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On Cronobacter sakazakii

By Dan Flynn and Suzanna Schreck

Cronobacter sakazakii is an environmental contaminant, which mostly affects only a small subset of the population, such as premature babies and infants under 1 year of age.  It is most notable for being associated with the deaths of neonates fed infant formula via naso-gastric tubing. Though it is ubiquitous in nature, only powdered infant formula and preparation equipment have been linked to C. sakazakii outbreaks among infants.

We studied 17 strains of C. sakazakii in our laboratories and found that 15 were able to form biofilms on various materials.  Biofilms are accumulations of cells attached to and growing on a surface; they produce extra-cellular polymeric substances (slime), which glue them to the surface and protect them from antibiotics and cleaning chemicals, so they are hard to control and remove.

Milk powder is manufactured by evaporating pasteurised milk to concentrate it before spray drying.  Temperatures in the milk vary throughout the process, but can reach around 90 C.

C. sakazakii  is heat-sensitive and is therefore unlikely to survive the high temperatures in the milk powder drying process; however it may be found in dry food products, such as skimmed milk powder, lactose, starch, lecithin and banana powder--all ingredients that can be added to powdered infant formula. Logically, this contamination is most likely to originate from the general environment after manufacture before or during the packing of the product.

The dairy industry takes extreme care to ensure that contamination during milk powder manufacture is minimized.  Access to the plant is strictly limited.  The High Care Area of the plant, (where the product is vulnerable to contamination), is protected by a "Red Line", which usually incorporates a physical barrier.  If operators cross the red line, hair must be covered, clothing and boots changed and hands sanitized before entry.  Anecdotal evidence suggests that contamination has occurred most often when standard operating procedures in the manufacturing process have not been strictly followed.

Though the contamination in infant formula has often been attributed to the dried powder, the origin could well be the environment in which the formula is prepared for feeding. Delays between reconstitution of the formula and the actual feeding may allow growth of the contaminants, particularly if the formula is kept warm.  Naso-gastric feeding tubes may be left in place for long periods and initially low numbers of C. sakazakii in the formula could attach to feeding tube and reach levels capable of threatening infant health through biofilm growth on the internal bore of the tubes. As the biofilms grow, individual cells or lumps can slough off and re-enter the formula stream.  The infant's digestive tract is thus continuously inoculated with the bacteria. Infection is likely to be exacerbated by the weak immune system and lack of mature competing intestinal microflora in neonates.

We tested 3 of our 17 C. sakazakii strains for their ability to grow on the surface of three types of naso-gastric feeding tubes in common use in neonatal units. In all tests, the growth on silicone tubing was slower than on the tubing made from polyvinyl chloride or polyurethane. While the difference in growth rate in these trials was not large, it was consistent and likely to be important in the neonatal infant feeding environment.

The risks of naso-gastric feeding may be reduced by selection of silicone tubing, chemical modification of the surface of the tubing to reduce biofilm growth, or by feeding sterilised formula.  It goes without saying that absolute cleanliness and avoidance of temperature abuse must be observed in the reconstitution and handling of formula for neonatal feeding.

Full post as published by Food Safety News on June 21, 2010 (boomark / email).