In this article, let's unravel the mystery of Cronobacter sakazakii, a bacterium that's causing concern due to its sneaky way of contaminating infant formula. We'll tackle why this particular germ is worrisome for babies and also discuss the dos and don'ts when preparing baby formula, all from the viewpoint of keeping those tiny tots safe.
Where Does It Belong?
Cronobacter sakazakii is part of the big family of bacteria known as Enterobacteriaceae. It's a Gram-negative, facultatively anaerobic bacterium. Until 2007, it was grouped under the Enterobacter genus, but then it got its own 'Cronobacter' label.
The name 'Cronobacter' is a bit of a dark nod to the Greek myth of Kronos, who had a bad habit of eating his children, referencing the bacterium's harmful effect on infants.
Where Can You Find It?
This sneaky bacterium isn't just hanging out in animal guts; it's pretty much everywhere in the environment. That means it can pop up in all sorts of places, making it hard to keep tabs on.
Foods that often get mixed up with Cronobacter sakazakii include herbal teas, spices, starchy foods, powdered soups, dried foods, and even salads. It's also been found lurking in food factories. While it doesn't typically cause a big fuss in adults, it's a different story for babies, especially in powdered infant formula.
The Trouble It Causes
Cronobacter sakazakii doesn't play favorites – it can infect anyone, though it's particularly tough on babies and older folks with weaker immune systems. In adults, it's usually linked to infections after surgery or in the urinary tract.
The above figure is taken from Emerging Infectious Diseases - www.cdc.gov/eid - Vol. 20, No. 9, September 2014. This image is posted here as is, in accordance with the public domain policy of this journal.
Above figure cited from Emerging Infectious Diseases, an open access journal published by the US Centres for Disease Control and Prevention - www.cdc.gov/eid - Vol. 20, No. 9, September 2014.This journal Published in the original figure due to the public domain policy of the
What's scary about this bug is that it's especially dangerous for newborns, especially those born a bit too early or small. They can get really serious infections like meningitis, a nasty intestinal condition called necrotizing enterocolitis, and even sepsis, with a tragically high chance of not making it. Even the survivors can face lifelong challenges like developmental delays.
As for how it does its dirty work, it seems to be good at sticking to certain cells in the gut and might use specific proteins to wreak havoc.
Interestingly, it's even sneakier in babies because their guts aren't fully developed, making it easier for the bacterium to get into places it shouldn't. The most common problems it causes in infants are sepsis and meningitis
Interestingly, there's a concern that Cronobacter sakazakii infections might be underestimated in the United States. This is because Minnesota is the only state that requires reporting when Cronobacter sakazakii is found in infants under one year old. In other states, this reporting isn't mandatory. So, it's feared that the frequency of Cronobacter sakazakii infections in the U.S. might be higher than what's currently known, as suggested by the CDC.
The Notorious MLST ST4 in Cronobacter sakazakii
In the world of bacteria, identifying the bad guys isn’t always straightforward. One method, known as MLST (Multi Locus Sequence Typing), helps us genetically type pathogens beyond just their species. For Cronobacter sakazakii, 66 sequence types are defined (www.pubMLST.org/cronobacter). In one study of 41 clinical isolates of C. sakazakii, half (20/41 strains) were found to be sequence type (ST) 4. What's more, 9 out of 12 strains causing meningitis were ST4. This suggests that ST4 is a particularly nasty genetic type (sequence type: ST) of C. sakazakii, prone to causing neonatal meningitis.
*Stay tuned for an upcoming blog post for an easy-to-understand explanation of MLST technology.
Furthermore, the presence of multidrug-resistant strains of Cronobacter sakazakii has been confirmed, raising concerns about infections that can’t be treated with antibiotics.
Cronobacter sakazakii's Strength in Dryness
One reason why Cronobacter sakazakii is problematic in infant formula and other dried foods is its resilience to dryness. Now, while it's a Gram-negative bacterium from the Enterobacteriaceae family and doesn't boast the dry-resistance superpowers of Gram-positive bacteria, it's still considered one of the toughest Gram-negative bacteria in dry conditions.
A study looking at various Enterobacteriaceae bacteria's survival under dry conditions revealed three groups based on their dry resistance:
- The weakest group in dry conditions includes Citrobacter koseri, Citrobacter freundii, and Enterobacter cloacae. These guys couldn't be detected on dry solid surfaces after six months.
- The intermediate group, which includes Salmonella Enteritidis, Escherichia coli, and Klebsiella pneumoniae, managed to survive up to 15 months in dry conditions.
- The toughest group under dry conditions, which includes Cronobacter sakazakii, Escherichia vulneris, Klebsiella oxytoca, and Pantoea spp. These bacteria could be recovered even two years later!
The figure above has been re-drawn from the data and graphs in the following paper.
Dry stress and survival time of Enterobacter sakazakii and other Enterobacteriaceae in dehydrated powdered infant formula
J Food Prot. 2007 Sep;70(9):2111-7.
So, Cronobacter sakazakii belongs to the top tier of dry-resistant bacteria in the Enterobacteriaceae family. This resilience to dryness is why it poses a risk of lingering in infant formula and other dried foods, potentially leading to infections.
The Role of Capsules in Cronobacter sakazakii's Resistance to Dryness
Why does Cronobacter sakazakii, a Gram-negative bacterium from the Enterobacteriaceae family, vary in its resistance to dryness? What mechanisms are behind this resilience?
Not all strains of Cronobacter sakazakii, but many, are known to form capsules and extracellular polysaccharides. Let’s quickly clarify the relationship between bacterial capsules, extracellular polysaccharides, and biofilms:
- The Capsule: The term 'capsule' might make you think of a tough, cell-like structure. However, it's not quite that. It's just a state where high-molecular polysaccharides secreted by bacteria uniformly coat the cell. These polysaccharides are essentially the same as those in biofilms.
- Extracellular Polysaccharides: As the name suggests, these are polysaccharides that bacteria secrete outside the cell. They aren’t as structurally defined around the cell as capsules.
- Biofilms: This is when cells secrete polysaccharides extensively between cells, forming a collective group.
Understanding that capsules, extracellular polysaccharides, and biofilms are essentially the same but differ in structure can be helpful.
These structures are not produced by all bacterial strains and can vary depending on environmental conditions. So, not all bacteria form capsules or produce extracellular polysaccharides, and their presence can depend on the environment. Their roles typically include defense against host immune cells and survival strategies in harsh environments. Many pathogens form capsules when infecting animals, though they might not do so in pure culture. Microbes also tend to adopt the biofilm strategy, secreting polysaccharides on solid surfaces when environmental conditions worsen.
Now, why is Cronobacter sakazakii so tough against dryness? As mentioned, many strains of C. sakazakii can form capsules and extracellular polysaccharides. Furthermore, researchers who compared the survival of Enterobacteriaceae in dry conditions found that:
- In the first 18 months of dry storage, there was no correlation between capsule formation and recovery.
- However, after two years, 4 out of 5 recovered strains were those forming capsules.
- After 2.5 years, only 2 strains with capsules could be recovered.
These findings suggest that the presence of a capsule plays a significant role in the long-term survival of Cronobacter sakazakii under dry conditions.
Contamination Patterns in Infant Formula
Internationally, it's generally tough to completely prevent Cronobacter sakazakii contamination in the manufacturing processes of infant formula factories (CDC). One reason is that the nutrients in infant formula are easily lost during heat sterilization, so they are often added in the final stages after heating. It's also considered impossible to completely prevent bacterial contamination at the final packaging stage.
*Note: Manufacturing processes vary by manufacturer. The above refers to a general international process.
Moreover, since Cronobacter sakazakii can also be found in everyday environments, there's a chance of contamination when preparing powdered infant formula with hot water at home.
Preparing with Water at 70°C
The WHO sets the following guidelines for preparing powdered infant formula at home: always use water hotter than 70°C to dissolve the powder. But why 70°C?
This is where understanding the concept of the D-value, related to microbial sterilization, becomes crucial.
The D-value in heat sterilization measures how long it takes to kill 90% of bacteria at a certain temperature. For instance, a D-value of 2 minutes at 70°C means in 2 minutes, 90% of bacteria are destroyed. This helps make food safe without harming its quality.
Let's do some calculations.
Research on heat experiments with Cronobacter sakazakii determined a D-value of 3.9 seconds at 70°C. Meanwhile, contamination levels of Cronobacter sakazakii inpowdered milk are often 1cfu/100g or less. Therefore, applying a 4D heat treatment at 70°C, which equals 3.9 seconds × 4 = 15.6 seconds, can reduce the bacteria to 0.0001cfu/100g. This calculation suggests that you can virtually eliminate the risk of infection by Cronobacter sakazakii by heating the milk powder at 70°C for the appropriate time.
Understanding this concept of D-value and doing the math ourselves can help us comprehend how effectively we can prevent Cronobacter sakazakii infections when preparing powdered milk at 70°C.
Liquid Milk is the Safest Option
Unlike powdered infant formula, liquid infant milk sold in bottles or cartons is either sterilized after packaging or adequately sterilized in liquid form and then packaged in a sterile environment. Therefore, the risk of Cronobacter sakazakii contamination in liquid milk is considered nonexistent. The CDC, especially for infants under three months, recommends using liquid milk whenever possible.
In Japan, following the revision and implementation of the 'Ministerial Ordinance on Component Standards of Milk and Dairy Products' and the 'Standards for Foods, Additives, etc.' by the Ministry of Health, Labour and Welfare in 2018, the domestic production and sale of liquid infant milk have become possible.
