E. coli strikes again

Five people in the United States have died since March and approximately 200 are sick as a result of an Escherichia coli infection apparently acquired from romaine lettuces grown in the Yuma region of Arizona.  However, the authorities have been unable to pinpoint the source of the contamination.

Though E. coli are found in the gut of man and animals, only certain strains cause serious illness.  These strains have assembled many genes that enable them to attach to the cells lining the intestine and produce very damaging toxins called Shigatoxins.  This is an example of the continuous evolution of bacteria.  Many of these new strains are the result of bacteriophage infection transferring genes between bacteria or possibly by direct transfer between strains.

Various posts responding to news reports of the outbreak have suggested washing the lettuces as a means of preventing illness.  Unfortunately, washing, even in chlorinated water, will not guarantee removal of the bacteria, as they can attach to the lettuce or even localise in the stomata of the leaves.  I obtained the following image from https://www.inverse.com/article/28938-e-coli-detection-lettuce-feces.

You can see bacteria within the stoma; it is clear that they would be very difficult to wash out.

This outbreak was originally thought to be caused by bagged and chopped lettuce, but inmates in a prison in Alaska also became ill after eating whole head lettuce.  In view of these facts, it is unfortunate that consumer advocates are urging the FDA to introduce new rules to speed up investigation of such outbreaks.  It is hard to see how new rules would influence the investigations, given that it has so far been impossible to tie the contamination to a single farm, processor or distributor.

The one fairly sure conclusion is that the lettuces have become contaminated with faeces, since this E. coli is not a natural inhabitant of the environment.

Giving bacteria a cold

I hope that title got your attention. I could equally have used "Fight fire with fire" or "Biological warfare".

Food Standards Australia New Zealand is calling for submissions on an application to use viruses to aid in the control of Listeria monocytogenes.  L. monocytogenes is a bacterium found in the environment that can infect food and may cause listeriosis.  The majority of the population is unaffected by Listeria, but it is a hazard for pregnant women, young children and the elderly, in addition to those who are immunocompromised.  Note that you don't have to have had an organ transplant to be immunocompromised - you might just have another infection stressing your immune system.

You might think that deliberately putting viruses into foods is crazy.  However, these viruses are special.  They exclusively target bacteria and are called bacteriophages.  Bacteriophages (or "phages") are extremely common; they occur at levels up to 9×10⁸ per mL in bacterial mats at the surface of the sea.  They were first identified in 1915 by Frederick Twort in England and, independently, in 1917 by Felix d'Herelle in France.

Bacteriophages are deceptively simple - essentially a piece of nucleic acid contained within a protein coat called a capsid.  They can't reproduce independently and have to have a means of infecting a living cell.  Perhaps the most famous phage is the T4 phage that infects Escherichia coli and always reminds me of the lunar lander.  (I sometimes wonder how something like this could have evolved by chance).  See image from Wikipedia below.  

 When a phage infects a bacterial cell, it injects its DNA into the cell.  The viral nucleic acid then takes over the bacterial synthetic machinery and makes copies of itself, and synthesises new phage coat and other components.  The components are then assembled into new phage particles, whereupon the bacterial cell is lysed and releases the phage.  Burst sizes may be around 100 viruses per bacterial cell.  Since these are all infective, the infection of the population proceeds rapidly, resulting in the death of the majority of the bacterial cells.  Phages cause the cheese-making industry a lot of trouble, because they kill the starter bacteria.

The application currently under consideration is for the use of a mixed bacteriophage preparation, sold commercially as LISTEX™ P100, as a processing aid.  This preparation was the first phage product to be classified as Generally Recognized as Safe (GRAS) by the FDA  and USDA.

A number of scientific papers have been published on the efficacy of the P100 preparation, showing that the phage significantly reduces the population of L. monocytogenes on foods, such as salmon fillets or surface-ripened cheeses*.

The use of bacteriophage to control pathogens, such as Listeria and Salmonella, can reduce the risk of food poisoning, though it is unlikly that it can be a total solution, as the pathogen population may not be totally destroyed.  However, when used in an integrated food safety programme, the processing aid can be a valuable tool, reducing the reliance on chemicals to inhibit the bacteria.

*  See:  doi:10.1016/j.yrtph.2005.08.005

Electron micrograph of T4 phage adsorbed to a bacterial cell.

Image by Elizabeth Kutter, Bacteriophage Ecology Group

http://www.mansfield.ohio-state.edu/~sabedon/beg_phage_images.htm

Odd Spot:  When I was researching this post, I searched on "Listeria virus".  I got many hits, predominantly in popular press and websites, where the Listeria bacteria were described as "viruses".  In modern microbiology terms, this is totally wrong.  However, "virus" is derived from the Latin word for poison and this apparently appeared in the English lexicon in 1392. "Virus" was also used in 1728 to describe an "agent that causes infectious diseases".  I'm sure that most of those press writers didn't know this, but, strictly, they were correct.

Evolution in action

Over the last three weeks, a rare form of Escherichia coli has made an appearance in Germany.

E. coli is a normal inhabitant of the gut of man and animals and is found there in very large numbers.  They are mostly harmless and actually help us by producing vitamin K.  However, some strains are pathogenic and cause diarrhoea.

Individual strains are recognised by their antigenic signature. The antigens are found on the surface of the cells, on the flagella and in the capsule that surrounds the cells.  Thus one of the strains that hits the news quite frequently is referred to as O157:H7.

The strain now causing havoc in Europe is E. coli O104:H4.  This has rarely been seen as a cause of disease.  However, the current outbreak is shaping up to be one of the most dangerous ever seen.

Why has this happened?

That's a question that none of us can answer yet, but the information on this strain is growing rapidly.  It can produce a very damaging toxin, called Shigatoxin.  The bacterium causes bloody diarrhoea, which is bad enough in itself.  But bacteria producing this toxin can also go on to produce Haemolytic Uraemic Syndrome, normally affecting around 2 to 10% of patients, who are often very young or immunocompromised.  However, O104:H4 appears to be very virulent and around 30% of patients, who were not in the high risk group, have developed this life-threatening syndrome.  So far, 18 people have died.

The fact is, bacteria evolve very rapidly.  We see one strain develop resistance to an antibiotic and soon other strains become resistant too.  This happens because bacteria can  exchange genetic information by a number of different mechanisms.  The rate of mutation might be very small, perhaps one in 10 million replications produces a mutant and most of these mutations are probably lethal.  However, a single cell can potentially produce a population of around 17 million cells in 8 hours.  That allows for a lot of mutations.  If even one of these mutants has some advantage over the rest of the population, or at least no disadvantage, the mutation will spread through the population.

This might surprise you, but bacteria also suffer from virus infections.  These viruses, or bacteriophages, invade the cells and cause them to make more virus particles before bursting open to restart the infection cycle.  Occasionally, the new virus particles contain a bit of bacterial DNA and transfer it to the next host.  Sometimes, whole genes can be transferred.

It appears that O104:H4 has not only the Shigatoxin gene, almost certainly transferred by bacteriophage, but has also picked up some other virulence factors.  This is why it is so dangerous - the toxin is very damaging to human cells, particularly in the kidney, and the bacterium appears very capable of initiating infection.

I believe we are seeing evolution in action.  A normally benign bacterium has become a killer.