have been reported. We discuss the origins of ES, the detection and pathogenesis of the disease, and potential prevention strategies.

CONCLUSIONS

The precise pathogenesis of ES remains a mystery. Appropriate measures by parents, infant formula manufacturers, and health care providers, as well as understanding of the pathogenesis, are important in the prevention of ES-related infections.

Enterobacter sakazakii in dried infant formulas and milk kitchens of maternity wards in São Paulo, Brazil

This study was the first conducted in Brazil to evaluate the presence of Enterobacter sakazakii in milk-based powdered infant formula manufactured for infants 0 to 6 months of age and to examine the conditions of formula preparation and service in three hospitals in São Paulo State, Brazil. Samples of dried and rehydrated infant formula, environments of milk kitchens, water, bottles and nipples, utensils, and hands of personnel were analyzed, and E. sakazakii and Enterobacteriaceae populations were determined. All samples of powdered infant formula purchased at retail contained E. sakazakii at <0.3 [corrected] most probable number (MPN)/100 g. In hospital samples, E. sakazakii was found in one unopened formula can (0.3 MPN/100 g) and in the residue from one nursing bottle from hospital A. All other cans of formula from the same lot bought at a retail store contained E. sakazakii at <0.3 [corrected] MPN/100 g. The pathogen also was found in one cleaning sponge from hospital B. Enterobacteriaceae populations ranged from 10(1) to 10(5) CFU/g in cleaning aids and <5 CFU/g in all formula types (dry or rehydrated), except for the sample that contained E. sakazakii, which also was contaminated with Enterobacteriaceae at 5 CFU/g. E. sakazakii isolates were not genetically related. In an experiment in which rehydrated formula was used as the growth medium, the temperature was that of the neonatal intensive care unit (25 degrees C), and the incubation time was the average time that formula is left at room temperature while feeding the babies (up to 4 h), a 2-log increase in levels of E. sakazakii was found in the formula. Visual inspection of the facilities revealed that the hygienic conditions in the milk kitchens needed improvement. The length of time that formula is left at room temperature in the different hospitals while the babies in the neonatal intensive care unit are being fed (up to 4 h) may allow for the multiplication of E. sakazakii and thus may lead to an increased health risk for infants.

[Enterobacter sakazakii in powdered infant formula]

Under inadequate hygienic conditions, opportunistic bacteria may multiply in powdered infant formula (PIF) and cause severe, often fatal neonatal infections. Enterobacter sakazakii has obtained Public Health relevance causing neonatal meningitis (often fatal), bacteremia and necrotizing enterocolitis. At highest risk are neonates up to two months of age. The new genus designation Cronobacter spp. nov. has been proposed to replace Enterobacter sakazakii. Enterobacter sakazakii is relatively resistant to osmotic and dry stress and may survive in PIF more than 2 years. (Inter)national organisations (EFSA, FAO, WHO, ESPGHAN, DGKJ, OGKJ, ISO) published their opinions recently. Manufacturers can minimize the risk of contamination of PIF by continuously improving technologies and by microbiological surveillance. Institutional and private consumers may reduce the risk of infection by using appropriate hygienic procedures.

Effect of desiccation, starvation, heat, and cold stresses on the thermal resistance of Enterobacter sakazakii in rehydrated infant milk formula

Enterobacter sakazakii is an opportunistic foodborne pathogen that has been isolated from powdered infant milk formula. This study determined the effect of desiccation, starvation, heat and cold stresses on the thermal inactivation of E. sakazakii in rehydrated infant milk formula (RIMF). Stressed cells were mixed with RIMF at 52, 54, 56, and 58 degrees C for various time periods. The D- and z-values were determined by using linear regression analysis. D-values for unstressed E. sakazakii at 52, 54, 56, and 58 degrees C were 15.33, 4.53, 2, and 0.53 min, respectively. Desiccation and heat stresses, but not starvation or cold stress, caused significant (P < 0.05) reduction in D-values. The z-values of desiccated, starved, heat stressed, and cold stressed E. sakazakii were not significantly different from unstressed cells (4.22 degrees C). Thermal resistance of E. sakazakii in RIMF is affected by the environmental stresses; that is, desiccation and heat stresses that may surround the bacterium prior to the contamination of infant formula. The results of this study may be of use to regulatory agencies, infant milk producers, and infant caregivers to design heating processes to eliminate E. sakazakii that may be present in infant milk formula.

Effects of extended dry storage of powdered infant milk formula on susceptibility of Enterobacter sakazakii to hot water and ionizing radiation

Infant milk formula has been identified as a potential source of Enterobacter sakazakii, which has been implicated in neonatal meningitis and necrotizing enterocolitis. This study was undertaken to determine whether the length of E. sakazakii storage in powdered infant milk formula (PIMF) affected the ability of the pathogen to survive subsequent reconstitution of the powder with hot water or treatment with gamma radiation. Five E. sakazakii strains were mixed individually with PIMF and kept for up to 12 months at 25 degrees C. After storage PIMF was reconstituted with water at 60 to 100 degrees C or was exposed to < or = 5 kGy of gamma radiation. Without any treatment secondary to drying, E. sakazakii counts decreased < 1 log/g after 1 month but decreased about 4 log/g during storage for 8 to 12 months. Dry storage decreased thermal resistance but increased resistance of E. sakazakii to ionizing radiation in PIMF. Reconstitution of contaminated powder with water at 70 degrees C after 1 month of dry storage reduced E. sakazakii viability slightly, > 2 log/g, and after powder was stored for 12 months all E. sakazakii strains were eliminated. In contrast, desiccation substantially increased the resistance of E. sakazakii strains to ionizing radiation. Although the D-value for E. sakazakii IMF1 following overnight storage in PIMF was 0.98 kGy, > 4 kGy was required to kill 1.5 log/g of the same strain that had survived 12 months in dry PIMF. Results suggested that low-dose irradiation will more effectively eliminate E. sakazakii from PIMF if the treatment is applied shortly after PIMF manufacture.

Fate of Enterobacter sakazakii attached to or in biofilms on stainless steel upon exposure to various temperatures or relative humidities

Survival of Enterobacter sakazakii dried on the surface of stainless steel and exposed to 43% relative humidity, as affected by temperature, was studied. Populations of E. sakazakii (7.4 to 8.6 log CFU per coupon) on coupons dried for 2 h at 22 degrees C decreased significantly (P < or = 0.05) at 4, 25, and 37 degrees C within 10, 3, and 1 day(s), respectively, but the pathogen remained viable for up to 60 days. At a given storage temperature and time, reductions were significantly greater when cells had been suspended in water rather than in infant formula before drying. Formation of biofilm by E. sakazakii on stainless steel immersed in M9 medium, which contains minimal concentrations of nutrients, and infant formula at 25 degrees C and subsequent survival of cells at 25 degrees C as affected by exposure to 23, 43, 68, 85, and 100% relative humidity were investigated. Some of the cells in these biofilms survived under all test relative humidities for up to 42 days. The overall order of survival as affected by relative humidity was 100 > 23 = 43 = 68 > 85% relative humidity, regardless of the medium in which the biofilm was formed. Reduction in viability of cells was significantly greater in biofilm that had formed in M9 medium than in biofilm formed in infant formula. Results indicate that infant formula provides protection for attached cells, as well as cells in biofilm, against lethality on exposure to desiccation. These results are useful when predicting the survival characteristics of E. sakazakii on stainless steel surfaces in processing and preparation kitchen environments.

Tracing of Enterobacter sakazakii isolates in infant milk formula processing by BOX-PCR genotyping

AIMS

Enterobacter sakazakii is an emerging food-borne pathogen that can cause rare but severe neonatal meningitis, bacteraemia and necrotizing enterocolitis. Contaminated powder infant formulae (PIF) have been identified as one of various infection routes. In this study, E. sakazakii was monitored in the processing environment of a PIF factory to identify possible dissemination routes.

METHODS AND RESULTS

The BOX-polymerase chain reaction (PCR), a fingerprinting technique which targets the repetitive BOX sequences, was used in routine to identify points of contamination and investigate clonal persistence. Two hundred E. sakazakii isolates were collected and typed. Most (70%) showed the same fingerprint that revealed the persistence of resident E. sakazakii strains in the processing environment. This method allowed to detect contamination of some PIF by dry-blending ingredients.

CONCLUSIONS

Environment was the major cause for contamination of PIF and facilities. Some raw materials delivered as powder were also implicated.

SIGNIFICANCE AND IMPACT OF THE STUDY

Routine BOX-PCR genotyping was very useful to trace and investigate in real-time dissemination of micro-organisms in the PIF plant and to implement a series of additional control measures to reduce the risk of final product contamination by E. sakazakii.

Development of a novel screening method for the isolation of "Cronobacter" spp. (Enterobacter sakazakii)

A differential medium, "Cronobacter" screening broth, has been designed to complement agars based on hydrolysis of chromogenic alpha-glucopyranoside substrates. The broth was evaluated using 329 Enterobacteriaceae strains (229 target isolates), spiked/naturally contaminated samples, and a parallel comparison with current methods for raw materials, line/end products, and factory environment samples.

Enterobacter sakazakii invasion in human intestinal Caco-2 cells requires the host cell cytoskeleton and is enhanced by disruption of tight junction

Enterobacter sakazakii is an opportunistic pathogen that causes systemic bacteremia and meningitis with high mortality, and powdered infant formula is a frequent source of this bacterium. However, the mechanisms that this organism uses to invade and translocate through the intestinal barrier are unknown. Using Caco-2 epithelial cells, we were able to demonstrate penetration of E. sakazakii and to determine invasion-associated properties. We found that E. sakazakii entry and invasion were dependent on the exposure time and multiplicity of infection and required bacterial de novo protein synthesis but was independent of cell polarity in the presence of tight junctions. Moreover, the presence of actin filaments and microtubule structures was required, and disruption of the tight junction significantly enhanced the initial association with Caco-2 cells and the efficiency of invasion, which provides a possible explanation for the preferential occurrence of this infection in babies and neonates. This is the first description of E. sakazakii invasion of host intestinal cells, and our findings suggest that this emerging pathogen employs a novel invasion mechanism for development of systemic infection.

Cellulose as an extracellular matrix component present in Enterobacter sakazakii biofilms

Cellulose was identified and characterized as an extracellular matrix component present in the biofilm of an Enterobacter sakazakii clinical isolate grown in nutrient-deficient (M9) medium. Using a bacterial artificial cloning approach in Escherichia coli and subsequent screening of transformants for fluorescence on calcofluor plates, nine genes organized in two operons were identified as putatively responsible for the biosynthesis of cellulose. In addition to the genes already described for cellulose production, two more genes were identified, putatively transcribed together with the genes from the first operon. Putative cellulose in E. sakazakii ES5 biofilm grown on glass coverslips was visualized by calcofluor staining and confocal fluorescence laser scanning microscopy. For the first time, the presence of cellulose in biofilms produced by E. sakazakii was confirmed by methylation analysis.

Prevalence and genetic diversity of Enterobacter sakazakii in ingredients of infant foods

Various food samples in Korea were examined for the presence of Enterobacter sakazakii. Dried shrimp had the highest contamination rate among the examined dried fish products. E. sakazakii isolates were confirmed using an API 20E kit and a polymerase chain reaction (PCR) targeting the rDNA operons. The isolates were subtyped by pulsed-field gel electrophoresis (PFGE) using XbaI to elucidate the genetic diversity of the organisms. Ten pulsotypes were identified using PFGE and 22 types were identified from the random amplified polymorphism DNA (RAPD) assay. An antibiotic resistance test was performed by disk diffusion assay using eight antibiotics: nalidixic acid, ciprofloxacin, chloramphenicol, ampicillin, tetracycline, gentamicin, kanamycin, and cephalothin. Most of the E. sakazakii isolates were resistant to ampicillin or cephalothin but susceptible to the other antibiotics. The analysis of E. sakazakii isolates using PFGE, RAPD, and the antibiotic resistance test identified 18 composite types from 113 isolates, suggesting diverse sources of contamination.

Growth of Enterobacter sakazakii in reconstituted infant formula as affected by composition and temperature

The ability of Enterobacter sakazakii to cause infections in infants, coupled with its documented presence in some lots of commercially manufactured powdered infant formula, raises a concern about the potential for its growth in reconstituted formula, with consequent increased safety risk. A study was done to determine these characteristics in four commercial milk-based powdered infant formulas and two soy-based formulas reconstituted with water and inoculated with a 10-strain mixture of E. sakazakii at populations of 0.02 and 0.53 CFU/ml (ca. 13 CFU/100 g and ca. 409 CFU/100 g of powdered formula, respectively). Reconstituted formulas were stored at 4, 12, 21, and 30 degrees C, and populations were monitored up to 72 h. E. sakazakii did not grow in formulas stored at 4 degrees C, although it was detected by enrichment of all formulas 72 h after reconstitution. Initially at a population of 0.02 CFU/ml, E. sakazakii grew to populations > or = 1 log CFU/ml of reconstituted formulas held at 12, 21, and 30 degrees C for 48, 12, and 8 h, respectively. At an initial population of 0.53 CFU/ml, the pathogen grew to populations > or = 1 log CFU/ml in reconstituted infant formula held at 12 and 21 degrees C for 24 and 8 h, respectively, and to populations 2.55 to 3.14 log CFU/ml when held at 30 degrees C for 8 h. Populations initially at 0.02 and 0.53 CFU/ml of reconstituted formula increased to < or = 0.25 and 0.4 log CFU/ml, respectively, when formulas were held at 30 degrees C for 4 h. Growth was not greatly influenced by the composition of formulas. Results show that the hang time for reconstituted infant formula held at temperatures in neonatal intensive care units should be no longer than 4 h. Portions of reconstituted infant formula not fed to infants should be stored at < or = 4 degrees C, a temperature at which E. sakazakii will not grow.

Dry stress and survival time of Enterobacter sakazakii and other Enterobacteriaceae in dehydrated powdered infant formula

Powdered infant formula is not a sterile product, and opportunistic pathogens could multiply in the reconstituted product, resulting in neonatal infections. In this study, the generation of sublethally injured Enterobacteriaceae during desiccation and their persistence in dehydrated powdered infant formula was assessed during a 2.5-year period. The study included 27 strains of Enterobacter sakazakii, Enterobacter cloacae, Salmonella Enteritidis, Citrobacter koseri, Citrobacter freundii, Escherichia coli, Escherichia vulneris, Pantoea spp., Klebsiella oxytoca, and Klebsiella pneumoniae. The number of sublethally injured cells generated during desiccation was lower for K. oxytoca, Pantoea spp., Salmonella Enteritidis, and capsulated strains of E. sakazakii than for the other Enterobacteriaceae. The Enterobacteriaceae could be divided into three groups with respect to their long-term survival in the desiccated state. C. freundii, C. koseri, and E. cloacae were no longer recoverable after 6 months, and Salmonella Enteritidis, K. pneumoniae, and E. coli could not be recovered after 15 months. Pantoea spp., K. oxytoca, and E. vulneris persisted over 2 years, and some capsulated strains of E. sakazakii were still recoverable after 2.5 years.

Inhibition of growth of Enterobacter sakazakii in reconstituted infant formula by the lactoperoxidase system

Neonatal bacteremia and meningitis caused by the opportunistic pathogen Enterobacter sakazakii have been associated with the consumption of reconstituted powdered infant formula. Lactoperoxidase (LPO), present in mammalian milk, is known to inhibit the growth of enteric pathogens. We undertook a study to determine if the lactoperoxidase system (LPOS) will inhibit the growth of E. sakazakii in a milk-based powdered infant formula reconstituted with water. Initially at 0.04 CFU/ml, E. sakazakii grew to 2.40 to 2.74 log CFU/ml in reconstituted infant formula held at 30 or 37 degrees C for 8 h and to 0.6 log CFU/ ml in formula held for 12 h at 21 degrees C. The pathogen was not detected (less than 1 CFU/227 ml) by enrichment of formula treated with 10 to 30 microg/ml LPO and stored for 24 h at 37 degrees C or 30 microg/ml LPO and stored for 24 h at 30 degrees C. Populations of E. sakazakii, initially at 4.40 log CFU/ml of reconstituted infant formula containing 5 microg/ml LPO, did not increase significantly (P > 0.05) for up to 12 h at 21 and 30 degrees C. Populations either decreased significantly or were unchanged in formula supplemented with 10 microg/ml LPO and stored at 21, 30, or 37 degrees C for up to 24, 8, and 8 h, respectively. Results indicate that LPOS can be used to control the growth of E. sakazakii in reconstituted infant formula, thereby potentially reducing the risk of neonatal infections resulting from consumption of formula that may be contaminated with the pathogen.

Comparison of three chromogenic media and evaluation of two molecular-based identification systems for the detection of Enterobacter sakazakii from environmental samples from infant formulae factories

Enterobacter sakazakii is an occasional contaminant of powdered infant formula that can cause rare but severe foodborne infections in infants. To determine optimal methods for the detection and identification of E. sakazakii, 38 naturally contaminated samples from infant formulae factories were analyzed by two PCR-based methods and by a method (TS 22964/RM 210) developed by the International Organization for Standardization and the International Dairy Federation (ISO-IDF) using three different commercial chromogenic agars. The ISO-IDF method includes two enrichment steps, plating of the second enrichment broth on E. sakazakii isolation agar (a chromogenic selective agar), picking of five typical colonies for transfer onto tryptone soy agar, and subsequent confirmation of yellow-pigmented colonies by biochemical characterization. Twenty-two of the 38 samples were positive by the culture method. E. sakazakii isolation agar (ESIA; AES Laboratoires), COMPASS agar (Biokar Diagnostics), and Druggan-Forsythe-Iversen agar (Oxoid) compared favorably with violet red bile glucose agar (VRBG, a selective medium for Enterobacteriaceae), with positive predictive values of 86.96, 88, and 74.07%, respectively, in contrast to 47.83% for VRBG. One additional positive sample was detected using the nonpatented real-time PCR method evaluated, and those results were in 97.3% concordance with the ISO-IDF results. Some discrepancies between the results of the DuPont Qualicon BAX system and those of the ISO-IDF method could be explained by heterogeneity of contamination and sampling. Thus, both PCR-based systems were suitable for detecting and specifically identifying E. sakazakii within 1 to 2 days, and COMPASS agar and ESIA could be used interchangeably as a first-step medium to isolate presumptive E. sakazakii colonies.

Survival and growth of Enterobacter sakazakii in infant cereal as affected by composition, reconstitution liquid, and storage temperature

Invasive infections caused by Enterobacter sakazakii have occurred predominantly in low-birth-weight neonates and infants younger than 2 months of age. However, infections have also occurred in healthy infants up to 8 months of age and in immunocompromised children up to 4 years of age. The ability of E. sakazakii to survive and grow in infant cereals as affected by composition of the cereal, composition of the reconstitution liquid, and temperature is unknown. A study was done to determine the survival and growth characteristics of E. sakazakii initially at populations of 0.005 and 0.52 CFU/ml of infant rice cereal, oatmeal cereal, or rice with mixed fruit cereal reconstituted with water, milk, or apple juice. Reconstituted cereals were stored at 4, 12, 21, and 30 degrees C, and populations were monitored for up to 72 h. Growth did not occur in reconstituted cereals stored at 4 degrees C or in cereals reconstituted with apple juice and stored at 12 degrees C. Populations (> or =1 CFU/ml) were detected in cereals reconstituted with water or milk and stored at 12, 21, and 30 degres C for 24, 8, and 4 h, respectively. The composition of infant cereals did not markedly affect the survival or growth of E. sakazakii in reconstituted cereals. Populations of E. sakazakii in reconstituted cereal decreased with increases in populations of mesophilic aerobic microflora up to 8 to 9 log CFU/ml, which was concurrent with decreases in pH. E. sakazakii, initially at 2.62 log CFU/ml of rice cereal reconstituted with apple juice (pH 4.32), survived at 40C for at least 14 days. The pathogen grew at 21 and 30 degrees C within 2 days and then decreased to undetectable levels (<1 CFU/10 ml) in cereal stored at 21 degrees C for 5 days or 30'C for 4 days. Initially, at 7.32 log CFU/ml, E. sakazakii was detected in rice cereal stored at 4 degrees C for 50 days. It is recommended that reconstituted infant cereals stored at 21 or 30 degrees C be discarded within 4 h after preparation or stored at -40C, temperatures at which E. sakazakii will not grow.

Characterization of the zinc-containing metalloprotease encoded by zpx and development of a species-specific detection method for Enterobacter sakazakii

Enterobacter sakazakii causes a severe form of neonatal meningitis that occurs as sporadic cases as well as outbreaks. The disease has been epidemiologically associated with consumption of reconstituted, dried infant formulas. Very little information is available regarding pathogenicity of the organism and production of virulence factors. Clinical and environmental strains were screened for production of factors which have activity against Chinese hamster ovary (CHO) cells in tissue culture. Polymyxin B lysate and sonicate preparations but not culture supernatants from the strains caused "rounding" of CHO cells. Subsequent studies showed that the CHO cell-rounding factor is a proteolytic enzyme that has activity against azocasein. The cell-bound protease was isolated by using a combination of polymyxin B lysis, followed by sonication of cells harvested from tryptone broth. The protease was purified to homogeneity by sequential ammonium sulfate precipitation, gel filtration chromatography with Sephadex G-100, hydrophobic interaction chromatography with phenyl-Sepharose CL-4B, and a second gel filtration with Sephadex G-100. In addition to activity against azocasein, the purified protease also exhibits activity against azocoll and insoluble casein but not elastin. The protease has a molecular weight of 38,000 and an isoelectric point of 4.4. It is heat labile and for maximal activity against azocasein has an optimum temperature of 37 degrees C and a pH range of 5 to 7. Proteolytic activity is inhibited by ortho-phenanthroline and Zincov but is not affected by phenylmethylsulfonyl fluoride, N-ethylmaleimide, and trypsin inhibitors, which demonstrates that the protease is a zinc-containing metalloprotease. The metalloprotease does not hemagglutinate chicken or sheep erythrocytes. Twenty-three to 27 of the first 42 N-terminal amino acid residues of the metalloprotease are identical to proteases produced by Serratia proteamaculans, Pectobacterium carotovorum, and Anabaena sp. PCR analysis using primers designed from a consensus nucleotide sequence showed that 135 E. sakazakii strains possessed the metalloprotease gene, zpx, and 25 non-E. sakazakii strains did not. The cloned zpx gene of strain 29544 consists of 1,026 nucleotides, and the deduced amino acid sequence of the metalloprotease has 341 amino acid residues, which corresponds to a theoretical protein size of 37,782 with a theoretical pI of 5.23. The sequence possesses three well-characterized zinc-binding and active-site motifs present in other bacterial zinc metalloproteases.

Application of pulsed-field gel electrophoresis to characterise and trace the prevalence of Enterobacter sakazakii in an infant formula processing facility

Enterobacter sakazakii (E. sakazakii) contamination of powdered infant formula (PIF) and its processing environment was monitored between April 2005 and March 2006. The purpose of the monitoring programme was to locate points of contamination, investigate clonal persistence, and identify possible dissemination routes along the processing chain. A total of 80 E. sakazakii isolates were recovered from the manufacturing facility. The overall frequency of isolation of E. sakazakii in intermediate and final product was 2.5%, while specific locations in the processing environment were contaminated at frequencies up to 31%. All E. sakazakii isolates were characterised by pulsed-field gel electrophoresis (PFGE). XbaI macrorestriction digests yielded 19 unique pulse-types that could be grouped into 6 clusters of between 5 and 32 isolates. The formation of large clusters was consistent with the presence of a number of clones in the manufacturing environment. While the majority of isolates were of environmental origin (72.5%), no cluster was confined to one specific location and indistinguishable PFGE profiles were generated from isolates cultured from the manufacturing environment, sampling points along the processing chain and from intermediate and final product. These findings suggest that the manufacturing environment serves as a key route for sporadic contamination of PIF. These data will support the development of efficient intervention measures contributing to the reduction of E. sakazakii in the PIF processing chain.

Effectiveness of disinfectants in killing Enterobacter sakazakii in suspension, dried on the surface of stainless steel, and in a biofilm

The effectiveness of 13 disinfectants used in hospitals, day-care centers, and food service kitchens in killing Enterobacter sakazakii in suspension, dried on the surface of stainless steel, and in biofilm was determined. E. sakazakii exhibited various levels of resistance to the disinfectants, depending on the composition of the disinfectants, amount and type of organic matrix surrounding cells, and exposure time. Populations of planktonic cells suspended in water (7.22 to 7.40 log CFU/ml) decreased to undetectable levels (<0.30 log CFU/ml) within 1 to 5 min upon treatment with disinfectants, while numbers of cells in reconstituted infant formula were reduced by only 0.02 to 3.69 log CFU/ml after the treatment for 10 min. The presence of infant formula also enhanced the resistance to the disinfectants of cells dried on the surface of stainless steel. The resistance of cells to disinfectants in 6-day-old and 12-day-old biofilms on the surface of stainless steel was not significantly different. The overall order of efficacy of disinfectants in killing E. sakazakii was planktonic cells > cells inoculated and dried on stainless steel > cells in biofilms on stainless steel. Findings show that disinfectants routinely used in hospital, day-care, and food service kitchen settings are ineffective in killing some cells of E. sakazakii embedded in organic matrices.

Enterobacter sakazakii in food and beverages (other than infant formula and milk powder)

The ubiqitous microorganism Enterobacter sakazakii is a rare contaminant of infant formula and may cause severe systemic infection in neonates. So far, other food is not known to cause E. sakazakii-infections. The scarce information about the ecology of E. sakazakii and the uncertainty concerning the source of infection in children and adults warrant a summary of the current knowledge about the presence of this opportunistic microorganism in food other than infant formula. This review systematizes publications on the presence of E. sakazakii in food and beverages until June 2006. Food other than infant formula has been rarely investigated for the presence of E. sakazakii. Nevertheless, this microorganism could be isolated from a wide spectrum of food and food ingredients. E. sakazakii was isolated from plant food and food ingredients like cereal, fruit and vegetables, legume products, herbs and spices as well as from animal food sources like milk, meat and fish and products made from these foods. The spectrum of E. sakazakii-contaminated food covers both raw and processed food. The kind of processing of E. sakazakii-contaminated food was not restricted to dry products. Fresh, frozen, ready-to-eat, fermented and cooked food products as well as beverages and water suitable for the preparation of food, were found to be contaminated by E. sakazakii. Although E. sakazakii-contaminated food do not have general public health significance, measures for prevention should consider the presence of E. sakazakii in food, food ingredients, their processing and preparation as possible source of contamination, colonization or infection.

Comparison of media for the isolation of Enterobacter sakazakii

Enterobacter sakazakii is associated with neonatal infections and is occasionally present at low levels (<1 CFU/g) in powdered infant formula milk (IFM). It has been previously reported that some E. sakazakii strains do not grow in standard media for Enterobacteriaceae and coliform bacteria; therefore, a reliable method is needed for recovery of the organism. Three E. sakazakii enrichment broths-Enterobacteriaceae enrichment broth (EE), E. sakazakii selective broth (ESSB), and modified lauryl sulfate broth (mLST)-were compared with a novel broth designed for maximum recovery of E. sakazakii, E. sakazakii enrichment broth (ESE). One hundred seventy-seven strains (100%) grew in ESE, whereas between 2 and 6% of strains did not grow in EE, mLST, or ESSB. E. sakazakii possesses alpha-glucosidase activity, and a number of selective, chromogenic agars for E. sakazakii isolation based on this enzyme have been developed. E. sakazakii isolation agar produced fewer false-positive colonies than did Druggan-Forsythe-Iversen agar. However, the latter supported the growth of more E. sakazakii strains. It was also determined that 2% of E. sakazakii strains did not produce yellow pigmentation on tryptone soya agar at 25 degrees C, a characteristic frequently cited in the identification of E. sakazakii. The recovery of desiccated E. sakazakii (0.2 to 2000 CFU/25 g) from powdered IFM in the presence of a competing flora was determined with various enrichment broths and differential selective media. Current media designed for the isolation and presumptive identification of E. sakazakii do not support the growth of all currently known E. sakazakii phenotypes; therefore, improvements in the proposed methods are desirable.

Enterobacter sakazakii and other bacteria in powdered infant milk formula

Recently there has been considerable concern related to the presence of bacteria, in particular Enterobacter sakazakii, in powdered infant formula milk. This paper considers the bacteria in these products at point of sale, with reference to current microbiological testing and the need for good hygienic practice in their subsequent preparation before feeding. The ingestion of raised numbers of E. sakazakii resulting from temperature abuse after reconstitution is highlighted as well as the uncertain routes of E. sakazakii product contamination.

Attachment of and biofilm formation by Enterobacter sakazakii on stainless steel and enteral feeding tubes

Enterobacter sakazakii has been reported to form biofilms, but environmental conditions affecting attachment to and biofilm formation on abiotic surfaces have not been described. We did a study to determine the effects of temperature and nutrient availability on attachment and biofilm formation by E. sakazakii on stainless steel and enteral feeding tubes. Five strains grown to stationary phase in tryptic soy broth (TSB), infant formula broth (IFB), or lettuce juice broth (LJB) at 12 and 25 degrees C were examined for the extent to which they attach to these materials. Higher populations attached at 25 degrees C than at 12 degrees C. Stainless steel coupons and enteral feeding tubes were immersed for 24 h at 4 degrees C in phosphate-buffered saline suspensions (7 log CFU/ml) to facilitate the attachment of 5.33 to 5.51 and 5.03 to 5.12 log CFU/cm(2), respectively, before they were immersed in TSB, IFB, or LJB, followed by incubation at 12 or 25 degrees C for up to 10 days. Biofilms were not produced at 12 degrees C. The number of cells of test strains increased by 1.42 to 1.67 log CFU/cm(2) and 1.16 to 1.31 log CFU/cm(2) in biofilms formed on stainless steel and feeding tubes, respectively, immersed in IFB at 25 degrees C; biofilms were not formed on TSB and LJB at 25 degrees C, indicating that nutrient availability plays a major role in processes leading to biofilm formation on the surfaces of these inert materials. These observations emphasize the importance of temperature control in reconstituted infant formula preparation and storage areas in preventing attachment and biofilm formation by E. sakazakii.