Enterobacter sakazakii is a gram-negative rod-shaped bacterium within the family Enterobacteriaceae. The organism was called "yellow-pigmented Enterobacter cloacae" until 1980 when it was renamed Enterobacter sakazakii. The majority of cases of infection reported in the peer-reviewed literature have described neonates with sepsis, meningitis, or necrotizing enterocolitis as a consequence of the infection. (1)

E. sakazakii is a rare cause of bloodstream and central nervous system infections. The organism has also been associated with necrotizing enterocolitis; however, it has not been firmly established as a causative agent. Reported outcomes are often severe: seizures; brain abscess; hydrocephalus; developmental delay; and death in as many as 40%–80% of cases. Premature infants are thought to be at greater risk than more mature infants, other children, or adults, and outbreaks of disease have occurred in hospital units for newborns. (2)

E. sakazakii was first implicated in a case of neonatal meningitis in 1958, and since then there have been around 70 reported cases of E. sakazakii infection. However, it is likely that E. sakazakii is significantly under-reported in all countries. Although E. sakazakii can cause illness in all age groups, infants are believed to be at greatest risk of infection. (3)

Experts from the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) met in 2004 to summarize information, and develop international guidelines and educational messages. The meeting confirmed that there is very little known about virulence factors and pathogenicity of E. sakazakii. The work done by Pagotto et al. (2003) was the first describing putative virulence factors for E. sakazakii. Enterotoxin-like compounds were produced by some strains. Using tissue cultures, some strains produced a cytotoxic effect. Two strains (out of 18 isolates) were capable of causing death in suckling mice by the peroral route. Therefore, there appear to be differences in virulence among E. sakazakii strains, and some strains may be non-pathogenic. (4)

Mortality rates from E. sakazakii infection have been reported to be as high as 50 percent or more, but this figure has declined to under 20 percent in recent years. Significant morbidity in the form of neurological deficits can result from infection, especially among those with bacterial meningitis and cerebritis. While the disease is usually responsive to antibiotic therapy, a number of authors have reported increasing antibiotic resistance to drugs commonly used for initial treatment of suspected Enterobacter infection. Long-term neurologic sequelae are well recognized. (4)

Enterobacter sakazakii kills 40%–80% of infected infants. In 2007, the CDC analyzed 46 cases of invasive infant E. sakazakii infection to define risk factors and guide prevention and treatment. Twelve infants had bacteremia, 33 had meningitis, and 1 had a urinary tract infection. Compared with infants with isolated bacteremia, infants with meningitis had greater birthweight (2,454 g vs. 850 g, p = 0.002) and gestational age (37 weeks vs. 27.8 weeks, p = 0.02), and infection developed at a younger age (6 days vs. 35 days, p<0.001). Among meningitis patients, 11 (33%) had seizures, 7 (21%) had brain abscess, and 14 (42%) died. (2)

Although E. sakazakii can cause illness in all age groups, infants (children <1 year) are at most risk, with neonates and infants under two months at greatest risk. The groups of infants at greatest risk includes in particular pre-term infants, low-birth-weight (<2.5 kg) infants or immunocompromised infants. However, infants who are compromised for any other reason may also be at greater risk of E. sakazakii infection. Infants of HIVpositive mothers are also at risk because they may be immunocompromised, and may specifically require powdered infant formula (PIF). (3)

There appear to be two distinct infant risk groups for E. sakazakii infection: prematureinfants who develop bacteraemia after one month of age, and term infants who develop meningitis during the neonatal period. Therefore, an FAO/WHO expert working group in 2006 concluded that while infants appear to be the group at particular risk, neonates and those less than two months of age are at greatest risk. (3)

In the United States of America, an incidence rate of 1 per 100 000 infants for E. sakazakii infection has been reported. This incidence rate increases to 9.4 per 100 000 in infants of very low birth weight, i.e. <1.5 kg. (3)

While the reservoir for E. sakazakii is unknown in many cases, a growing number of reports have established powdered infant formula as the source and vehicle of infection. In several investigations of outbreaks of E. sakazakii infection that occurred among neonates in neonatal intensive care units, investigators were able to show both statistical and microbiological association between infection and powdered infant formula consumption. These investigations included cohort studies which implicated infant formula consumed by the infected infants. In addition, there was no evidence of infant-to-infant or environmental transmission; all cases had consumed the implicated formula. The stomach of newborns, especially of premature babies, is less acidic than that of adults: a possible important factor contributing to the survival of an infection with E. sakazakii in infants. (4)

Limited information was available on the numbers of E. sakazakii organisms that ill patients were exposed to in any of the various outbreaks. It is therefore not possible to develop a dose-response curve for E. sakazakii. However, it is possible that a small number of cells present in PIF could cause illness. This risk increases rapidly when bacteria in the reconstituted formula are allowed to multiply, such as by holding at inappropriate temperatures for an extended period. (3)

The frequency of intrinsic E. sakazakii contamination in powdered infant formula is of concern, even though intrinsic concentration levels of E. sakazakii appear to be typically very low. In a study of the prevalence of E. sakazakii contamination in 141 powdered infant formulas, 20 were found culture-positive, yet all met the microbiological specifications for coliform counts in powdered infant formula (<3 cfu/g) of the current Codex code. Such formula has been linked to outbreaks. (4)

Furthermore, outbreaks have occurred in which the investigators have failed to identify lapses in formula preparation procedures. Thus, it seems that neither high levels of contamination nor lapses in preparation hygiene are necessary to cause infection from E. sakazakii in powdered infant formula. While it can be assumed that lapses in preparation hygiene or extended holding at non-refrigerated temperatures could lead to increases in the levels of contamination at the time of consumption, it is not possible to assess the contribution that these factors have on the cases of infection that have been associated with powdered infant formula that contained low levels of E. sakazakii. Thus it must be currently assumed that low levels of E. sakazakii in infant formula (<3 cfu/100 g) can lead to infections. (4)

In the April 12th 2002 issue of Morbidity and Mortality Weekly Report, the Centers for Disease Control and Prevention (CDC) reported on a fatal case of meningitis in an intensive care nursery in Tennessee. The infecting organism was E. sakazakii, an unusual but often fatal, invasive pathogen. In the fatal Tennessee case, the infection was traced to contaminated powdered infant formula. Other infants in the same nursery were screened for E sakazakii. Of 49 screened infants, 10 events were discovered (1 proven infection, 2 assumed infections, and 7 colonizations). This report detailed for the first time a direct link to an unopened product. The manufacturer voluntarily recalled the contaminated batch of powdered formula identified as the source. (5)

In 2004, PIF was microbiologically linked to two E. sakazakii outbreaks, in New Zealand and in France. The French outbreak involved nine cases, and resulted in the death of two infants. While eight of the cases were in premature infants of low birth weight (<2 kg), one case was in an infant born at 37 weeks and weighing 3.25 kg. The outbreak involved five hospitals, and a review of practices in the hospitals revealed that one hospital was not following recommended procedures for the preparation, handling and storage of feeding bottles, and four were storing reconstituted formula for >24 hours in domestic-type refrigerators, with no temperature control or traceability. (3)

The FDA points out that powdered infant formulas are not commercially sterile products. Powdered milk-based infant formulas are heat-treated during processing, but unlike liquid formula products they are not subjected to high temperatures for sufficient time to make the final packaged product commercially sterile. FDA has noted that infant formulas nutritionally designed for consumption by premature or low birth weight infants are available only in commercially sterile liquid form. However, so-called "transition" infant formulas that are generally used for premature or low birth weight infants after hospital discharge are available in both non-commercially sterile powder form and commercially sterile liquid form. Some other specialty infant formulas are only available in powder form. (1)

The FDA has become increasingly aware that a substantial percentage of premature neonates in neonatal intensive care units are being fed non-commercially sterile dry infant formula. In light of the epidemiological findings and the fact that powdered infant formulas are not commercially sterile products, FDA recommends that powdered infant formulas not be used in neonatal intensive care settings unless there is no alternative available. If the only option available to address the nutritional needs of a particular infant is a powdered formula, risks of infection can be reduced by:

• Preparing only a small amount of reconstituted formula for each feeding to reduce the quantity and time that formula is held at room temperature for consumption; Recognizing differences in infant formula preparation among hospitals, individual facilities should identify and follow procedures appropriate for that institution to minimize microbial growth in infant formulas;

• Minimizing the holding time, whether at room temperature or while under refrigeration, before a reconstituted formula is fed; and

• Minimizing the "hang-time" (i.e., the amount of time a formula is at room temperature in the feeding bag and accompanying lines during enteral tube feeding), with no "hang-time" exceeding 4 hours. Longer times should be avoided because of the potential for significant microbial growth in reconstituted infant formula. (1)

WHO recommends that infants should be exclusively breastfed for the first six months of life to achieve optimal growth, development and health. Thereafter, to meet their evolving nutritional requirements, infants should receive nutritionally adequate and safe complementary foods while breastfeeding continues for up to two years of age or beyond (WHO/UNICEF, 2003). It is important to support breastfeeding and promote its benefits to infants and young children. (3)

There are, however, instances where breast milk is not available, where the mother is unable to breastfeed, where they have made an informed decision not to breastfeed, or where breastfeeding is not appropriate, e.g. where the mother is taking medication that is contraindicated for breastfeeding or the mother is HIV-positive. Similarly, some very low-birth-weight babies may not be able to breastfeed directly, and in some cases expressed breast milk may not be available at all or available in insufficient quantities. Infants who are not breastfed require a suitable breast-milk substitute, for example, an infant formula prepared in accordance with the present guidelines. WHO Guidelines for infant formula preparation, storage, and handling (2007), in both care settings and at home, are specified at http://www.who.int/foodsafety/publications/micro/pif (3)

In January 2006, a second meeting (after the initial one in 2004) of experts from the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) took place to summarize information, and develop international guidelines and educational messages. The meeting participants first re-endorsed the recommendations made by the 2004 FAO/WHO meeting on this issue. The additional recommendations made by the expert meeting to member countries included the following:

• Develop prevention strategies for E. sakazakii infections caused by contaminated PIF that address the different stages of production and preparation and use of PIF, taking into consideration the risk to infants ?both within and beyond the neonatal period and of any immune status.

• Develop educational messages on the safe handling, storage and use of powdered infant formula, including the health hazards of inappropriate preparation and use; target healthcare workers, parents and other caregivers, in both hospitals and the community, since E. sakazakii infections have occurred in hospital and home settings.

• Review and revise product labels, as appropriate, to enable caregivers to handle, store and use the product safely, and to make clear the health hazards of inappropriate preparation.

• Encourage member countries to establish surveillance and rapid response networks, and facilitate coordinated investigation by clinicians, laboratorians, and public health and regulatory officials, to enable the timely recognition and cessation of outbreaks of illness associated with E. sakazakii and the identification of contaminated PIF. (6)

(1) “Health Professionals Letter on Enterobacter sakazakii Infections Associated With use of Powdered (Dry) Infant Formulas in Neonatal Intensive Care Units”, U. S. Department of Health and Human Services, U. S. Food and Drug Administration, April 11, 2002; Revised October 10, 2002.
(2)“Invasive Enterobacter sakazakii Disease in Infants”, Emerging Infectious Diseases, Volume 12, Number 8–August 2006.
(3) “Safe Preparation, Storage and Handling of Powdered Infant Formula Guidelines” (2007), World Health Organization, in collaboration with Food and Agriculture Organization of the United Nations.
(4) “Enterobacter sakazakii and other microorganisms in powdered infant formula” Microbiological Risk Assessment Series 6, World Health Organization (2004).
(5) “Enterobacter sakazakii Infections Associated With the Use of Powdered Infant Formula—Tennessee, 2001”, JAMA. 2002; 287:2204-2205, Vol. 287 No. 17, May 1, 2002.
(6) “Enterobacter sakazakii and Salmonella in powdered infant formula: Meeting report, MRA Series 10”, Microbiological Risk Assessment Series 10, World Health Organization (2006).