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The Bug Formerly Known as Enterobacter – the one that can live in powdered infant formula

Enterobacter sakazakii was recently reclassified into eight distinct taxa of a new genus – Cronobacter. All have been linked retrospectively to clinical cases in adults and infants. To avoid confusion the organism will be referred to here as Cronobacter spp. (E. sakazakii). This confusion is caused by microbiologists renaming bugs solely to irritate clinicians. Just as they continue to do with classifying strep every ten minutes. Anyway, the bacterium is Gram-negative, motile, non-spore-forming rod and will grow in aerobic and anaerobic conditions. It is considered an opportunistic pathogen. Enterotoxin-like compounds are produced by some strains. Cronobacter can survive for long periods as a contaminant in powdered infant formulae. For this reason they have been identified as high-risk foods – the risks of Cronobacter spp. (E. sakazakii) in this regard are summarised below.

Growth and Control Survival Infant Formulae


Range 5.5-45°C. Optimum 39.4°C. Generation time 5 h at 10°C, 40 min at 23°C, 20 min at optimum. It has been shown to grow in breast milk and breast milk with fortifiers (calorie and/or nutrient supplements) at 23°C and 37°C. The addition of fortifiers slowed growth at both temperatures, the effect was especially pronounced at 10°C.


Minimum 3.89. Optimum 5-9. No consistent maximum established.


Grows in aerobic and anaerobic conditions.

Water Activity

Maximum salt concentration permitting growth: 9.1%

Favoured in powdered infant formula at low aw and temperature. In a long-term survival experiment the organism was inoculated into powdered infant formula to achieve a final reconstituted concentration of 106 cfu/ml and the powdered infant formula stored in screw-capped bottle at room temperature for 2 years. A final concentration of approximately 300 cfu/ml was measured in the reconstituted product (a 3.4 log10 reduction). Most of the reduction occurred in the first 5 months.


Survived 6 months of freezing in reconstituted powdered infant formula without a decrease in concentration. Ten strains did not grow in reconstituted powdered infant formula stored at 4°C but could be detected by enrichment 72 h after preparation.


Ability to survive moderate acid conditions – ten of twelve strains reduced by less than 1 log10 during a 5-hour challenge at pH 3.5 (at 36°C).

Water Activity

Survives in powdered infant formula (aw = 0.2). Survived better in powdered infant formula at aw 0.25-0.30 than in PIF at aw 0.43-0.50 at both 21°C and 30°C Exponential-phase cells are more sensitive to drying than stationary-phase cells in low Water Activity environments. Dried stationary phase cells survived 46 days at 25°C and 47°C, reducing by around 2 log10 CFU/ml in the first 20 days then remained stable.


No synergistic interactions between inhibitory factors such as weak acids, pH, salt and temperature.

IMPORTANT: Cronobacter infections are incredibly rare. The highest risk group is immunocompromised neonates – who will also be at risk from plenty of other infections too.

Powdered formulae can be used to supplement or replace human breast milk. There are a number of types of powdered formulae for infants and young children, including powdered infant formulae and infant formulae for special medical purposes, follow-up formula, and human milk fortifiers used to supplement breast milk. As a powder it has advantages of cost and storage over liquid forms, however liquid (ready-to-use) infant formula is commercially sterile and is rarely implicated in human illness. In general, powdered formulae have been identified as high-risk foods for the growth of Cronobacter spp. (E. sakazakii) although only powdered infant formula has been implicated in cases of Cronobacter spp. (E. sakazakii) infection. Powdered infant formula is intended for newborns to weaning infants. Its composition closely resembles human breast milk. It is subject to stringent hygiene controls and microbial criteria in its manufacture. Current international standards require Cronobacter spp. (E. sakazakii) to be absent in 30 samples of 10 grams. Follow up formula is a liquid food (derived from milk and/or other constituents of animal/plant origin) that is suitable for weaning infants from their 6th to 12th month. Follow up formula may contain a wider variety of dry-mix ingredients that diversify the diet, e.g. cocoa powder, fruit / vegetable powders or flakes and flavours. Follow up formula generally has a higher protein, iron and mineral content and a higher renal solute load compared to powdered infant formula . International evidence suggests that follow up formula has been consumed by infants <6 months old, and occasionally <1 month old. A general consensus has been reached by the Codex Alimentarius Commission not to establish a microbial criterion for Cronobacter spp. (E. sakazakii) in follow up formula . This is mostly due to a lack of evidence associating illness with follow up formula , but also because feeding follow up formula to infants <6 months old contradicts manufacturers’ directions. Unintended use or misuse of follow up formula has led to calls for clearer labelling and education of caregivers and healthcare professionals regarding the appropriate preparation and use of powdered infant formula and follow up formula.

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