Part of the foodborne pathogens pillar of the TAAG Learning Center. If you are interested in the other pathogen that accompanies Cronobacter in powders, read Salmonella in low-moisture foods.
Table of contents
A pathogen of low incidence and serious consequences
Cronobacter is a genus of Gram-negative bacteria of the Enterobacteriaceae family that lives in the environment and can reach dry foods and preparation equipment. The CDC has found it in powdered infant formula, milk powder, herbal teas and starches. In most people it goes unnoticed, but in newborns it can cause meningitis and sepsis, and that is why food safety standards treat it differently from other bacteria of the same group.
The Codex code of hygienic practice for powdered formulae says it bluntly: the incidence of these infections in infants appears to be low, but the consequences can be severe. This article goes through its risk, its biology, the criteria that regulate it and the ways of detecting it, and connects it with what we already saw in Salmonella in low-moisture foods, because both pathogens share niches in powder plants.
Taxonomy: from Enterobacter sakazakii to a genus with seven species
Until 2007 the microorganism was known as Enterobacter sakazakii. A study by Iversen and colleagues in BMC Evolutionary Biology, based on phenotypic and molecular methods, reclassified it into a new genus, Cronobacter. A 2023 review recalls that what was previously described as yellow-pigmented Enterobacter cloacae was in fact another genus, and that seven species are now recognized: C. sakazakii, C. malonaticus, C. turicensis, C. muytjensii, C. dublinensis, C. universalis and C. condimenti.
This distinction matters for two reasons. First, not all species have the same clinical weight: in the Iversen study, the neonatal meningitis isolates belonged to C. sakazakii. Second, the older literature and many official documents, including the 2008 Codex code itself, still use the name Enterobacter sakazakii or the transitional formula "E. sakazakii (Cronobacter species)". When you read an older criterion or method, it refers to the same organism.
Why it is a risk for infants
Codex gathers the assessment of two FAO and WHO expert meetings, which ranked the microorganisms in powdered formulae according to the evidence that they cause illness in infants. In category A, with clear causality, were Salmonella enterica and Enterobacter sakazakii. EFSA reached a similar conclusion in 2004: Salmonella and E. sakazakii are the microorganisms of greatest concern in infant formulae.
- Population at risk. All infants under 12 months; among them, neonates (less than 28 days), premature infants, low birth weight infants (less than 2,500 g), immunocompromised infants and those under 2 months.
- Main conditions. Meningitis, which tends to appear in the neonatal period, and bacteremia, more frequent in premature infants outside that period. The CDC adds fever, poor feeding, excessive crying, lethargy and, in some cases, seizures.
- Severity. Codex cites highly variable case fatality rates, up to 50% in at least one outbreak, and permanent sequelae such as developmental delay and neurological conditions in some of the survivors. According to the CDC, about 20% of U.S. infants with meningitis or bloodstream infection from Cronobacter died.
- Frequency. Between January 2002 and July 2022, the CDC received 76 reports of severe Cronobacter illness in infants, in 32 states.
Healthy adults are much less susceptible, although immunocompromised adults and older people can become ill. A 2023 review points to the case of people with difficulty swallowing who consume rehydrated powdered protein supplements. Even so, Codex notes that all known outbreaks have affected infants, and that only powdered formula has been linked to outbreaks among the foods in which the bacterium has been detected.
Resistance to desiccation: it does not need to grow
A powdered infant formula has a water activity of between 0.2 and 0.5, and under those conditions there is not enough water for the bacterium to multiply. As with Salmonella, the key to Cronobacter is not multiplying in the powder but surviving in it. A 2023 review indicates that C. sakazakii tolerates osmotic stress and desiccation better than other Enterobacteriaceae.
Fei and colleagues measured the survival of isolates from formula and process environments at 21 °C and 20.7% relative humidity for one year. The mean reduction was 3.02 log units, most of it in the first 60 days (about 1.55 log), and the most resistant strain lost only 2.43 log. That is, the population declines, but does not disappear, and the difference between strains is large. The authors also found that C. sakazakii resisted osmotic stress and desiccation better than C. malonaticus, which could explain why the former predominates in formula environments.
Reconstituted, the story changes
The problem worsens when the powder is rehydrated. Codex warns that improper handling and storage of reconstituted formula can favor the growth of Cronobacter and Salmonella initially present at low levels. That is why preparation guidelines recommend reconstituting with water above 70 °C (EFSA, Codex), preparing each feed at the moment and discarding what is left over. The CDC also recommends considering ready-to-feed liquid formulas for higher-risk infants, because they are manufactured sterile. A powder is not sterile, and no product criterion changes that.
How it arrives and stays in a powder plant
Codex identifies four routes by which Cronobacter and Salmonella can enter a powdered formula: ingredients added in dry blends, contamination from the process environment during or after drying, contamination after opening the container and that which occurs during or after reconstitution by the caregiver. The first two depend on the plant, and are the focus of control.
- It cannot be completely eliminated. Codex acknowledges that current technology does not allow the organism to be eliminated from the manufacturing environment and that low levels of Enterobacteriaceae, including Cronobacter, may be present even with good hygiene.
- Drying is not a lethal step. A 2023 review recalls that the heat treatments in dairy plants, such as spray drying, are not designed to eliminate bacteria and may even induce an adaptive response. C. sakazakii was isolated from roller dryers, drying towers and tanker discharge areas, and from floors and adjacent soil, where it persisted for long periods.
- More frequent than Salmonella in the dry zone. Codex notes that Cronobacter is found more frequently than Salmonella in dry areas, and regularly when appropriate sampling and testing methods are used. This connects with what was seen in Salmonella in low moisture: both pathogens exploit the same niches.
- Water is the risk factor. According to the code, the increase of Cronobacter in high hygiene areas is favored by water, even in minimal quantities such as condensation, and wet cleaning has been linked to its appearance and spread. Construction work or changes in equipment layout can release organisms from previously hidden niches.
The Codex recommendation is to keep high hygiene zones as dry as possible, use dry cleaning as routine practice, restrict wet cleaning to equipment that can be taken to a dedicated room or dried immediately, and design equipment without cracks, rough welds or hollow tubes where product accumulates. Codex also asks for environmental monitoring programs with Enterobacteriaceae as indicators of process hygiene and testing for Salmonella and Cronobacter in relevant samples. Its own Annex III specifies that there is no universal correlation between Enterobacteriaceae and Cronobacter counts, although it has been shown in individual plants that reducing the former reduces the latter. More on how that program is designed in environmental monitoring in food plants.
Microbiological criteria: Codex and Regulation (EC) 2073/2005
Product criterion
Annex I of Codex (CAC/RCP 66-2008) sets for Cronobacter in powdered infant formula a two-class plan with n = 30, c = 0 and m = absence in 10 g, applied to the finished product after primary packaging. For Salmonella the plan is n = 60, c = 0, absence in 25 g. Regulation (EC) 2073/2005 agrees for Cronobacter: in powdered infant formulae and powdered dietary foods for special medical purposes intended for infants under 6 months, 30 sample units, none with the bacterium detected in 10 g, with EN ISO 22964 as the reference method, throughout the shelf life of the product.
Codex itself explains how much a plan like this tells. With n = 30, the mean concentration that would be detected with 95% probability (with an assumed standard deviation of 0.8) is 1 CFU in 340 g. That is, the plan does not see even lower contaminations, hence Codex insists that the limitations of finished product testing also require an environmental monitoring program. To understand what n, c, m and M mean in a plan, review attribute sampling plans.
Process hygiene criterion
The Regulation also defines a process hygiene criterion for Enterobacteriaceae at the end of manufacturing: in powdered infant formulae, 10 sample units and absence in 10 g (EN ISO 21528-1), and in follow-on formulae, 5 units. It provides for running Enterobacteriaceae and Cronobacter analyses in parallel, unless the manufacturer demonstrates a correlation between the two in its plant; if Enterobacteriaceae are detected in any of the samples, the batch must be tested for Cronobacter. It is the logic of hygiene indicators: an indicator alerts, but does not replace the search for the pathogen.
The reference method: ISO 22964
The horizontal method for the detection of Cronobacter is the standard ISO 22964, whose 2017 version replaced the ISO/TS 22964:2006 cited by Codex. It relies on culture: a 2023 review notes that it uses a non-selective enrichment, and the confirmation phase is biochemical. A 2026 systematic review estimates that the full procedure takes between 5 and 7 days and is labor-intensive.
The 2017 version was validated under European Commission Mandate M381. The interlaboratory study involved 17 laboratories from nine countries, with five matrices: two powdered infant formulae (with and without probiotics), lactose, starch and environmental samples taken with swabs. Specificity was between 99 and 100%, and sensitivity between 65 and 100% depending on the matrix, the inoculation level and the accompanying microbiota. The LOD50 of the three food matrices (the two formulae and the starch) was 0.8 to 1.1 CFU per sample. That study matters for two reasons: it includes ingredients, not just finished product, and it includes surfaces, which supports its use in environmental monitoring.
Culture or PCR: what each contributes
Reviews compile a wide variety of alternatives to classic culture: chromogenic media that give blue-green colonies through the degradation of a glucosidic substrate in about 24 hours, immunological methods, biosensors and PCR. For real-time PCR, the 2023 review gathers assays targeting genes such as ompA or the MMS operon (rpsU, dnaG, rpoD), with times of about 24 hours and limits of detection of 10 to 100 CFU/mL in the studies it cites. Those figures come from different research assays and are not comparable with the LOD50 of ISO 22964 or with a method validated in your matrix.
A practical distinction helps to decide:
- To release a batch against the regulatory criterion, the reference method is ISO 22964. Codex admits other validated methods that offer equivalent sensitivity and reliability; how that equivalence is demonstrated is explained by the ISO 16140 series. The formal decision is not made with an unvalidated method.
- To screen many environmental samples, a validated molecular method shortens the response and allows action before product accumulates. It must be preceded by enrichment, as explained in microbiological enrichment, because cells stressed by desiccation need to recover.
- A presumptive positive calls for confirmation with an isolate, either by the reference procedure or by another confirmatory method. That stage is described in the procedure of the standard itself.
Not only infant formula
The regulatory criterion covers formulae for infants, but the organism is not limited to them. The CDC has detected it in milk powder, herbal teas and starches. In the ISO 22964 validation study, lactose and starch appear as matrices of interest, common ingredients of dry blends. Codex observes that, although the microorganism has been detected in other foods and environments, only powdered formula has been linked to outbreaks. Even so, powdered ingredients intended for more vulnerable people deserve the same attention in the design of the environmental program: the hygiene of the dry zone and the absence of water weigh more than the name of the product.
What this asks of a laboratory and a plant
Four questions organize the work of those who produce or analyze powders:
- Does the environmental program measure Enterobacteriaceae as an indicator and also look for Cronobacter and Salmonella in high hygiene zones, with priority on known niches?
- Is dry cleaning the rule and is there a written procedure, with drying and verification, for when water is used?
- Are the methods used to decide validated in the real matrix, including surfaces?
- Is there a defined action plan for a finding: investigate the root cause, hold batches and review trends?
On TAAG kits, it is worth being precise: the published catalog does not currently include a specific kit for Cronobacter. For surface hygiene verification, Elevia 2.9 Salmonella spp. and Enterobacteria detects Salmonella and Enterobacteriaceae with AiGOR™ RT-qPCR technology, with post-sanitized stainless steel as the validated matrix and the dairy industry among the industries indicated. It provides process hygiene information, but does not replace Cronobacter analysis by the reference method or by a method validated for this pathogen.
Conclusion
Cronobacter is a rare but very serious pathogen for neonates and premature infants, and that is why regulations set a strict criterion for powdered infant formulae: absence in 10 g in 30 units, with ISO 22964 as the reference method. It resists desiccation, persists in dry zone niches and is favored by water, even in minimal quantities. That is why control depends as much on the environmental program, dry cleaning and the use of validated methods as on the analysis of the finished product, which Codex itself acknowledges is limited on its own.
About TAAG
Consult TAAG on how to use Elevia 2.9 Salmonella spp. and Enterobacteria, based on AiGOR™ RT-qPCR, to verify the hygiene of post-sanitized stainless steel surfaces in your dairy or powder plant.
Frequently asked questions
Are Cronobacter and Enterobacter sakazakii the same microorganism?
Yes. Until 2007 the bacterium was called Enterobacter sakazakii. A study by Iversen and colleagues reclassified it into the genus Cronobacter, which now includes seven species. Older documents, including the 2008 Codex hygiene code, still use the former name or the formula E. sakazakii (Cronobacter species).
Why is Cronobacter a special risk for infants?
Because in neonates, premature infants and those under 2 months it can cause meningitis and bacteremia. Codex cites case fatality rates of up to 50% in at least one outbreak and permanent neurological sequelae in some of the survivors. Although the incidence is low, the CDC reported 76 severe cases in infants between 2002 and July 2022 in the U.S.
What does Regulation 2073/2005 require for Cronobacter?
For powdered infant formulae and powdered dietary foods for special medical purposes intended for infants under 6 months, 30 sample units with absence of Cronobacter in 10 g, reference method EN ISO 22964, throughout the shelf life. In addition, a process hygiene criterion for Enterobacteriaceae (absence in 10 g) at the end of manufacturing.
Is it enough to test for Enterobacteriaceae to control Cronobacter?
Not entirely. Codex indicates that there is no universal correlation between Enterobacteriaceae and Cronobacter counts, although in individual plants it can be demonstrated. That is why Regulation 2073/2005 provides for analyzing both in parallel, unless the manufacturer demonstrates that correlation in its plant.
Can Cronobacter be detected by PCR?
Real-time PCR assays are described in the literature, with times of about 24 hours. But the regulatory decision on a batch is made with ISO 22964 or with an alternative method validated against it. A molecular method must be validated in the specific matrix being analyzed.