Saturday, April 30, 2011

Walls have ears and bacteria have antibiotic resistance

Recently, I was having my lunch at a conference and couldn't help overhearing a conversation between two people sitting alonside me.  I suppose I should have tried not to listen, but their discussion was in my own field.

It appeared that one of them had read a survey conducted on the bacteria isolated from poultry in USA.  Even "organically grown" chicken contained bacteria that displayed multiple-antibiotic resistance.  This implies that the chicken farms may be using antibiotics in the flock rearing operation, though it doesn't necessarily prove the point.

Without getting into the rights and wrongs of using antibiotics in rearing of "organically grown" poultry, this does point to a wider concern: that farmers may be using sub-therapeutic levels of antibiotics widely to improve growth rates and yields of all meat animals.  Any bacteria that survive and grow in the presence of antibiotic are likely to be resistant to the chemicals and if these bacteria should go on to infect humans, those antibiotics will be useless for treatment.

I'm not sure if these people were talking about a survey*** conducted by the Translational Genomics Research Institute (TGen) in Arizona.  These scientists tested 136 samples of 80 brands of beef, chicken, pork and turkey collected from 26 retail stores in five US cities.  They found that nearly half of the samples contained Staphylococcus aureus and that just over half of these isolates were resistant to at least three classes of antibiotics.  Further analysis suggested that the bacteria were from the animals and not from the processing factories, again indicating that antibiotics had probably been used in rearing the animals.

It might make meat more expensive, but I believe that governments must force the reduction in the amount of antibiotics used in animal rearing if we are to avoid catastrophic consequences for the human population in the near future.  Remember: it has been estimated that the cost of getting a new drug onto the market is now between 2 and 3 billion dollars.  We aren't going to get many new drugs at that rate.

*** The full report is available at:

(The report is a technical report, intended to be read by other scientists, but anyone should be able to follow the discussion).

Friday, April 15, 2011

Tasty but occasionally dangerous

If I asked you to name a food group that might be implicated in food poisoning, you would probably respond with "meat".  I did a survey (n=1) at home today and got that answer.  My wife thought a bit longer and said "Custard, but that's only because you have influenced me over the years".

Baked goods probably don't spring to mind as potential carriers of food poisoning bacteria.

In the last week, there have been reports from Rhode Island of zeppole, or St. Joseph's Day cakes, causing food poisoning by Salmonella.  I had never heard of zeppole before this report, but they are apparently popular in Italy, Sicily and Malta and in the Italian-American communities in the United States and date from the early 19th century.  They are deep-fried dough balls, or sometimes choux pastry, topped with powdered sugar or filled with custard, pastry cream or a mixture of butter and honey.  They sound delicious, though perhaps dangerous for the figure!

At the time of writing, there have been 76 cases and 29 hospitalisations.  Two people have died from Salmonella-associated illness.  It is too soon to say how this happened, but the possibilities include a carrier of Salmonella working as a food handler, or the use of ingredients contaminated with Salmonella.  The deep-fried pastry would probably be sterile immediately after frying, but could be contaminated by the food handler during the filling process.

Breads, fruit cakes and biscuits are usually pretty safe by virtue of low water activity, but filled pastries receive quite a lot of handling, while real cream may be contaminated with bacteria that rapidly grow under abuse temperatures to the point where they can cause food poisoning, either by infection or intoxication.

Monday, April 4, 2011

Will spoiled food make you sick?

I recently received a message from Jo, asking me to comment on this subject.  Jo actually raised a lot of questions and, of course, the answers are not simple.

First question:  If the food smells off, tastes bad, has a poor colour etc. is that a good indicator it will make you sick?

Humans have been on the Earth for a long time and one of the reasons they have survived for so long is that they have evolved self preservation mechanisms.  If the food smells off or doesn't look right, there is a chance that it is poisonous in some way and we tend to avoid it.  Sometimes I wonder what the first people to taste durian or blue cheese thought they were doing!  So, clearly, the answer has to be a qualified "Yes, it might make you sick, but some sort of tribal wisdom suggests that a few foods can look and smell awful but still be OK to eat".

Jo then went on to say that she thought that the food was unlikely to make you sick, as the changes were caused by spoilage bacteria and enzymatic reactions, which are not the same as pathogenic bacteria, which she thought do not alter the food in the same way.

This is where it gets complicated.  Spoilage bacteria do cause some changes, which, by definition, make the food unacceptable to most people.  Other bacteria, such as Salmonella, may grow alongside the spoilage bacteria.  When the food is eaten, the salmonellae  set up an infection in the gut and produce the familiar food poisoning symptoms.

Second question: Is it true that pathogenic bacteria don't alter the food?  Again, it depends on the bacteria.  Generally, clostridia alter the food a lot, producing many smelly compounds and gas.  This would put most people off eating the food, so they would be safe.  But some Clostridium botulinum strains are non-proteolytic.  That is, they don't break down proteins and they don't produce the foul smells that the proteolytic strains make.  So you could find improperly processed canned foods that appear quite normal, but could kill you.

Jo's third question was perhaps the most fascinating: she was most interested in how the food could potentially have both types of bacteria on it near to the time it was made and correctly identified contamination of the food as the cause, possibly by cross-contamination from some source.  A good example would be a careless food handler.

At this point, the food could cause food poisoning if consumed - Salmonella transferred from raw meat to a cream pastry would be a good example here.

Jo went on "However as time goes on and assuming the food gives them all the things they both need to grow, is it as simple as the pathogens might be around for a short time until the spoilage guys, who being better competitors for resources, take over; so by the time the food
is showing signs of being spoiled, the pathogens have been killed off?"

That's a really insightful comment.  Many fermented foods are actually safe because of this pattern.  Take raw cabbage and make sauerkraut:  we shred the cabbage and add about 2.5% salt to it and then press it into a container and seal it.  The salt draws out the tissue fluids from the shredded leaves and bacteria naturally on the leaves begin to produce lactic acid.  If we sample on the first day, we can find all sorts of bacteria, including Escherichia coli and possibly Salmonella.  However, as more acid is produced, the pH falls and the potential pathogens die off.  Finished sauerkraut has a pH around 3.1 to 3.7 and is perfectly safe to eat, though there has been a lot of bacterial growth in it.

So, to return to Jo's original question, "Is spoiled food potentially safe to eat, even though it looks and tastes awful?"    Sorry Jo, I can't answer that.   But if it tastes awful, why would you want to eat it?

One last point:  there is an anecdote that a lady opened a can of peas and thought that they looked a bit different.  She tasted one and cooked the rest.  She died of botulism.  Ironically, if she had cooked them and then tasted one, she would have lived, as the botulin toxin is heat labile.  I don't remember where I read this - it was somewhere around 1980, but it's a good example with which to finish this article.