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 stuff while pissed at conferences to irritate clinicians. Just as we continue to do with classifying strep.
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.
Principal Risk: Baby Formula
Powdered formulae (PF) can be used to supplement or replace human breast milk. As a powder, it has advantages of cost and storage over the liquid form, however liquid (ready-to-use) infant formula is commercially sterile and is rarely implicated in human illness. PF includes all types of powdered formulae for infants and young children, including powdered infant formulae (PIF) and infant formulae for special medical purposes, follow-up formula (FUF), and human milk fortifiers used to supplement breast milk.
In general, PF products have been identified as high-risk foods for the growth of Cronobacter spp. (E. sakazakii) although only PIF has been implicated in cases of Cronobacter spp. (E. sakazakii) infection.
PIF 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 (CAC, 2008) require Cronobacter spp. (E. sakazakii) to be absent in 30 samples of 10 grams.
Follow up formula (FUF) 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. FUF may contain a wider variety of dry-mix ingredients that diversify the diet, e.g. cocoa powder, fruit/vegetable powders or flakes and flavours. FUF generally has a higher protein, iron and mineral content and a higher renal solute load compared to PIF.
International evidence suggests that FUF 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 FUF. This is mostly due to a lack of evidence associating illness with FUF, but also because feeding FUF to infants <6 months old contradicts manufacturers’ directions. Unintended use or misuse of FUF has led to calls for clearer labelling and education of caregivers and healthcare professionals regarding the appropriate preparation and use of PIF and FUF.
Growth and Control
Temperature: 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.
pH Minimum 3.89. Optimum 5-9. No maximum value found in the literature.
Atmosphere Grows in aerobic and anaerobic conditions.
Water Activity Maximum salt concentration permitting growth: 9.1% (Lambert and Bidlas, 2007).
Favoured in PIF at low aw and temperature. In a long-term survival experiment the organism was inoculated into PIF to achieve a final reconstituted concentration of 106 cfu/ml and the PIF 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.
Temperature Survived 6 months of freezing in reconstituted PIF without a decrease in concentration. Ten strains did not grow in reconstituted PIF stored at 4°C but could be detected by enrichment 72 h after preparation.
pH Ability to survive moderate acid conditions is pH-dependent. 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 PIF (aw = 0.2). Survived better in PIF 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.